Living Environment

Notes

Mrs. J. Frydberg

2

Living Environment Notes

© 2015 J. Frydberg

3

Table of Contents

Notes

Chapter 1 – The Science of Biology 5

Chapter 18 – Classification 11

Chapter 2 – The Chemistry of Life 15

Chapter 7 – Cell Structure and Function 24

Chapter 8 – Photosynthesis 35

Chapter 9 – Cellular Respiration 40

Chapter 10 – Cell Growth and Division 45

Chapter 35 – Nervous System 49

Chapter 36 – Skeletal, Muscular, and Integumentary Systems 57

Chapter 37 – Circulatory and Respiratory Systems 63

Chapter 38 – Digestive and Excretory Systems 72

Chapter 39 – Endocrine and Reproductive Systems 81

Chapter 40 – Immune System and Disease 92

Chapter 11 – Introduction to Genetics 99

Chapter 12 – DNA and RNA 105

Chapter 13 – Genetic Engineering 115

Chapter 14 – The Human Genome 120

Chapter 15 – Darwin’s Theory of Evolution 125

Chapter 16 – Evolution of Populations 130

Chapter 17 – The History of Life 135

Chapter 3 – The Biosphere 143

Chapter 4 – Ecosystems and Communities 151

Chapter 5 – Populations 160

Chapter 6 – Humans in the Biosphere 163

4

5

Chapter 1 – The Science of Biology

1-1 What is Science?

Biology – the study of life. It is the science that seeks to understand the living world.

The goal of science is to investigate and understand the natural world, to explain events in

the natural world, and to use those explanations to make useful predictions.

Science is an organized way of using evidence to learn about the natural world.

Observation is the process of gathering information about events or processes in a careful,

orderly way. Data is the information gathered from observations. An inference is a logical

interpretation based on prior knowledge or experience. A hypothesis is a proposed scientific

explanation for a set of observations. A hypothesis must be in the form of a statement, not in

the form of a question.

Whenever possible, a hypothesis should be tested by an experiment in which only one

variable is changed at a time. All other variables should be kept unchanged, or controlled.

1-2 How Scientists Work

Designing an Experiment:

Identify the problem to be solved by asking a question

Form a hypothesis (educated guess) – must be in the form of a statement

Set up a controlled experiment – the independent variable is the one that is

manipulated or changed; the dependent variable is what we observe changing in

response to the manipulated variable

Record and analyze results – collect observations and data

Draw a conclusion – use data to evaluate the hypothesis and make a conclusion

Repeating Investigations

It is necessary to repeat experiments/investigations to show the results are the same

each time and ensure the results are reliable. Experiments should have a large sample size to

increase precision.

Example: Redi’s experiment on spontaneous generation (life from nonliving). In

1668, Francesco Redi challenged the idea of spontaneous generation using jars with meat

(uncovered, covered, and with a gauze covering). It showed that maggots appeared on the

meat when uncovered but not on meat when covered.

Redi’s experiment:

6

In the 1700s, John Needham challenged Redi’s work, claiming spontaneous

generation could occur under the right conditions. He boiled gravy to kill microorganisms

but days later the gravy was filled with microorganisms. Lazzaro Spallanzani read about

both expriements and showed microorganisms will not grow in boiled gravy when the flask

is sealed but will grow when the flask is open. Many scientists argued his experiment was

unfair because no air = no life.

In the 1800s, Louis Pasteur used a curved neck flask to prove Spallanzani was

correct. He boiled a broth which remained free of microorganisms for a year; once the

curved neck was broken off the flask, microorganisms began to grow in the broth.

A hypothesis can be considered a theory if it is well supported by observations and

data from many investigations.

Pasteur’s experiment:

In science, the word theory applies to a well-tested explanation that unifies a broad range

of observations.

1-3 Studying Life

Characteristics of Living Things

Describing what makes something alive is not easy. No single characteristic is

enough and many nonliving things share traits with living things.

Living things share the following characteristics:

Made of units called cells (unicellular and multicellular)

Reproduce (sexual and asexual)

7

Genetic code (DNA – deoxyribonucleic acid)

Grow and develop

Obtain and use materials and energy (metabolism)

Respond to the environment (stimulus)

Maintain stable internal environment (homeostasis)

As a group, change over time (evolution)

Some properties of life:

Unicellular means single-celled. Multicellular means many-celled. Sexual

reproduction involves cells from two different parents uniting to produce the new organism.

Asexual reproduction involves only one parent. Metabolism is the combination of chemical

reactions through which an organism builds up or breaks down materials as it carries out its

life processes; it is all the activities an organism must perform to sustain life. A stimulus is a

signal to which an organism responds. Homeostasis is the process by which organisms

maintain a constant internal environment. Evolution is the ability of a group of organisms to

change over time.

Branches of Biology

Various branches exist: zoologists – animals, botanists – plants, paleontologists –

ancient life. Some fields focus on living systems at different levels of organization:

Biosphere – any part of earth where life can exist

Ecosystem – community and the nonliving environment

Community – populations that live together in an area

Population – group of same species in an area

Organism – individual

Groups of cells – tissues, organs, and organ systems

8

Cells – basic units of life

Molecules – groups of atoms; smallest unit of compounds

Some of the levels at which life can be studied include molecules, cells, organisms,

populations of a single kind of organism, communities of different organisms in an area, and

the biosphere. At all these levels, smaller living systems are found within larger systems.

Levels of organization:

8 Life Functions/Processes

1. Regulation – maintains homeostasis; control and coordination of all life processes

2. Nutrition – take materials from environment and change them to useful forms for

growth and repair; includes ingestion, digestion, and egestion

3. Transport – substances move into, are distributed within, and move out of cells

4. Respiration – releasing chemical energy stored in nutrients

5. Synthesis – building complex substances from smaller ones

6. Growth – increase in organism size due to increase in cell size and/or cell number

7. Excretion – removal of metabolic wastes (CO2, water, salt, and nitrogen compounds)

from chemical reactions

8. Reproduction – new individuals are made; this is the only life function that is NOT

needed for the survival of the individual (but is necessary for survival of the species)

1-4 Tools and Procedures

Common Measurement System

Scientists need a common system of measurement in order to replicate each other’s

experiments. The metric system is a decimal system of measurements whose units are absed

9

on certain physical standards and are scaled on multiples of 10. Length = meter, mass =

gram, volume = liter, temperature = Celsius.

10

Metric conversion chart:

Most scientists use the metric system when collecting data and performing experiments.

Microscopes

Microscopes are devices that produce magnified images of structures that are too

small to see with the unaided eye.

Light microscopes produce magnified images by focusing visible light rays. Electron

microscopes produce magnified images by focusing beams of electrons.

Parts of a compound light microscope:

The compound light microscope allows light to pass through the specimen and uses

two lenses to form an image (magnification of about 1000 times). The electron microscope

uses beams of electrons to produce an image that is usually 1000 times better than the light

microscope.

11

Size range of cells and microscopy:

Lab Techniques

A cell culture is a group of cells grown in a nutrient solution. Cell fractionation is

used to separate different cell parts. Cells are broken by a special blender, added to liquid in

a tube, and then placed in a centrifuge which spins to separate the cell parts by density.

Cell fractionation:

12

Chapter 18 – Classification

18-1 Finding Order in Diversity

Why Classify?

Taxonomy is the discipline in which scientists classify organisms and assign each a

universally accepted name.

To study the diversity of life, biologists use a classification system to name organisms and

group them in a logical manner.

Assigning Scientific Names

Common names were too confusing because of the variations in languages and/or

regions. 18th

century scientists used Latin and Greek for scientific names. Early efforts

described physical characteristics but the names were too long and it was hard to standardize

names because different characteristics were being described.

In binomial nomenclature, each species is assigned a two-part scientific name.

Binomial nomenclature is a two-word naming system developed by Carolus Linnaeus

(Carl von Linné). Each species is assigned a 2 part name: 1st part is capitalized (Genus), 2

nd

part is lowercased (species). A genus is a group of closely related species. A species name is

a specific character. Ex: grizzly bear = Ursus arctos; polar bear = Ursus maritimus; black

bear = Ursus americanus.

Linnaeus’s System of Classification

Linnaeus’s hierarchical system of classification includes seven levels. They are – from

smallest to largest – species, genus, family, order, class, phylum, and kingdom.

Hierarchical system of classification:

13

Each level in taxonomic nomenclature is called a taxon (category). The species is the

smallest and least inclusive (most specific) of the taxa and the kingdom is the largest and

most inclusive.

18-2 Modern Evolutionary Classification

In Linnaeus’s time, organisms were grouped according to visible similarities and

differences.

Evolutionary Classification

Phylogeny is the study of evolutionary relationships among organisms. Organisms

that look similar may not have a recent common ancestor. Evolutionary classification is the

strategy of grouping organisms based on evolutionary history.

Biologists now group organisms into categories that represent lines of evolutionary

descent, or phylogeny, not just physical similarities.

Classification Using Cladograms

Derived characters are characteristics that appear in recent parts of a lineage but not

in its older members. A cladogram is a diagram showing evolutionary relationships among a

group of organisms; it is constructed using derived characters. Phylogenetic trees can also be

used to show evolutionary relationships but the lengths of branches can be proportional to the

length of evolutionary time.

Cladogram and phylogenetic tree:

Similarities in DNA and RNA

All organisms use DNA and RNA to pass on information and to control growth and

development. Genes are the most basic level of comparison.

14

The genes of many organisms show important similarities at the molecular level.

Similarities in DNA can be used to help determine classification and evolutionary

relationships.

Molecular Clocks

A molecular clock is a model that uses DNA comparisons to estimate the length of

time that two species have been evolving independently. The degree of dissimilarity

indicates the length of time.

18-3 Kingdoms and Domains

The Tree of Life Evolves

In Linnaeus’s time there were only animals (mobile organisms that used food for

energy) and plants (green, photosynthetic organisms that used energy from the sun) as

categories for classification: kingdoms Animalia and Plantae.

As microorganisms were being discovered, scientists agreed that new kingdoms were

needed. The five kingdom system included: Monera, Protista, Fungi, Plantae, and Animalia.

In recent years, evidence from microorganisms led to the split of Monera into 2 distinct

groups, and so there were six kingdoms.

The six-kingdom system of classification includes the kingdoms Eubacteria,

Archaebacteria, Protista, Fungi, Plantae, and Animalia.

Most recent evolutionary trees are produced using comparative studies of a small

subunit of ribosomal RNA. A new taxonomic category, larger and more inclusive than

kingdom, is the domain.

The three domains are the domain Eukarya, which is composed of protists, fungi, plants,

and animals; the domain Bacteria, which corresponds to the kingdom Eubacteria; and the

Archaea, which corresponds to the kingdom Archaebacteria.

Domain Bacteria

Bacteria are unicellular and prokaryotic. They have thick, rigid cell walls around the

cell membrane. They have peptidoglycan in their cell walls. This domain corresponds to the

kingdom Eubacteria, ranging from free-living soil organisms to deadly parasites.

Domain Archaea

Archaea are also unicellular and prokaryotic; they live in extreme environments such

as volcanic hot springs, brine pools, and black organic mud. Their cell walls lack

peptidoglycan. This domain corresponds to the kingdom Archaebacteria.

Domain Eukarya

Eukarya consists of all organsisms that have a nucleus and is organized into four

remaining kingdoms: Protista, Fungi, Plantae, and Animalia.

15

The kingdom Protista has great variety. Most are unicellular but some are

multicellular or colonial. Some are photosynthetic and others are heterotrophic. They can be

plant-like, animal-like, or fungus-like.

The kingdom Fungi consists of heterotrophs. They mostly feed on dead, decaying

matter by secreting digestive enzymes into the food source and then absorbing the nutrients.

Most are multicellular (mushrooms) but some are unicellular (yeast).

The kingdom Plantae is made up of multicellular, photosynthetic autotrophs. They

are nonmotile and have cellulose in their cell walls. There are various types of plants

including mosses, ferns, cone-bearing plants (gymnosperms), and flowering plants

(angiosperms).

The kingdom Animalia consists of multicellular heterotrophs that lack cell walls.

Most animals can move about. This kingdom has incredible diversity.

Three domains of life:

16

Chapter 2 – The Chemistry of Life

2-1 The Nature of Matter

Atoms

Atoms are the basic unit of matter. Democritus, a Greek philosopher, first referred to

matter as atomos which means “cannot be cut”. Subatomic particles in atoms are: protons

(positive), neutrons (no charge), and electrons (negative).

The subatomic particles that make up atoms are protons, neutrons, and electrons.

Elements and Isotopes

An element is a pure substance of entirely one type of atom. It cannot be broken

down into simpler substances. The most common elements in living things are: carbon (C),

oxygen (O), hydrogen (H), and nitrogen (N).

Isotopes are atoms of the same element that have the same number of protons and

electrons but a different number of neutrons and therefore different mass numbers (the

number of protons plus neutrons). Ex: carbon-12, carbon-13, and carbon-14 are different

isotopes of carbon. Atomic mass is the weighted average of all naturally occurring isotopes

of an element.

Radioactive isotopes have unstable nuclei and break down at a constant rate over

time. By analyzing the isotopes found in rocks and fossils, scientists can determine their age.

Because they have the same number of electrons, all isotopes of an element have the same

chemical properties.

Chemical Compounds

A compound is a pure substance formed by combining two or more different

elements in definite proportions. Ex: H2O, NaCl, CO2. Physical and chemical properties of

compounds are usually very different from the properties of the elements from which they

form.

Chemical Bonds

Atoms in compounds are held together by chemical bonds which involve valence

electrons, the outermost electrons in atoms.

The main types of chemical bonds are ionic bonds and covalent bonds.

An ionic bond forms when one or more electrons are transferred from one atom to

another. An atom that loses electrons becomes positively charged and an atom that gains

electrons becomes negatively charged; charged atoms are called ions. Ex: a sodium atom

loses 1 valence electron to become Na+; a chlorine atom gains 1 valence electron to become

Cl–.

A covalent bond forms when electrons are shared between atoms. Single covalent

bonds result from the sharing of two electrons, double bonds from sharing four electrons, and

17

triple bonds from sharing six electrons. A molecule, the smallest unit of most compounds, is

formed by covalent bonding. Molecules are neutral.

Intermolecular forces, which are slight attractions between oppositely charged regions

of molecules, are called van der Waals forces. They are weaker than ionic and covalent

bonds but can hold molecules together.

2-2 Properties of Water

Water covers ¾ of Earth and is the single most abundant compound in most living

things.

The Water Molecule

Polarity is caused by an uneven distribution of charge. The oxygen end has a slight

negative charge and the hydrogen end has a slight positive charge.

A water molecule is polar because there is an uneven distribution of electrons between the

oxygen and hydrogen atoms.

A hydrogen bond is not actually a bond; it is an intermolecular force or attraction

between the hydrogen of one water molecule and the oxygen of another. Cohesion is an

attraction between molecules of the same substance. Adhesion is an attraction between

molecules of different substances. Ex: water droplets are connected to each other by

cohesion and attach to a glass by adhesion.

Hydrogen bonds between water molecules:

Solutions and Suspensions

A mixture is a material composed of two or more elements or compounds physically

mixed but not chemically combined. Ex: air is a mixture of gases. A solution is a mixture in

which components are evenly distributed (homogeneous). Ex: salt dissolves in water to

make salt water. Solutions consist of two parts: solute (the substance that dissolves) and

solvent (the substance in which the solute dissolves). In salt water, the salt is the solute and

water is the solvent.

18

A suspension is a mixture in which the materials do not dissolve but separate into

pieces that do not settle out. Ex: milk. Some important fluids are both a solution and

suspension. Ex: blood.

Acids, Bases, and pH

Water can react to form ions as shown here: H2O ↔ H+ + OH

– (water ↔ hydrogen

ion + hydroxide ion). The double arrow means the reaction can happen in either direction.

The pH scale indicates concentration of H+ ions in solution. Lower pH = more H

+ ions;

higher pH = less H+ ions (more OH

– ions).

An acid is a compound that forms hydrogen ions in solution and has pH values from 0

up to 7.

Acidic solutions contain higher concentrations of H+ ions than pure water and have pH

values below 7.

A base is a compound that forms hydroxide ions in solution and has a pH above 7

through 14.

Basic, or alkaline, solutions contain lower concentrations of H+ ions than pure water and

have pH values above 7.

Buffers are weak acids or bases that react with strong acids or bases to prevent sharp,

sudden pH changes.

pH scale:

19

2-3 Carbon Compounds

The Chemistry of Carbon

Organic compounds are created by organisms and must have carbon in them. Carbon

has four valence electrons so it can form four covalent bonds. The bonds can be single,

double, or triple. Carbon bonds with many elements (O, H, N, P, and S).

Macromolecules

A macromolecule is a giant molecule made from thousands or hundreds of thousands

of smaller molecules; many are found in living cells. Monomers are smaller units joined to

form polymers during polymerization.

Four groups of organic compounds found in living things are carbohydrates, lipids,

nucleic acids, and proteins.

Carbohydrates

Carbohydrates are compounds made of C, H, and O (usually in a 1:2:1 ratio).

Organisms use carbohydrates as their main source of energy. The breakdown of sugars

(glucose) supplies energy and excess sugar is stored as complex carbohydrates (starches).

Monosaccharides are simple sugars (Ex: glucose, galactose, and fructose). Polysaccharides

are complex carbohydrates (Ex: glycogen, plant starch, cellulose).

Glucose:

Living things use carbohydrates as their main source of energy. Plants and some animals

also use carbohydrates for structural purposes.

Carbohydrates serve as fuel and building material:

20

Lipids

Lipids are compounds made of C, H, and O (not in a 1:2:1 ratio). Lipids include fats,

oils, and waxes; they are not soluble in water. They can be used to store energy. Glycerol

combines with three fatty acids to produce a lipid. Fatty acids are considered saturated when

there are all single bonds because they have the maximum number of hydrogen atoms. They

are considered unsaturated if there is at least one double or triple bond present because there

is less than the maximum number of hydrogen atoms; they are polyunsaturated when there is

more than one double or triple bond present.

Lipid:

Lipids can be used to store energy. Some lipids are important parts of biological

membranes and waterproof coverings.

Lipids are a diverse group of hydrophobic molecules:

Nucleic Acids

Nucleic acids are compounds made of C, H, O, N and P; they are polymers made up

of monomers called nucleotides. A nucleotide has three parts: a 5-carbon sugar, a phosphate

group, and a nitrogenous base. Nucleic acids store and transmit genetic information.

21

There are two kinds of nucleic acids: DNA (deoxyribonucleic acid) and RNA

(ribonucleic acid). They differ by the type of sugar (deoxyribose or ribose), general structure

(double-stranded or single-stranded), and the type of bases present. DNA has A (adenine), T

(thymine), C (cytosine), and G (guanine). RNA has A, C, G but not T; instead it has U

(uracil).

DNA forms a ladder shape:

Nucleic acids store and transmit hereditary, or genetic, information.

Nucleic acids store, transmit, and help express hereditary information:

Proteins

Proteins are compounds made of C, H, O, and N; they are polymers made up of

monomers called amino acids. Amino acids have an amino group (–NH2) and a carboxyl

group (–COOH). There are twenty different amino acids.

The amino group of one joins to the carboxyl group of another amino acid. The R

group differs in each amino acid (Ex: acidic or basic; polar or nonpolar). Each protein has a

22

specific role: control reaction rate, regulate cell processes, form bones and muscles, transport

substances into or out of cells, and help fight diseases. There are up to four levels of

organization: 1st = sequence of amino acids; 2

nd = amino acid within chain can be twisted

(alpha helix) or folded (pleated sheet) due to hydrogen bonds; 3rd

= three-dimensional folding

of chains due to attraction between helices and sheets; 4th

= protein consisting of more than

one amino acid chain.

General structure of amino acids:

Levels of protein structure:

Some proteins control the rate of reactions and regulate cell processes. Some proteins

build tissues such as bone and muscle. Others transport materials or help to fight disease.

23

Proteins include a diversity of structures, resulting in a wide range of functions:

2-4 Chemical Reactions and Enzymes

Chemical Reactions

A chemical reaction is a process that changes, or transforms, one set of chemicals into

another. Mass and energy are conserved. Reactants enter a chemical reaction and products

are produced by a chemical reaction. Several chemical reactions occur in the body. For

example, carbon dioxide reacts with water to produce carbonic acid so the blood can carry

carbon dioxide to the lungs. Ex: CO2 + H2O → H2CO3. The reverse reaction happens in the

lung to produce carbon dioxide we exhale. Ex: H2CO3 → CO2 + H2O.

Chemical reactions always involve changes in the chemical bonds that join atoms in

compounds.

Energy in Reactions

Reactions either absorb or release energy. Reactions that absorb energy are called

endothermic; reactions that release energy are called exothermic. Activation energy is the

amount of energy needed to get a reaction started.

Chemical reactions that release energy often occur spontaneously. Chemical reactions

that absorb energy will not occur without a source of energy.

Enzymes

A catalyst is a substance that speeds up the rate of a chemical reaction by lowering a

reaction’s activation energy. Enzymes are proteins that act as biological catalysts. Enzymes

are very specific.

Effect of enzyme on reaction rate:

24

Enzymes speed up chemical reactions that take place in cells.

Enzyme Action

Substrates are reactants of enzyme-catalyzed reactions. An enzyme-substrate

complex forms when an enzyme binds to the substrate at the active site. They are specific

and must fit together. Two models explain how enzymes work with substrates: the lock and

key model and the induced fit model.

Lock and key model:

Induced fit model:

Enzyme activity can be affected by pH, temperature change, and the amount of

enzyme or substrate present. Most cells have proteins that turn enzymes “on” or “off” at

critical times or stages in the cell’s life cycle.

Factors affecting enzyme action:

25

Chapter 7 – Cell Structure and Function

7-1 Life is Cellular

The Discovery of the Cell

Without microscopes, no one knew about the microscopic world and its

microorganisms until the mid 1600s.

Robert Hooke used an early compound light microscope in 1665 to look at cork and

called the tiny room-like structures “cells” because they looked like monastery rooms.

Around that time, Anton van Leeuwenhoek used a single-lens microscope to observe pond

water and discovered a world of tiny organisms he called “animacules”.

Hooke’s cork cells:

Leeuwenhoek’s microscope and animacules:

26

Numerous observations made it clear that cells were the basic units of life. Matthias

Schleiden concluded that all plants were made of cells (1838) and Theodor Schwann

concluded that all animals were made of cells (1839). Rudolph Virchow concluded that new

cells only come from the division of existing cells (1855). These discoveries led to the cell

theory.

The cell theory states:

All living things are composed of cells

Cells are the basic units of structure and function in living things

New cells are produced from existing cells.

There are three exceptions to the cell theory: the first cell did not come from a

preexisting cell; viruses are acellular and therefore nonliving; some organelles (mitochondria

and chloroplasts) have their own DNA and divide on their own.

Prokaryotes and Eukaryotes

Despite great differences in cells (such as size) there are two common characteristics:

the cell membrane and DNA. Eukaryotes are cells that have nuclei; prokaryotes are cells that

do not have nuclei. The nucleus is a large membrane-enclosed structure that controls many

of the cell’s activities and contains the cell’s genetic material (DNA).

Prokaryotes are smaller and simpler. Some have internal membranes but they are not

as complex as eukaryotes. Prokaryotes are bacteria. Eukaryotes are larger and more

complex with dozens of specialized structures and membranes.

Prokaryotic cells have genetic material that is not contained in a nucleus.

Eukaryotic cells contain a nucleus in which their genetic material is separated from the

rest of the cell.

Prokaryotic cell:

27

Eukaryotic animal cell:

Eukaryotic plant cell:

28

7-2 Eukaryotic Cell Structure

Comparing the Cell to a Factory

A eukaryotic cell is like a small factory and has many processes similar to the human

body. Organelles (little organs) are internal structures that act like organs. The cytoplasm is

the portion of the cell outside the nucleus that holds the organelles.

The nucleus has most of the cell’s DNA; like a main office controls a factory, the

nucleus is the control center of the cell. It is surrounded by a nuclear envelope which is

made up of two membranes. Granular material in the nucleus is chromatin, which consists of

DNA bound to proteins and is spread throughout the nucleus. Chromatin condenses to form

chromosomes which are threadlike structures that contain genetic information. The

nucleolus is a small, dense region of the nucleus where assembly of ribosomes begins.

The nucleus contains nearly all the cell’s DNA and with it the coded instructions for

making proteins and other important molecules.

Ribosomes are small particles of RNA and protein found throughout the cytoplasm.

Like small machines, they produce proteins based on the coded instructions from the boss

(nucleus).

Proteins are assembled on ribosomes.

The nucleus, nuclear envelope, and ribosomes:

The endoplasmic reticulum (or ER) is an internal membrane system that makes

proteins to be exported from the cell. One type of endoplasmic reticulum makes membranes

and secretory proteins. The other type of ER makes lipids and helps to detoxify, or remove

harmful substances. The rough ER, which has ribosomes on its surface, is where proteins are

synthesized. The smooth ER, which lacks ribosomes, has enzymes that perform specialized

tasks.

29

The endoplasmic reticulum is the site where lipid components of the cell membrane are

assembled, along with proteins and other materials that are exported from the cell.

Endoplasmic reticulum:

The Golgi apparatus is like a customization shop in a factory. Proteins are processed,

packaged, and stored until they are ready to be shipped either to another part of the cell or to

be secreted out of the cell.

The function of the Golgi apparatus is to modify, sort, and packages proteins and other

materials from the endoplasmic reticulum for storage in the cell or secretion outside the cell.

Golgi apparatus:

30

Lysosomes are small organelles filled with enzymes that act like the factory cleanup

crew. One function is the digestion of lipids, carbohydrates, and proteins into small

molecules the cell can use; another function is to remove “junk” organelles that are no longer

useful. Vacuoles are saclike structures used for storage of materials such as water, salts,

proteins, and carbohydrates. Many plants have a single, large central vacuole filled with

water. Contractile vacuoles, found in single-celled organisms like paramecia, are used for

pumping excess water out of the cell.

Lysosomes:

Mitochondria, found in nearly all eukaryotic cells, are like the energy source in a

factory. The mitochondrion is the “powerhouse” of the cell, which converts chemical energy

from food into ATP (adenosine triphosphate) during cellular respiration. Mitochondria are

enclosed by two membranes.

Mitochondria are organelles that convert the chemical energy stored in food into

compounds that are more convenient for the cell to use.

Mitochondrion:

Chloroplasts, found in plants, algae, and some other organisms, are like solar power

plants. The green pigment chlorophyll absorbs sunlight and converts it into chemical energy

during photosynthesis. Chloroplasts are enclosed by two membranes.

Chloroplasts are organelles that capture the energy from sunlight and convert it into

chemical energy in a process called photosynthesis.

31

Chloroplast:

Chloroplasts and mitochondria are the only organelles that have DNA. Margulis

suggested they are descendants of ancient prokaryotes that developed a symbiotic

relationship with eukaryotes because they have different DNA from the nucleus of they cell

in which they are found.

The cytoskeleton, which is like the supporting structure and transportation system of

a factory, helps support the cell. It is a network of protein filaments that helps the cell

maintain its shape and is also involved in cell movement. The two protein filaments in the

cytoskeleton are microfilaments and microtubules. Microfilaments are made of actin,

support the cell, and help it move. Microtubules are hollow, made of tubulin, critical for

maintaining cell shape, and needed in cell division (they form the mitotic spindle). In animal

cells, tubulin forms centrioles which are located near the nucleus and help organize cell

division. Cilia and flagella, used for swimming, are also made of microtubules.

The cytoskeleton is a network of protein filaments that helps the cell to maintain its shape.

The cytoskeleton is also involved in movement.

Structure and function of the cytoskeleton:

32

7-3 Cell Boundaries

Cell Membrane

All cells have a cell membrane which regulates what goes into or out of the cell; it

also provides protection and support for the cell. It is composed of a double layered sheet

called a lipid bilayer that gives it a flexible structure. Proteins embedded in the bilayer can

have carbohydrates attached. It is called the fluid mosaic model; fluid because proteins can

move throughout the membrane and mosaic because it is composed of so many types of

molecules. Channels and pumps can help move material across the membrane;

carbohydrates act as identification cards so cells can identify each other.

The cell membrane regulates what enters and leaves the cell and also provides protection

and support.

Plasma membrane:

Cell Walls

Cell walls provide additional support and protection for the cell. They are outside the

cell membrane and are found in many organisms including plants, algae, fungi, and many

prokaryotes. They are porous enough to let water, O2, and CO2 pass through.

The main function of the cell wall is to provide support and protection for the cell.

Cell wall and plasma membrane:

33

Diffusion Through Cell Boundaries

Regulating the movement of dissolved molecules from liquid on one side of the cell

membrane to the other side is one of the most important functions of the cell membrane.

Concentration of a solution is the mass of solute in a given volume of solution. Diffusion is

the process by which particles move from an area of high concentration to an area of low

concentration without the use of energy. Equilibrium occurs when concentration of the

solute is the same throughout a system.

Because diffusion depends upon random particle movements, substances diffuse across

membranes without requiring the cell to use energy.

Diffusion:

Osmosis

Osmosis is the diffusion of water. Water tends to move across the membrane until

equilibrium is reached. When concentrations of materials and water are the same on both

sides of the membrane, there is an isotonic (same strength) solution. A hypertonic (above

strength) solution occurs when the solution has a higher solute concentration than the cell,

causing water to leave the cell. A hypotonic (below strength) solution occurs when the

solution has a lower solute concentration than the cell, causing water to enter the cell.

Osmosis is the diffusion of water through a selectively permeable membrane.

Osmosis:

34

Osmotic pressure can cause problems for a cell. A cell is usually hypertonic to fresh

water (because they are filled with salts, sugars, proteins, and other molecules) so its volume

will increase until it swells or bursts. Multicellular organisms have cells bathed in isotonic

fluids like blood. Those that contact fresh water have cell walls that prevent the cell from

expanding.

Water balance in cells:

Facilitated Diffusion

Some molecules are too large o too strongly charged to cross the cell membrane but

they diffuse quickly because of hundreds of different protein channels that allow substances

to cross. Substances move from high to low concentration and energy is not needed.

Active Transport

To move materials against the concentration gradient (from low to high

concentration), cells use a process called active transport which requires energy. Small

molecules or ions cross through transport proteins or “pumps”. Larger molecules and clumps

of material can be transported across the membrane through the processes of endocytosis and

exocytosis; this can involve changes in membrane shape.

Proteins act like energy-requiring pumps to move small molecules and ions such as

calcium, potassium, and sodium ions; this involves changes in protein shape.

Endocytosis is the process of taking material into the cell by infoldings or pockets of

the cell membrane. In phagocytosis (“cell eating”), extensions of cytoplasm surround a

particle and package it within a food vacuole so the cell can engulf it. Pinocytosis is a

process in which tiny pockets form along the cell membrane, fill with liquid, and pinch off to

form vacuoles within the cell. Exocytosis occurs when the membrane of the vacuole fuses

with the cell membrane and forces contents out of the cell.

35

7-4 The Diversity of Cellular Life

Unicellular Organisms

The cell is the basic unit of life and with single-celled organisms, the cell is the

organism. Single-celled organisms are called unicellular; they dominate life on Earth.

Multicellular Organisms

A multicellular organism is made up of many cells; they depend on communication

and cooperation among specialized cells. Cell specialization is the process in which cells

develop in different ways to perform different tasks.

Cells throughout an organism can develop in different ways to perform different tasks.

Levels of Organization

Multicellular organisms have levels of organization that make it easier to describe

cells. Tissues are groups of similar cells that perform a particular function. An organ is

composed of groups of tissues that work together. An organ system is a group of organs that

work together to perform a specific function.

The levels of organization in a multicellular organism are individual cells, tissues, organs,

and organ systems.

36

Chapter 8 – Photosynthesis

8-1 Energy and Life

Autotrophs and Heterotrophs

Where does energy come from? Food. Energy is the ability to do work. Some

organisms, like plants and algae, can use light energy from the sun to make food. Autotrophs

make their own food. Other organisms cannot use light energy. Heterotrophs obtain energy

from foods they consume. Some eat plants, others eat other animals, and some decompose

organisms or their wastes.

Plants and some other types of organisms are able to use light energy from the sun to

produce food.

Chemical Energy and ATP

Energy has many forms: heat, light, electricity, and energy stored in chemical

compounds. Adenosine triphosphate (ATP) is a compound that cells use to store and release

energy. ATP = adenine + ribose + 3 phosphates.

ATP:

Adenosine diphosphate (ADP) has 2 phosphates. When cells have energy to store,

they can add a phosphate to ADP to form ATP. Breaking the high-energy bond between the

second and third phosphate groups allow cells to release energy as needed.

Hydrolysis of ATP:

37

The characteristics of ATP make it exceptionally useful as the basic energy source of all

cells.

Using Biochemical Energy

ATP is used for active transport in cells, protein pumps in the cell membrane,

movement, protein synthesis, responses to chemical signals, and to produce light. ATP is

great for transferring energy but not very good for storage. Glucose stores 90 times more

energy than ATP.

8-2 Photosynthesis: An Overview

Investigating Photosynthesis

In photosynthesis, plants use the energy of sunlight to convert water and carbon

dioxide into high-energy carbohydrates (sugars and starches) and oxygen (a waste product).

Van Helmont’s experiment (1643) was to find out if plants took materials from the

soil. He planted a seed, watered it, and after five years found the mass of the plant was 75 kg

but the mass of the soil was unchanged. He concluded the change in mass of the plant was

due to water.

Priestley’s experiment (1771) showed that something in air was needed to keep a

candle burning. When he placed a bell jar over the candle, the flame went out because there

was no oxygen left inside. When he placed a live plant under the jar with the candle, it

stayed lit for days. He concluded that the plant produced the substance (oxygen) needed to

keep the candle burning.

Ingenhousz’s experiment (1779) showed that the effect observed by Priestley only

worked when the plant was exposed to light.

The experiments performed by van Helmont, Priestley, and Ingenhousz led to work by

other scientists who finally discovered that in the presence of light, plants transform carbon

dioxide and water into carbohydrates, and they also release oxygen.

The Photosynthesis Equation

The overall equation for photosynthesis is: 6CO2 + 6H2O C6H12O6 + 6O2

(carbon dioxide + water sugars + oxygen)

Photosynthesis uses the energy of sunlight to convert water and carbon dioxide into high

energy sugars and oxygen.

Light and Pigments

Sun energy travels to Earth as light. White light is actually a mixture of wavelengths

(colors). Pigments are light-absorbing molecules. Chlorophyll is the main pigment in plants.

There are two types: chlorophyll a and chlorophyll b. Chlorophyll absorbs well in the red

and blue-violet regions but not green (green is reflected). Other pigments include carotene

(orange) and xanthophyll (yellow).

38

Absorption of light:

In addition to water and carbon dioxide, photosynthesis requires light and chlorophyll, a

molecule in chloroplasts.

8-3 The Reactions of Photosynthesis

Inside a Chloroplast

Photosynthesis occurs in the chloroplast.

Thylakoids are saclike membranes arranged in

stacks called grana. Proteins in the thylakoid

organize pigments into clusters called

photosystems (light-collecting units). Light-

dependent reactions occur in the thylakoid.

Light-independent reactions (the Calvin cycle)

occur in the stroma (space outside thylakoid).

Chloroplast:

39

1 = stroma, 2 = grana, 3 = thylakoid

Electron Carriers

Sunlight excites electron in chlorophyll

and they gain energy. Electron carriers

transport high energy electrons from

chlorophyll to other molecules in a process

called electron transport; the carriers make up

the electron transport chain.

NADP+ (nicotinamide adenine

dinucleotide phosphate) accepts and holds 2

electrons and an H+ ion. This converts

NADP+ into NADPH which can then carry

40

high-energy electrons from light absorption to

other reactions in the cell.

41

Photosynthesis Overview:

1=sunlight, 2=H2O, 3=CO2, 4=glucose

(sugars), 5=O2, 6=chloroplast, 7=grana,

8=light-dependent reactions, 9=Calvin Cycle

(light-independent reactions), 10=stroma

Light-Dependent Reactions

Light-dependent reactions require light to

produce oxygen, convert ADP into ATP, and

convert NADP+ into NADPH. This involves

several steps. A) Pigments in photosystem II

absorb light; electrons absorb the energy and

42

are passed to the electron transport chain. As

plants remove electrons from water, oxygen is

released. B) Electrons move through the

chain from photosystem II to photosystem I

and energy is used to move H+ ions from the

stroma into the thylakoid. C) Pigments in

photosystem I use light energy to reenergize

the electrons; the electrons and H+ ions are

picked up by NADP+ to become NADPH. D)

As electrons from chlorophyll are passed to

NADP+, more H

+ ions are pumped in making

the inside positive while the outside is

negative; this great difference in charges

provides energy to make ATP. E) ATP

synthase (a protein that spans the membrane)

allows H+ ions to pass through it, binding

ADP and a phosphate group to produce ATP.

The light-dependent reactions produce

oxygen gas and convert ADP and NADP+ into

the energy carriers ATP and NADPH.

The Calvin Cycle

43

The Calvin Cycle uses ATP and NADPH

from light-dependent reactions to produce

high-energy sugars which can store energy.

This involves several steps. A) Six carbon

dioxide molecules enter the cycle from the air

and combine with six 5-carbon molecules; this

makes twelve 3-carbon molecules. B) The

twelve 3-carbon molecules are converted to

higher-energy forms. C) Two of the twelve 3-

carbon molecules are removed from the cycle

to produce sugars, lipids, amino acids, and

other compounds. D) The remaining ten are

converted back into 5-carbon molecules to

combine with new carbon dioxide in the next

cycle.

The Calvin cycle uses ATP and NADPH

from the light-dependent reactions to produce

high-energy sugars.

Factors Affecting Photosynthesis

Many factors can affect the rate of

photosynthesis. Shortage of water can slow or

44

stop photosynthesis. Temperatures above or

below the normal enzyme range (between 0°C

and 35°C) can damage (denature) enzymes;

this can also slow or stop photosynthesis.

Increasing light intensity can increase the rate

of photosynthesis up to a certain maximum

rate.

Light-dependent reactions:

Calvin cycle:

45

46

Chapter 9 – Cellular Respiration

9-1 Chemical Pathways

Chemical Energy and Food

A calorie is the amount of energy needed

to raise the temperature of 1 gram of water

1°C. One gram of glucose releases 3811

calories when burned. Each Calorie (with a

capital “C”) on a food label actually represents

a kilocalorie (1000 calories).

Glycolysis is a process that releases a

small amount of energy. When oxygen is

present, it leads to two other pathways that

release a lot of energy but without oxygen it is

followed by a different pathway.

Overview of Cellular Respiration

Cellular respiration releases energy by

breaking down glucose and other food

molecules in the presence of oxygen; it

includes glycolysis, the Krebs cycle (citric

47

acid cycle), and the electron transport chain.

Each of the three main stages captures some

of the chemical energy available in food and

uses it to make ATP.

The equation for cellular respiration is:

6O2 + C6H12O6 → 6CO2 + 6H2O + energy

(oxygen + glucose → carbon dioxide + water + energy)

Overview of cellular respiration:

48

Cellular respiration is the process that

releases energy by breaking down glucose and

other food molecules in the presence of

oxygen.

49

Glycolysis

2 ATP are used to produce 4 ATP

resulting in a net gain of 2 ATP per glucose

molecule. Four high-energy electrons are

removed and passed to the electron carrier

NAD+ (nicotinamide adenine dinucleotide).

The NAD+ accepts 2 electrons making

NADH, which holds the electrons until they

are transferred. Glycolysis does not need

oxygen and it happens very quickly (1000s of

ATP are made in milliseconds) but it uses

NAD+ so when all of the NAD

+ have

electrons, ATP production stops.

Glycolysis:

50

Glycolysis is the process in which one

molecule of glucose is broken in half,

producing two molecules of pyruvic acid, a 3-

carbon compound.

Fermentation

When oxygen is not present, glycolysis is

followed by fermentation which also releases

energy from food to make ATP. Cells convert

NADH to NAD+ by passing electrons back to

pyruvic acid. It is an anaerobic process

because it occurs without oxygen. There are

two types of fermentation.

51

Alcohol fermentation, carried out by yeast,

forms ethyl alcohol and carbon dioxide as

wastes; the reaction is: pyruvic acid + NADH

→ alcohol + carbon dioxide + NAD+.

Lactic acid fermentation regenerates

NAD+ so that glycolysis can continue; the

reaction is: pyruvic acid + NADH → lactic

acid + NAD+. During rapid exercise, lactic

acid is produced in muscles causing a painful,

burning sensation. Unicellular organisms also

produce lactic acid to make certain foods like

cheese, yogurt, and pickles.

The two main types of fermentation are

alcoholic fermentation and lactic acid

fermentation.

9-2 The Krebs Cycle and Electron Transport

The Krebs Cycle

Cellular respiration requires oxygen so it

is considered aerobic; oxygen is the final

electron acceptor. The Krebs cycle (also

52

called the citric acid cycle) breaks down

pyruvic acid into carbon dioxide. This

involves several steps. A) Pyruvic acid (the 3-

carbon compound from glycolysis) enters the

mitochondrion; one carbon becomes part of

carbon dioxide and the other two join to

coenzyme A, forming acetyl-CoA. Acetyl-

CoA adds the 2-carbon acetyl group to a 4-

carbon molecule to make the 6-carbon

molecule citric acid. B) Citric acid is broken

down into a 4-carbon compound, more carbon

dioxide is released, and electrons are

transferred to carriers, changing NAD+ to

NADH and changing FAD (flavine adenine

dinucleotide) to FADH2.

Krebs cycle (citric acid cycle):

53

During the Krebs cycle, pyruvic acid is

broken down into carbon dioxide in a series of

energy-extracting reactions.

Electron Transport

The electron transport chain consists of a

series of proteins that uses electrons from the

Krebs cycle to make ATP. This involves

several steps. A) Electrons are transferred

from one carrier to the next. An enzyme at the

end of the chain combines these electrons with

54

H+ ions and oxygen to form water. Oxygen is

the final electron acceptor. B) Energy from

two electrons is used to transport H+ ions

across the membrane. C) As H+ ions escape

through channels, ATP synthases spin. The

spinning picks up low energy ADP and

attaches a phosphate group to make ATP.

Electron transport:

The electron transport chain uses the high-

energy electrons from the Krebs cycle to

convert ADP into ATP.

The Totals

55

Glycolysis produces 2 ATP per glucose.

The Krebs cycle and electron transport chain

produce 34 more ATP per glucose molecule.

A total of 36 ATP can be produced, per

glucose, in the presence of oxygen.

Complete breakdown of glucose in cellular

respiration:

Energy and Exercise

Quick energy: cells have small amounts of

energy from glycolysis and cellular

respiration, good only for a few seconds.

Muscles turn to lactic acid fermentation,

56

usually enough for 90 seconds. Fermentation

builds up an oxygen debt.

Long-term energy: cellular respiration

supplies ATP but is slower than fermentation.

The body stores energy in muscles and other

tissues in the form of glycogen, which is

usually good for 15 to 20 minutes. After that,

the body begins to break down other stored

molecules including fats.

Comparing Photosynthesis and Cellular

Respiration

Photosynthesis Cellular

Respiration

Function Use sun’s energy

to create FOOD

(glucose)

Convert food

energy into

ATP

Location Chloroplasts Mitochondria

Reactants Carbon dioxide +

water

Oxygen +

glucose

Products Oxygen + glucose Carbon dioxide

+ water + ATP

57

Equation A B

The reactions are the reverse of each other

A: 6CO2 + 6H2O C6H12O6 + 6O2

B: 6O2 + C6H12O6 → 6CO2 + 6H2O + energy

58

Chapter 10 – Cell Growth and Division

10-1 Cell Growth

Limits to Cell Growth

There are many reasons why cells divide. If cells grow without limit (indefinite), the

DNA would not be able to serve the increasing needs of the growing cell. The rate at which

food and oxygen are used and wastes are produced is based on cell volume, but the rate at

which the exchange takes place is based on surface area. As a cell grows, the volume

increases much more rapidly than the surface area; this leads to a decrease in the ratio of

surface area to volume which causes problems for the cell.

The larger a cell becomes, the more demands the cell places on its DNA. In addition, the

cell has more trouble moving enough nutrients and wastes across the cell membranes.

Division of the cell

Cell division is the process by which a cell divides into two new daughter cells.

Before division, the cell copies (replicates) its DNA.

10-2 Cell Division

In prokaryotes, cell division is simple; it separates into two parts. In eukaryotes, cell

division is complex and occurs in two stages. The first stage is mitosis, the division of the

nucleus; it is asexual reproduction and results in two genetically identical daughter cells. The

second stage is cytokinesis, the division of the cytoplasm.

Chromosomes

Chromosomes are made of DNA and protein. Each organism has a specific number

of chromosomes. Ex: humans have 46; fruit flies have 8. Most of the time, chromosomes

are not visible; they condense into compact, visible structures when division begins.

Each chromosome has two identical sister chromatids attached at an area called the

centromere which is usually near the middle of the chromosome.

Chromosome:

59

The Cell Cycle

Interphase is a period of growth that occurs between division cycles. The cell cycle is

the series of events that cells go through as they grow and divide. The cell cycle includes

four phases:

G1 phase – gap phase between M and S phase; growth of cell occurs

S phase – synthesis; chromosome replication

G2 phase – gap phase between S and M phase; preparation for mitosis

M phase – mitosis and cytokinesis

During the cell cycle, a cell grows, prepares for division, and divides to form two

daughter cells, each of which then begins the cycle again.

Events of the Cell Cycle

Cell cycle:

Mitosis

Biologists divide the events of mitosis into four phases: prophase, metaphase, anaphase,

and telophase.

Prophase is the first and longest (50 – 60% of total time). Chromosomes become

visible. Centrioles separate and take positions on opposite sides of the nucleus to help

organize the spindle, a fan-like microtubule structure that helps separate chromosomes.

Spindle fibers attach to the centromere. Plants do the same thing even though they do not

have centrioles. Chromosomes coil tightly near the end of prophase, the nucleolus

disappears, and the nuclear envelope breaks down.

Metaphase is second and lasts only a few minutes. Chromosomes line up along the

center of the cell and microtubules connect centromeres to the two poles of the spindle.

Anaphase is third. Centromeres joining the sister chromatids split and they become

individual chromosomes that move apart to opposite poles. When chromosomes stop

moving, anaphase ends.

60

Telophase is fourth. Chromosomes disperse into a tangle of dense material and the

nuclear envelope reforms. The spindle breaks apart and the nucleolus becomes visible in

each daughter cell. Mitosis is now complete, but not cell division. Two nuclei, each with a

duplicate set of chromosomes, are formed within the cytoplasm of one cell.

Cytokinesis

Cytokinesis, or division of the cytoplasm, usually occurs at the same time as

telophase. In animals, the cell pinches inward until there are two parts. In plants, a cell plate

forms between the divided nuclei and eventually develops into a cell membrane and then the

cell wall appears in the cell plate.

Phases of mitosis and cytokinesis:

61

10-3 Regulating the Cell Cycle

Controls on Cell Division

Cells grow until they contact other cells and then stop because growth and division

can be turned on and off.

Cell Cycle Regulators

Cyclins are a family of closely related proteins that regulate the timing of the cell

cycle. There are two types of regulatory proteins: those inside and those outside the cell.

Internal regulators respond to events inside the cell; they only allow the cycle to continue

after certain events have occurred. External regulators respond to events outside the cell;

they direct cells to speed up or slow down the cycle. Growth factors stimulate growth and

division. Molecules on the surfaces of cells cause the cycle to slow down or stop.

Cyclins regulate the timing of the cell cycle in eukaryotic cells

Uncontrolled Cell Growth

Cancer is a disorder in which some of the body’s cells lose the ability to control

growth. Cancer cells divide uncontrollably and form tumors (masses of cells) that can

damage surrounding cells and tissues.

Many forms of cancer cells have a defect in gene p53 which normally stops the cell

cycle until all of the chromosomes have been properly replicated.

Cancer cells do not respond to the signals that regulate the growth of most cells.

62

Chapter 35 – Nervous System

35-1 Human Body Systems

Organization of the Body

There are 11 systems of the human body: nervous, integumentary, skeletal, muscular,

circulatory, respiratory, digestive, excretory, endocrine, reproductive, and

lymphatic/immune. The eleven systems work together to maintain homeostasis.

Human organ system:

63

The levels of organization in a multicellular organism include cells, tissues, organs, and

organ systems.

Specialized cells perform unique functions. A group of cells that perform a single

function is called a tissue. There are four basic types of tissue:

Epithelial – glands and tissues that cover the interior and exterior body surfaces

Connective – provides support for the body and connects its parts

Nervous – transmits nerve impulses throughout the body

Muscle – works with bones to make the body move

Four types of tissue:

Maintaining Homeostasis

Homeostasis means keeping things balanced. Feedback inhibition (negative

feedback) is a process in which a stimulus produces a response that opposes the original

stimulus. Maintenance of homeostasis requires the integration of all organ systems at all

times.

Homeostasis is the process by which organisms keep internal conditions relatively

constant despite changes in external environments.

35-2 The Nervous System

Specialized cells carry messages from one cell to another so that communication

among all body parts is smooth and efficient.

The nervous system controls and coordinates functions throughout the body and responds

to internal and external stimuli.

Neurons

Neurons are nerve cells; they transmit electrical signals called impulses. Receptors

are sense organs which are sensitive to changes (stimuli). Effectors respond to commands of

the nervous system. Three basic events occur: a stimulus activates a receptor, the impulse is

started, and the effector responds to the stimulus.

64

The neuron is the basic structure of the nervous system; it can send electrical and

chemical (together called electrochemical) impulses. The neuron has three basic parts: cell

body, dendrites, and axon. The cell body has the nucleus and carries out metabolic activities.

The dendrites are branched fibers that receive impulses. The axon is a long thin fiber that

carries the impulses away from the cell body. Some axons are covered in an insulating

membrane called the myelin sheath. Gaps in the myelin are called the nodes of Ranvier; the

impulse jumps from one node to the next in a process called saltatory conduction. A synapse

is the space between terminal branches of one neuron and dendrites of another. Each axon

can have one or more synapses with as many as one thousand other neurons.

Neuron:

There are three basic types of neurons:

Sensory – from sense organs (receptors) to brain and spinal cord

Interneurons – associative; connect sensory and motor neurons; relay impulses from

one to another

Motor – from the brain and spinal cord to muscles and glands (effectors)

The Nerve Impulse

A resting neuron, one not transmitting an impulse, has a net positive charge outside

and a net negative charge inside. This difference in charge makes the impulse possible. The

nerve cell membrane pumps Na+ out and K

+ in by active transport. The electrical charge

across the cell membrane of a neuron in its resting state is called the resting potential.

A moving impulse begins when a neuron is stimulated by another neuron or by the

environment. A nerve impulse, which occurs when charges reverse from negative to

postivie, is called an action potential. Na+ outside the cell rapidly move into the cell

reversing polarization; then K+ diffuses out, restoring the negative potential and returning

polarity to normal.

The minimum level of a stimulus required to activate a neuron is called the threshold.

An impulse follows the all-or-none principle: a stimulus is either strong enough to cause an

impulse or too weak to cause an impulse.

An impulse begins when a neuron is stimulated by another neuron or by the environment.

65

The Synapse

The location at which a neuron can transfer an impulse to another cell is called a

synapse. The synaptic cleft is the space that separates the axon terminal from dendrites of an

adjacent cell. An impulse gets to the synaptic knob, goes across the gap to the next cell by a

chemical process. Tiny sacs called synaptic vesicles are filled with neurotransmitters which

are chemicals used by neurons to transmit impulses.

Different neurons release different neurotransmitters. Some are excitatory and start

impulses in neighboring cells. Ex: acetylcholine, norepinephrine, histamine, and glutamic

acid. Others are inhibitory and block impulses. Ex: serotonin, epinephrine, and glycine.

The neuromuscular junction is where motor neurons meet muscles. Motor end plates

have neurotransmitters which are released, cross the gap, and combine with receptors on the

muscle cell membrane.

Adaptations

Protists have no nervous system yet respond to stimuli in a coordinated way.

Filaments and organelles sense chemical stimuli. Hydra have no brain or organized center;

they have a nerve net, which is an irregular network connecting receptors with muscles and

glands. Earthworms are similar to complex organisms. They have a central nervous system

(CNS) consisting of two fused ganglia (brain) connected to ventral nerve cords that enlarge

into ganglia. They also have a peripheral nervous system (PNS) with branching nerve fibers

from the central nervous system; they have both sensory and motor neurons. Grasshoppers

are also similar to complex organisms. They have a CNS consisting of a brain, two ventral

nerve cords, and ganglia; they have a PNS of branched nerves. Their sense organs are more

developed; they have eyes, antennae, and feelers, and are sensitive to sound.

Adaptations:

35-3 Divisions of the Nervous System

The human nervous system is split into two major parts: the central nervous system

(CNS), which is the control center of the body, and the peripheral nervous system (PNS),

which gets information from the environment and relays commands from the CNS to organs

and glands.

The central nervous system relays messages, processes information, and analyzes

information.

66

The Central Nervous System

The CNS consists of the brain and spinal cord. It relays messages, processes

information, and analyzes information. The meninges are three layers of connective tissue

around the CNS. Cerebrospinal fluid is a shock absorber that bathes the brain and spinal

cord.

The Brain

The cerebrum is the largest and most prominent region of the brain. It is responsible

for voluntary activities, intelligence, learning, and judgment. It has two hemispheres

connected by the corpus callosum. Each hemisphere has four regions/lobes: frontal, parietal,

occipital, and temporal. The left brain controls the right side of the body and the right brain

controls the left. The cerebrum has two layers; the outer layer is the cerebral cortex made of

gray matter; the inner layer is white matter.

The cerebellum is the back of the brain. It coordinates and balances actions of the

muscles.

The brain stem, which includes the pon and medulla oblongata, connects the brain

and spinal cord. It controls some of the body’s most important functions, such as blood

pressure, heart rate, breathing, and swallowing.

The thalamus and hypothalamus are between the brain stem and cerebrum. The

thalamus receives messages from all sensory receptors and relays information to the proper

region in the cerebrum. The hypothalamus is the control center for recognition and analysis

of hunger, thirst, fatigue, anger, and body temperature.

The brain:

The Spinal Cord

The spinal cord is the main link between the brain and the rest of the body. Thirty-

one pairs of spinal nerves branch out from the spinal cord. A reflex is a quick, automatic

response to a stimulus; it allows the body to respond to danger immediately without thinking

about a response.

67

The Peripheral Nervous System

The PNS has two divisions: the sensory division (sense organs to CNS) and the motor

division (CNS to muscles and glands). The motor division is broken down into the somatic

and autonomic systems. The somatic regulates activities that are under conscious control and

some reflexes. A reflex arc is the pathway that a reflex travels, from the sensory receptor, to

a sensory neuron, then to a motor neuron, and then to an effector. The autonomic regulates

automatic or involuntary activities. The autonomic system is subdivided into the sympathetic

and parasympathetic systems which have opposite effects. Ex: the sympathetic system

increases heart rate and the parasympathetic decreases heart rate.

The sensory division of the peripheral nervous system transmits impulses from sense

organs to the central nervous system. The motor division transmits impulses from the central

nervous system to the muscles or glands.

Peripheral nervous system:

35-4 The Senses

Sensory receptors are neurons that react to a specific stimulus; they are located

throughout the body but are concentrated in sense organs. There are five types: pain

receptors (located everywhere except the brain) respond to chemicals released by damaged

cells; thermoreceptors (in the skin, body core, and hypothalamus) detect temperature

variations; mechanoreceptors (in the skin, skeletal muscles, and inner ear) are sensitive to

touch, pressure, stretching muscles, sound, and motion; chemoreceptors (in the nose and taste

buds) detect chemicals in the external environment; photoreceptors (in the eyes) detect light.

68

There are five general categories of sensory receptors: pain receptors, thermoreceptors,

mechanoreceptors, chemoreceptors, and photoreceptors.

Vision

Light enters the cornea, a tough, transparent layer that helps the eye focus, and then

moves through a fluid called aqueous humor. It then goes through the pupil, an opening in

the center of the colored part of the eye, called the iris. The size of the pupil regulates the

amount of light that enters the eye. Next, it moves through the lens, behind the iris, which

changes shape to adjust focus to near or far objects. It then moves through a large chamber

with transparent, jellylike fluid called vitreous humor.

The lens focuses light onto the retina which contains the photoreceptors. The two

types are: rods which are sensitive to light and cones that respond to light of different color.

There is a blind spot that lacks photoreceptors, where the optic nerve goes through the back

of the eye.

Hearing and Balance

The ears can distinguish pitch and loudness of sound vibrations in the air. Sound

enters the auditory canal, makes the eardrum (tympanum) vibrate, which causes three tiny

bones called the hammer, anvil, and stirrup to vibrate. This passes the vibration to the oval

window that creates waves in the fluid-filled cochlea which makes hair cells move.

The semicircular canals, three tiny canals at right angles to each other, help maintain

equilibrium (balance).

Smell and Taste

Chemoreceptors detect chemicals. Much of what we taste is actually smell. Taste

buds are the sense organs that detect tastes such as salty, bitter, sweet, and sour.

Touch and Related Senses

All parts of the skin are sensitive to touch but not equally; fingers, toes, and the face

have the most receptors. Skin also has receptors for temperature, pressure, and pain.

35-5 Drugs and the Nervous System

Drugs That Affect the Synapse

A drug is any substance other than food that changes the structure or function of the

body. Stimulants speed things up. Ex: amphetamines, cocaine, nicotine, and caffeine.

Depressants slow things down. Ex: tranquilizers and barbiturates. Cocaine, a powerful

stimulant, acts on pleasure centers of the brain causing a sudden release of dopamine.

Opiates mimic endorphins which help relieve pain. Ex: morphine and codeine. Marijuana

contains tetrahydrocannabinol (THC) which creates a temporary feeling of euphoria or

disorientation. Alcohol, a depressant, slows CNS function. Fetal alcohol syndrome (FAS) is

a group of birth defects caused by alcohol’s effects on the fetus.

Stimulants increase heart rate, blood pressure, and breathing rate. In addition, stimulants

increase the release of neurotransmitters at some synapses in the brain.

69

Depressants slow down heart rate and breathing rate, lower blood pressure, relax muscles,

and relieve tension.

Cocaine causes the sudden release in the brain of a neurotransmitter called dopamine.

Opiates mimic natural chemicals in the brain known as endorphins, which normally help

to overcome sensations of pain.

Alcohol is a depressant that slows down the rate at which the central nervous system

functions.

Drug Abuse

Drug abuse is the intentional misuse of any drug for nonmedical purposes. Addiction

is an uncontrollable dependence on a drug.

70

Chapter 36 – Skeletal, Muscular, and Integumentary Systems

Locomotion

Locomotion is the abilty to move from place to place. Motile organisms are capable

of locomotion. Contractile proteins can change length; they are the basis for the movement

of nearly all organisms. Sessile organisms cannot move from place to place. Advantages of

locomotion:

Easier to get food

Find places to live and move away from harm

Escape enemies or seek shelter

Find mates and reproduce

The skeleton and muscles are used for locomotion. An exoskeleton is on the outside

of an organism; it cannot grow so they shed or molt. It encloses soft parts and is found in

protists and invertebrates. Ex: carbs, spiders, insects, arthropods. An endoskeleton is inside

the body walls of organisms; it is made of living cells of bone and cartilage that grow and

divide. Muscles are fastened to bones to allow for movement.

Adaptations for Locomotion

Protists have pseudopods, cilia, or flagella to move. The hydra has cells that secrete

mucus so it glides along its base; it can also somersault its base over its tentacles, pull itself

by the tentacles, or float from a bubble at its base. The earthworm has two musle layers:

circular around the worm and longitudinal along its length. They have four pairs of setae

(tiny bristles) on most segments that hook into the ground for traction. The grasshopper has

a chitin exoskeleton separated by flexible joints allowing it to walk, jump, and fly. Their

muscles work in pairs; one muscle contracts to bend the joint and the other relaxes.

Adaptations:

71

36-1 The Skeletal System

The human skeleton:

72

The Skeleton

The skeletal system has many important functions: it supports the body, protects

internal organs, allows movement, stores minerals, and is a site for blood cell formation.

There are 206 bones in an adult human. The skeleton has two parts: the axial skeleton

consists of the skull, vertebral column, and rib cage and it supports the center of the body; the

appendicular skeleton consists of the arms, legs, pelvis, and shoulders.

The skeleton supports the body, protects internal organs, provides for movement, stores

mineral reserves, and provides a site for blood cell formation.

Structure of Bones

Bones are solid of network of living cells and protein fibers that are surrounded by

calcium deposits. Bone is surrounded by tough layer of connective tissue called periosteum.

Compact bone is under it. Haversian canals, which contain blood vessels and nerves, run

through dense compact bone. Spongy bone is inside the outer layer of compact bone, at ends

of long bones and in the middle of short flat bones.

Osteocytes are mature bone cells, embedded in the bone matrix. Two other cell types

are osteoclasts which break down old bone cells and osteoblasts which produce new bone

cells; they are found in Haversian canals and surfaces of compact and spongy bone. Bone

marrow is found within cavities of bones. There are two kinds: red marrow produces red

blood cells, white blood cells, and platelets; yellow marrow is primarily fat cells.

Bones are a solid network of living cells and protein fibers that are surrounded by

deposits of calcium salts.

Development of Bones

A human embryo (before 8 weeks) is almost entirely cartilage, a flexible connective

tissue. Cartilage has no blood vessels, is dense and fibrous, it can support weight and is very

flexible. Cartilage is found in the nose, ears, where ribs attach to the sternum, and between

vertebrae. Ossification is the process by which cartilage is replaced by bone.

Types of Joints

A joint is the place where one bone attaches to another. Depending on its type of

movement, a joint is classified as immovable, slightly movable, or freely movable.

Immovable joints are fixed and allow no movement; bones are interlocked and held by

connective tissue or they are fused together. Example: the skull. A slightly movable joint

allows for small amounts of restricted movement. Example: joints between adjacent

vertebrae. Freely movable joints allow bones to move in one or more directions. Ball-and-

socket joints allow motion in many directions. Example: the shoulders. Hinge joints allow

for back-and-forth motion. Example: the knees. Pivot joints allow rotation. Examples: the

wrists and elbows. Saddle joints allow one bone to slide in two directions. Example: the

hands.

Depending on its type of movement, a joint is classified as immovable, slightly movable,

or freely movable.

73

Structure of Joints

Cartilage covers surfaces where two bones come together. A fibrous joint capsule,

which holds bones together, is made of two layers. One layer forms ligaments which are

strips of tough connective tissue. The other layer produces synovial fluid so surfaces slide

smoothly (lubricant). Bursae are small sacs of fluid that reduce friction and absorb shock.

Skeletal System Disorders

Bones and joints can be damaged like any other tissue in the body. Strain can lead to

inflammation in which excess fluid causes swelling, heat, redness, and pain.

Bursitis is inflammation of a bursa.

Arthritis is inflammation of a joint.

Osteoporosis is a weakening of bones that can cause serious fractures.

36-2 The Muscular System

Types of Muscle Tissue

There are three types of muscle tissue: skeletal, smooth, cardiac. Each type has its

own function.

Skeletal muscle is usually attached to bones by tendons to allow for voluntary

movement. They are also called striated muscle because they appear to have stripes due to

the alternating bands of light and dark muscle fibers. Skeletal muscle cells, which are called

muscle fibers, are large, have many nuclei, and vary in length from 1 mm to 30 cm. A

complete skeletal muscle consists of muscle fibers, connective tissue, blood vessels and

nerves. Smooth muscle is usually not voluntary. A smooth muscle cell is spindle shaped,

has one nucleus, and is not striated. They are found in internal organs and are connected by

gap junctions. Cardiac muscle is only found in the heart. It is striated like skeletal muscle

but the cells are smaller and usually have one nucleus (or sometimes two). They are

connected by gap junctions and usually not under central nervous system control.

There are three different types of muscle tissue: skeletal, smooth, and cardiac.

Types of muscle:

Muscle Contraction

Muscle fibers are made of myofibrils which are made of filaments. There are two

kinds: thick contain myosin; thin contain actin. They form striations. Filaments are arranged

in units called sarcomeres which are separated by Z lines. A muscle contracts when thin

filaments slide over thick filaments. Myosin must form a cross-bridge with actin to pull it

towards the center.

74

Sliding filament model:

A muscle contracts when the thin filaments in the muscle fiber slide over thick filaments.

Control of Muscle Contraction

Impulses from motor neurons control the contraction of skeletal muscle fibers. A

neuromuscular junction is the point of contact between a motor neuron and a skeletal muscle

cell. Vesicles (pockets) release acetylcholine, a neurotransmitter, which produces the

impulse causing calcium (Ca2+

) to be released; this is what causes the sliding of filaments.

How Muscles and Bones Interact

Skeletal muscles generate force and produce movement by contracting or pulling on

body parts. Muscle is connected to bone by tendons which are what make bones work like

levers. Most skeletal muscles work in antagonistic pairs (opposites).

Exercise and Health

Resting muscle tone is a state of partial contraction. It keeps the back and legs

straight and the head upright. Exercise helps muscles stay firm and it increases their size and

strength by adding actin and myosin filaments.

Regular exercise is important in maintaining muscular strength and flexibility.

36-3 The Integumentary System

The integumentary system is made up of the skin, hair, nails, and some glands.

The Skin

The skin is the single largest organ in body. Integument means “to cover”. The skin

is a barrier against infection and injury, helps regulate body temperature, removes wastes in

sweat, and protects against UV radiation from the sun. The skin has many receptors which

are a gateway to the nervous system. It is made of two main layers: the epidermis (outer) and

75

the dermis (inner). There is also the hypodermis, a subcutaneous layer of fat below the

dermis, which helps insulate the body.

The epidermis is the outer layer of skin. It has two layers: the outer layer, which

contacts the environment, is made up of dead cells and the inner layer is made of living cells.

Rapid cell division pushes older cells towards the skin surface; they flatten, and begin

making keratin, a tough fibrous protein. When keratin-making cells die, they form a tough,

flexible, waterproof covering. Melanocytes produce melanin which makes the skin

pigmented to protect the skin from UV rays. There are no blood vessels in the epidermis.

The dermis is the inner layer which has collagen fibers, blood vessels, nerve endings,

glands, sensory receptors, smooth muscle and hair follicles. It helps maintain homeostasis by

regulating body temperature. There are two major types of glands: sweat glands which

produce perspiration (sweat) to remove metabolic wastes and cool the body, and sebaceous

glands which produce oil (sebum) that spreads along the skin to keep the keratin flexible and

waterproof.

The integumentary system serves as a barrier against infection and injury, helps to

regulate body temperature, removes waste products from the body, and provides protection

against ultraviolet radiation from the sun.

The skin:

Hair and Nails

The basic structure of hair and nails is keratin. Hair covers almost every part of

exposed surface of the body. Hair protects the scalp from UV radiation and provides

insulation from the cold; hair in the nostrils, ears and around the eyes prevents particles from

getting in. A hair follicle produces hair as large columns of cells that fill with keratin and

then die.

Nails grow from the nail root near the tips of fingers and toes. Cells fill with keratin

and produce a tough, platelike nail. They grow an average of 3 mm per month.

76

Chapter 37 – Circulatory and Respiratory Systems

Adaptations for Transport

Transport is the process by which substances move into, out of, or are distributed

within cells. Simple organisms do not have a special system because they are in direct

contact with their environment. Complex organisms have circulatory system consisting of

three parts:

A fluid to carry materials (blood).

Vessels for the fluid to move through (veins, arteries, and capillaries).

A pump to move the fluid through the vessels (the heart).

Protists such as ameba and paramecia and organisms like the hydra use diffusion and

active transport. The earthworm has a closed circulatory system; blood moves in vessels

(tubes) so pressure allows the fluid to move faster. The blood is red because it has the

pigment hemoglobin. It has five pairs of aortic arches (hearts) which pump blood from the

dorsal vessel (top) to the ventral vessel (bottom). Exchange of materials occurs at the

capillaries; materials include nutrients, water, wastes, gases and other substances. The

grasshopper has an open circulatory system; blood is not always in the vessels because it

flows directly through body spaces and bathes tissues and organs. The blood has no color

because it has no hemoglobin; it can carry nutrients and wastes but cannot carry gases. It has

a single vessel, the aorta, and a tubular heart.

Adaptations:

77

37-1 The Circulatory System

Functions of the Circulatory System

Large organisms need a transportation system to efficiently move materials

throughout the body. Humans and other vertebrates have a closed circulatory system with

blood contained within vessels.

The human circulatory system consists of the heart, a series of blood vessels, and the

blood that flows through them.

The Heart

The heart is made almost entirely of muscle. It is a hollow organ about the size of a

closed fist. The heart is surrounded by the pericardium, a protective sac of tissue. The

myocardium is a thick layer of muscle that contracts to pump blood through the circulatory

system. On average, the heart contracts 72 times per minute, pumping 70 milliliters of blood

each time.

The septum divides the heart into a left and right side, which prevents the mixing of

oxygen-rich and oxygen-poor blood. Each side has two chambers: an atrium and a ventricle.

Atria are the upper chambers that receive bloos. Ventricles are the lower chambers that

pump blood out of the heart.

Heart:

Circulation throughout the body includes two pathways. Pulmonary circulation, on

the right side of the heart, pumps blood from the heart to the lungs; carbon dioxide leaves the

blood and oxygen is picked up. Systemic circulation, on the left side, pumps blood from the

78

heart to the rest of the body; oxygen is brought to all parts of the body and picks up carbon

dioxide.

Circulation through the heart begins when blood enters the heart at the right and left

atria. Valves between the atria and ventricles close when the ventricles contract to prevent

backflow into the atria. They are called atrioventricular valves because they are between an

atrium and a ventricle; the right valve is called the tricuspid valve; the left valve is called the

bicuspid or mitral valve. Valves at the ventricles prevent the blood flowing out of the heart

from coming back into the heart. They are called semilunar valves; the right side has the

pulmonary valve and the left side has the aortic valve.

We can trace the overall pathway of blood as follows: right atrium (RA) → tricuspid

atrioventricular valve (AV valve) → right ventricle (RV) → pulmonary semilunar valve (SL

valve) → pulmonary arteries (PA) → lungs → pulmonary veins (PV) → left atrium (LA) →

bicuspid atrioventricular valve (AV valve) → left ventricle (LV) → aortic semilunar valve

(SL valve) → aorta → body → superior vena cava from upper body (SVC) and inferior vena

cava from lower body (IVC) → right atrium (RA).

Circulation:

Heartbeat makes a “lub dup” sound. Each contraction begins at the sinoatrial node

(SA node) in the right atrium. The SA node is known as the pacemaker of the heart because

it sets the pace of the heartbeat by starting the wave of muscle contraction. An impulse from

the SA node goes through fibers in atria and is picked up by the atrioventricular node (AV

node) and is then carried to fibers in the ventricles so they contract.

Blood Vessels

79

Blood vessels include veins, arteries and capillaries. The aorta, a large artery, is the

first of a series of vessels that carry blood to the body. Arteries carry blood from the heart to

the lungs and to the body; all have O2-rich blood except the pulmonary arteries. Veins carry

blood from the lungs and the body back to the heart; all have O2-poor blood except the

pulmonary veins. Veins have valves that help keep blood moving towards the heart;

contraction of muscles also helps blood move through the veins. Capillaries are the smallest

vessels; some are only one cell thick. Capillaries are the sites of exchange of nutrients,

wastes, and gases between cells and blood.

As blood flows through the circulatory system, it moves through three types of blood

vessels – arteries, capillaries, and veins.

Blood Pressure

Without pressure, blood would stop flowing. A sphygmomanometer (blood pressure

cuff) is used to measure blood pressure. A worker pumps air into the cuff until blood flow

through an artery is blocked; the person listens for a pulse as the pressure is released and

records numbers from a pressure gauge. The first number is systolic, the force felt in arteries

when the ventricles contract. The second number is diastolic, the force of blood in the

arteries when ventricles relax. Typical blood pressure is 120/80.

Regulation of blood pressure occurs in two ways. Sensory receptors in the body can

detect blood pressure and send impulses to the brain. When too high, the autonomic nervous

system releases neurotransmitters that relax walls of blood vessels. When too low,

neurotransmitters elevate pressure by causing vessel wall contractions. The kidneys also

help regulate pressure by removing more water when pressure is too high.

Diseases of the Circulatory System

Cardiovascular diseases are common and are among the leading causes of death and

disability in the United States. Circulatory disorders include:

Atherosclerosis is a condition in which fat deposits (plaque) build up on inner walls

of arteries; arteries harden from cholesterol build up. It can cause blood clots to form,

break free, travel through, and get stuck in the blood vessels; this can block the blood

supply to vital organs.

Hypertension is high blood pressure, which makes the heart work harder to pump

blood throughout body.

A heart attack occurs when the blood supply to the heart is blocked (by plaque build-

up or a blood clot), causing part of the heart muscle to die from a lack of oxygen.

A stroke occurs when the blood supply to the brain is blocked; this can cause brain

cells to die resulting in paralysis, loss of speech or other functions, coma, and death.

A pulmonary embolism occurs when the blood supply to the lungs is blocked.

37-2 Blood and the Lymphatic System

Blood Plasma

The body has 4 to 6 liters of blood (about 8% of total body mass). About 45%

consists of cells and the other 55% is plasma, a straw-colored fluid. Plasma is 90% water

and 10% dissolved gases, salts, nutrients, enzymes, hormones, waste products, and proteins.

80

There are three types of plasma proteins: albumin regulates osmotic pressure and blood

volume; globulin helps fight viral and bacterial infections; fibrinogen helps form blood clots.

Blood Cells

Red blood cells (RBCs) are erythrocytes; they are the most numerous (5 million in

one milliliter of blood); they transport oxygen and carbon dioxide; they are red because they

contain hemoglobin (iron-containing pigment) that binds to oxygen or carbon dioxide to

transport it; they are produced by cells in the red bone marrow, live about 120 days, and are

then destroyed in the liver and the spleen.

White blood cells (WBCs) are leukocytes; they have no hemoglobin, are less

numerous than RBCs (the ratio of RBCs:WBCs is 1000:1); they have nuclei; their lifespan

depends on the type of WBC – some live for days, months or years. They are an army that

guards against infection, fights parasites, and attacks bacteria. There are many types of

WBCs: phagocytes (neutrophils and monocytes) are eating cells that engulf bacteria or

pathogens and digest them; lymphocytes produce antibodies (proteins) that help destroy

pathogens; eosinophils release clot digesting enzymes and combat allergy causing

substances; basophils release an anticoagulant (heparin) and histamine which causes

inflammation.

Clotting is made possible by plasma proteins and platelets (cell fragments). When

blood vessels are injured, platelets clump at the site and release thromboplastin which

converts fibrinogen into fibrin. Fibrin causes a clot which prevents further loss of blood.

Red blood cells transport oxygen.

White blood cells are the “army” of the circulatory system – they guard against infection,

fight parasites, and attack bacteria.

Blood clotting is made possible by plasma proteins and cell fragments called platelets.

Types of blood cells:

81

The Lymphatic System

The lymphatic system consists of a network of vessels, nodes and organs; it collects

fluid lost by the blood and returns it back to the circulatory system. The fluid is called

lymph. The vessels have valves to prevent backflow. Lymph nodes are small, bean shaped

enlargements which act as filters, trapping bacteria and other microorganisms that cause

disease. Lymph vessels also play a role in nutrient absorption. The thymus and spleen also

play an important role in the lymphatic system. T cell lymphocytes, which recognize

invaders, mature in the thymus (beneath the sternum). The spleen helps cleanse blood and

removes damaged blood cells from the circulatory system; it also has phagocytes that engulf

and destroy pathogens.

A network of vessels, nodes, and organs called the lymphatic system collects the fluid that

is lost by the blood and returns it back to the circulatory system.

Blood and Lymphatic Disorders

There are many disorders that can affect the blood and the lymphatic system. Some

of the disorders are hereditary and others are infectious.

Sickle cell anemia is a genetic disorder in which RBCs have a sickle shape because

one DNA base is different making abnormal hemoglobin; the RBCs carry less oxygen

than they should so people with sickle cell anemia suffer from fatigue, the inability to

move around a lot, and pain.

Leukemia is a group of cancers that starts in the bone marrow and results in high

levels of WBCs.

Anemia is hereditary disease in which people have insufficient amounts of

hemoglobin or low numbers of RBCs both of which lead to reduced oxygen transport.

Hemophilia is a hereditary disease in which people lack a blood clotting factor.

Swollen glands occur when the lymph nodes fill with bacteria or other foreign

substances, become enlarged, and feel sore.

37-3 The Respiratory System

82

What Is Respiration?

The word respiration is used in two ways: cellular respiration, which occurs in the

mitochondria, releases energy from the breakdown of food in the presence of oxygen;

respiration at the organism level is the process of gas exchange, the release of carbon dioxide

and uptake of oxygen, between the lungs and the environment.

Adaptations for Respiration

Respiratory surfaces, through which gas exchange occurs, must be thin walled for the

rapid diffusion of gases; they must be moist because O2 and CO2 must be in solution

(dissolved); they must be in contact with O2 in environment or surroundings and in contact

with the transport system (circulatory system).

Small organisms have direct diffusion, movement of substances from high to low

concentrations, through their body surfaces; O2 from water goes in and CO2 leaves. Protists,

such as ameba and paramecia, have diffusion through their cell membranes and hydra carry

out diffusion through their cell layers.

The earthworm’s moist skin is its respiratory surface. O2 from the air in soil goes

through the skin into capillaries. In dry weather they burrow deeper to prevent their skin

from drying out. Rain can flood the burrows and drown them so they come out of the

ground. The grasshopper does not use their circulatory system for respiration because there

is no hemoglobin to carry gases. They have tracheal tubes which are branching air tubes that

carry air to all their cells. Air goes in and out of openings called spiracles and is pumped by

air sacs. Fish and some other aquatic animals use gills for respiration.

Plants have tiny openings on their leaves called stomata (or stomates) through which

carbon dioxide enters the plant and oxygen is released from the plant. A stoma (or stomata)

is surrounded by guard cells which regulate the opening and closing of the stoma.

Adaptations for gas exchange:

83

The Human Respiratory System

The basic function of the respiratory system is gas exchange between the blood, air,

and tissues. Air moves through the nose (or mouth) to the pharynx (throat) which is a tube at

back of mouth. From the pharynx, air moves into the trachea (windpipe). The trachea

divides into two cartilage-ringed tubes called bronchi which divide into smaller passages

called bronchioles; these lead to millions of tiny air sacs called alveoli. A network of

capillaries surrounds each alveolus. The epiglottis covers the trachea when swallowing.

Air must be warmed, moistened and filtered to keep the lungs healthy; hairs and

mucus in the nose do this. Cells in the respiratory system produce mucus to moisten air and

trap inhaled particles and germs. Cilia sweep particles and mucus away from the lungs

towards the pharynx so mucus and trapped particles can be swallowed or coughed out. The

larynx (voicebox) has two highly elastic folds of tissue called vocal cords which vibrate to

produce sounds.

The basic function performed by the human respiratory system is remarkably simple – to

bring about the exchange of oxygen and carbon dioxide between the blood, the air, and

tissues.

Human respiratory system:

84

Gas Exchange

The process of gas exchange involves four stages or physical processes:

Breathing is the movement of air into and out of the lungs.

External respiration is the exchange of O2 and CO2 between air in the environment

and the blood.

Internal respiration is the exchange of O2 and CO2 between blood in the capillaries

and the cells of the body.

O2 and CO2 transport is the movement of gases between the lungs and the body.

Breathing

Breathing is the movement of air into and out of the lungs. Air pressure forces air

into the lungs, which are sealed in two sacs called pleural membranes. A large, flat muscle

called the diaphragm is at the bottom of the chest cavity. When you inhale, the diaphragm

contracts, the rib cage rises up, the volume of the chest cavity expands, and a vacuum forms

inside the cavity causing the lungs to fill with air. When you exhale, the rib cage lowers, the

diaphragm relaxes, and air rushes back out of the lungs.

How Breathing Is Controlled

Although you can control your breathing, it is such an important function that your

nervous system will not allow you to have complete control over it. Autonomic nerves from

the medulla oblongata to the diaphragm and chest muscle produce contractions that bring air

into the lungs.

Disorders of the Respiratory System

There are many disorders that can affect the respiratory system.

85

Asthma is an allergic reaction when bronchioles spasm; it can cause wheezing and

breathing difficulty.

Bronchitis is the irritation and swelling of the bronchial tube lining; mucus clogs

passages causing severe coughing and making it hard to breathe.

Pneumonia occurs when the alveoli fill with fluid and prevent gas exchange.

Emphysema is the loss of elasticity in lung tissue because the alveoli are damaged; it

causes shortness of breath and difficulty in removing excess carbon dioxide.

Lung cancer is a disease in which tumors form in the lungs; it is particularly deadly

because the cancer cells can easily spread to other places in the body.

Smoking can cause such respiratory diseases as chronic bronchitis, emphysema, and lung

cancer.

86

Chapter 38 – Digestive and Excretory Systems

38-1 Food and Nutrition

Food and Energy

Living things need food and all food contains nutrients. Nutrients are substances that

provide energy and materials needed for metabolism. Nutrition is the process by which

organisms get food and break it down for metabolism. Ingestion is the process of taking in

food. Digestion is the breakdown of food or nutrients. Egestion is the elimination of solid

wastes. Absorption is the process by which nutrients go through the cell membrane to be

used by an organism.

Energy from the breakdown of nutrients is stored as ATP. Energy released by

cellular respiration is the same as burning it. A calorimeter is used to measure energy

content of food. A calorie is a unit used to measure energy in food; it is the amount of heat

needed to raise the temperature of one gram of water by 1°C.

There are two ways to get nutrients:

Aututrophs make their own food; they can make organic nutrients from simple,

inorganic substances by photosynthesis or chemosynthesis. Examples: green plants,

algae, and microorganisms.

Heterotrophs cannot make their own food; they must take in food from their

environment. Examples: animals and some microorganisms.

Nutrients

Nutrients include carbohydrates, fats, proteins, vitamins, minerals, and water. Simple

and complex carbohydrates (sugars and starches) are the main source of energy for the body.

Sugars that are not used right away are stored as glycogen. Many foods have cellulose

(fiber) which we need but are unable to breakdown. Fats or lipids are needed to produce cell

membranes, myelin sheaths, and certain hormones. Proteins have many roles in the body

such as supplying materials for repair and growth, being used in regulation and transport, and

being broken down into essential amino acids. Vitamins are organic coenzymes needed for

reactions. Minerals are chemical elements (iron, calcium, magnesium, phosphorus, and

iodine) that organisms need for normal functioning.

The nutrients that the body needs are water, carbohydrates, fats, proteins, vitamins, and

minerals.

Every cell in the human body needs water because many of the body’s processes,

including chemical reactions, take place in water.

Nutrition and a Balanced Diet

The Food Guide Pyramid classifies food into six groups and indicates how many

servings from each group should be eaten each day. When you choose food to eat, it should

be high in nutrition and low in Calories.

87

Food Guide Pyramid:

Adaptations for Nutrition

Protists are attracted to nutrients through chemical stimuli and have intracellular

digestion. Ameba can engulf nutrients using pseudopods; paramecia use cilia to move

nutrients towards their oral grooves; euglena can carry out photosynthesis and consume

nutrients from the environment.

The hydra carries out intracellular and extracellular digestion. Its tentacles have

cnidoblasts (stinging cells) to paralyze prey and move food into its mouth.

The earthworm has a “tube-within-a-tube” body plan which is an alimentary canal

with two openings: a mouth and an anus. Food is broken down mechanically and

chemically. The crop stores food and the gizzard grinds food.

The grasshopper also has a tubular digestive system. Mechanical breakdown of food

occurs using its mouth parts and chemical breakdown occurs as the chewed food mixes with

saliva. The gizzard is used to grind food and the glands secrete digestive enzymes into the

stomach for chemical digestion and absorption.

88

Adaptations for nutrition:

38-2 The Process of Digestion

The human digestive system is responsible for breaking down food into small

molecules that can be passed to cells in the body that need them. It is built around an

alimentary canal, a one-way tube.

The digestive system includes the mouth, pharynx, esophagus, stomach, small intestine,

and large intestine. Several major accessory structures, including the salivary glands, the

pancreas, and the liver, add secretions to the digestive system.

The Mouth

The teeth carry out mechanical digestion, which is the physical breakdown of food

into smaller pieces. Enzymes in the saliva carry out the chemical digestion of some food.

Amylase breaks chemical bonds in starches and releases sugars. Lysozyme, an enzyme that

fights infection by digesting bacterial cell walls, is also in saliva.

89

The function of the digestive system is to help convert foods into simpler molecules that

can be absorbed and used by the cells of the body.

The Esophagus

The tongue and throat muscles push a chewed clump of food called a bolus down the

throat. The epiglottis closes over the trachea (windpipe) to prevent food from blocking the

air to our lungs. The esophagus is the food tube through which the bolus moves by a process

called peristalsis, a series of wavelike smooth muscle contractions. The cardiac sphincter,

between the esophagus and stomach, closes after food enters the stomach.

Esophagus and epiglottis:

The Stomach

The stomach is a large, muscular sac that continues mechanical and chemical

digestion of food. Mechanical digestion occurs as the stomach muscles contract to churn and

mix stomach contents to create a thin, soupy liquid called chyme. Gastric glands release

mucus, to lubricate and protect the stomach, and they produce hydrochloric acid, to activate

pepsin; pepsin is an enzyme that digests proteins. From the stomach, chyme moves through

the pyloric sphincter into the small intestine.

The Small Intestine

The first part of the small intestine, the duodenum, is where most digestive enzymes

enter. Most chemical digestion and absorption of nutrients occur here. Accessory structures

include the pancreas and the liver. The pancreas produces hormones to regulate blood

glucose levels, produces a base to neutralize stomach acid, and makes enzymes to digest

carbohydrates, proteins, and lipids. The liver produces bile which emulsifies fats and the

gallbladder stores the bile.

Absorption in the Small Intestine

The next parts of the small intestine, the jejunum and ileum, are together about six

meters long. The lining of the small intestine has folds with fingerlike projections called villi

which have their own projections called microvilli. All of these features increase surface

area to increase the absorption of nutrients leaving only water, cellulose, and other

undigested materials to continue through the system. These substances pass the appendix on

90

the way to the large intestine. In humans, the appendix does not play a role in digestion but

in other mammals, the appendix stores cellulose and other materials that enzymes cannot

digest.

The Large Intestine

The main function of the large intestine is to remove water by absorption, leaving

concentrated waste to be eliminated by the body. Bacteria in the large intestine produce

compounds that the body can use, such as vitamin K.

Human digestive system:

Digestive System Disorders

There are many disorders that can affect the digestive system.

A peptic ulcer is a hole in the stomach lining resulting from decreased mucus

production, increased acidity levels, or by the infectious bacterium Heliobacter

pylori.

Diarrhea is a condition that occurs when not enough water is absorbed by the large

intestine.

Constipation is a condition that occurs when too much water is absorbed from

undigested materials.

Vomiting is reverse peristalsis that is caused by irritation of the cardiac sphincter or

stomach.

Heartburn occurs when acid backs up into the esophagus.

91

38-3 The Excretory System

Adaptations for Excretion

Protists remove wastes such as CO2, salts, and ammonia through simple diffusion and

active transport to maintain homeostasis. Contractile vacuoles pump out excess H2O. The

hydra removes wastes such as CO2, salts, and ammonia through diffusion but they do not

have contractile vacuoles. The earthworm removes urine and CO2. They have nephridia in

pairs on each segment. Wastes leave as a dilute solution of urine which consists of H2O,

salts, ammonia, and urea (ammonia and CO2) through openings called nephridiopores; CO2 is

excreted through their moist skin. The grasshopper removes uric acid and CO2 through

excretory organs called malpighian tubules. Uric acid, a dry nitrogenous waste, is excreted

with feces (from insects, reptiles, and birds). CO2 diffuses from tissues to tracheal tubes and

out of the grasshopper through openings called spiracles.

Adaptations:

Functions of the Excretory System

Wastes build up that must be removed from the body. Excretion is the prcess by

which metabolic wastes and excess substances are removed. Wastes from cellular processes

include:

CO2 and H2O from cellular respiration.

Nitrogen compounds such as ammonia, urea, and uric acid which result from the

breakdown of amino acids.

Mineral salts such as sodium chloride and potassium sulfate from metabolic

processes.

The kidneys play an important role in maintaining homeostasis. They remove waste

products from the blood; maintain blood pH; and regulate the water content of the blood and,

therefore, blood volume.

92

Human Excretory System

The human excretory system can be broken down into four major parts:

The skin removes excess water, salts, and small amounts of urea in sweat.

The lungs remove CO2 and water vapor.

The liver converts amino acids into useful compounds and nitrogen wastes are

converted into less toxic urea.

The kidneys remove wastes from the blood, maintain pH levels, regulate blood

volume and water content.

The skin keeps foreign materials out of the body and is waterproof to prevent drying

out. It helps remove exceszs heat and excretes wastes in sweat. When our body is too hot,

sweat evaporates to cool the body and we become flsuehd because the blood vessels open

wider to increase bloodflow so more heat can be released. When our body is too cold, the

blood vessels narrow to lower the amount of blood in surface capillaries or we sweat less or

we shiver to produce heat through muscle tension.

The lungs help release CO2 and H2O, the products of cellular respiration.

The liver is responsible for detoxification of the blood by removing harmful things

like bacteria, drugs, and hormones. The liver helps with the excretion of bile and the

formation of urea which goes into the blood to the kidneys which then filter it out to make

urine for excretion.

Human excretion:

The Kidneys

93

The kidneys filter urea, excess water, and other wastes from the blood. From the

kidneys, two ureters carry urine to the urinary bladder which stores urine until it leaves the

body through the urethra.

The outer portion of the kidney is called the renal cortex and the inner portion is

called the renal medulla. Nephrons are the functional units of the kidneys which process and

filter. Blood enters the neprhon through an arteriole. The nephron filters out impurities

which are then passed to the collecting duct. Pure blood exits the nephron through a venule.

Passing a liquid or gas through a filter to remove wastes is called filtration. Wastes

are filtered from the blood in the glomerulus (a capillary network) which is enclosed in the

Bowman’s capsule. Filtrate is made of water, urea, glucose, salts, amino acids, and some

vitamins but not all filtrate is excreted. Liquid, consisting of mostly water and nutrients, is

taken back into the blood by the process of reabsorption. All the blood in the body is filtered

every 45 minutes.

Urea, salts, water, and some other substances remain in the filtrate and become urine.

The loop of Henle reabsorbs about 99% of the water to conserve water and minimize the

volume of urine.

As blood enters a nephron through the arteriole, impurities are filtered out and emptied

into the collecting duct. The purified blood exits the nephron through the venule.

The kidney and a nephron:

Control of Kidney Function

Kidney activity is regulated by blood composition and hormones. Water reabsorption

by the kidneys decreases as the amount of water in the blood increases so that more water

goes into the urine and is stored in the urinary bladder. When salt increases in the blood, less

is returned to the blood by the kidneys and comes out with urine.

The kidneys make about 180 liters of filtrate per day but through homeostasis and

reabsorption only about 1 to 1.5 liters of urine are created from this amount of filtrate.

94

Homeostasis by Machine

Humans have two kidneys but can survive with only one. If both are damaged by

injury or disease, a person may need a kidney transplant or use a kidney dialysis machine.

Blood is removed from the body through a tube inserted in the arm. The blood is pumped

through dialysis tubing which acts like nephrons; tiny pores in the tubing allows salts and

small molecules like nitrogen wastes to pass through and purified blood is returned to the

body. Dialysis can be expensive and time-consuming.

Disorders of the Excretory System

There are many disorders that can affect the excretory system.

Kidney stones form when substances such as calcium, uric acid, or magnesium

crystallize into stones.

Cirrhosis of the liver results from toxic overloading which leads to excess tissue

overgrowing the liver; this cuts bloodflow and decreases purification due.

Jaundice is yellowing of the skin which occurs when hemoglobin reabsorbed from

bile is not excreted properly.

Diabetes is a disease in which blood sugar levels are so high that glucose is found in

the urine.

Kidney failure occurs when the kidneys do not filter out wastes; a person in kidney

failure may need a transplant or dialysis.

95

Chapter 39 – Endocrine and Reproductive Systems

39-1 The Endocrine System

The endocrine system is made up of glands that release chemical messages called

hormones into bloodstream. These products deliver messages throughout the body.

The endocrine system is made up of glands that release their products into the

bloodstream. These products deliver messages throughout the body.

Hormones

Hormones are chemicals released in one part of the body that act on another part.

They travel through bloodstream and affect activities of cells in other parts of the body.

They bind to specific receptors on target cells. The body’s response to hormones is slower

but lasts longer than nervous responses.

Glands

Glands are organs that produce and release chemical secretions called hormones.

Exocrine glands release secretions through tubes or ducts, directly to the organs that use

them. Examples: gallbladder, pancreas. Endocrine glands release hormones into the

bloodstream.

Endorine glands:

96

Hormone Action

Hormones can be classified into two general groups: steroid hormones and non-

steroid hormones.

Steroid hormones are produced from lipids (cholesterol). They enter cells by crossing

the cell membrane and then bind to a receptor protein to form a hormone-receptor complex

which then enters nucleus and binds to a DNA control sequence. This initiates transcription

of genes to messenger RNA (mRNA) which then moves into the cytoplasm and directs

protein synthesis. They affect gene expression directly and can produce dramatic changes in

cell and organism activity.

Non-steroid hormones cannot pass through cell membrane so they bind to receptors

on the cell membrane (outside the cell) which activates an enzyme on the inner surface of

cell membrane. This activates a secondary messenger which carries the message of the

hormone into the cell. Secondary messengers can be Ca+ ions, cAMP (cyclic adenosine

monophosphate), nucleotides and fatty acids; they can activate or inhibit a wide range of

activities.

Hormone action:

Prostaglandins

Prostaglandins are produced by all cells except RBCs. They are modified fatty acids

that are produced by a wide range of cells. They are local hormones that only affect nearby

cells.

Control of the Endocrine System

The endocrine system is regulated by feedback mechanisms to maintain homeostasis

(like a thermostat). One way the endocrine system is regulated by internal feedback

mechanisms is by maintaining the rate of metabolism. One example of this would be when

the hypothalamus detects that the level of thyroxine in the blood is low it secretes

thyrotropin-releasing hormone (TRH), which then stimulates the anterior pituitary to secrete

97

thyroid-stimulating hormone (TSH). TSH then stimulates the thyroid to release thyroxine.

Higher levels of TSH and thyroxine then inhibit TRH secretion by the hypothalamus.

Controlling metabolism:

Like most systems of the body, the endocrine system is regulated by feedback

mechanisms that function to maintain homeostasis.

Complementary Hormone Action

Sometimes two hormones with opposite effects act to regulate the body’s internal

environment. This is called complementary hormone action. For example, calcium

concentration in the blood can be lowered by calcitonin produced by the thyroid or increased

by parathyroid hormone (PTH) produced by the parathyroid glands. Another example

involves the regulation of sugar levels in the blood. Insulin lowers blood glucose levels

whereas glucagon increases blood glucose levels.

39-2 Human Endocrine Glands

Pituitary Gland

The pituitary is the master gland. It is a bean sized gland divided into two parts:

anterior and posterior. It secretes 9 hormones that directly regulate body functions and

controls other endocrine glands.

The posterior pituitary produces: antidiuretic hormone (ADH) which makes kidneys

reabsorb water; oxytocin which makes the uterus contract during childbirth and releases milk

in nursing mothers.

The anterior pituitary produces: follicle-stimulating hormone (FSH) which produces

eggs and sperm; luteinizing hormone (LH) which stimulates ovaries to release eggs, testes to

98

release sperm, and prepares the uterus for implantation; thyroid-stimulating hormone (TSH)

which makes the thyroid work by synthesizing and releasing thyroxine; adreno-corticotropic

hormone (ACTH) which stimulates the release of hormones from the adrenals; growth

hormone (GH) which stimulates protein synthesis and growth in cells; prolactin which

stimulates milk production; melanocyte-stimulating hormone (MSH) which stimulates

melanocytes to increase production of the skin pigment melanin.

The pituitary gland secretes nine hormones that directly regulate many body functions and

controls the actions of several other endocrine glands.

Hypothalamus

The hypothalamus controls secretions of the pituitary. The close connection between

the hypothalamus and the pituitary shows the close relationship between the nervous and

endocrine systems.

The hypothalamus controls the secretions of the pituitary gland.

Thyroid Gland

The thyroid plays a major role in regulating the body’s metabolism with thyroxine

which increases the rate of protein, carbohydrate, and fat metabolism as well as the rate of

cellular respiration. Hyperthyroidism occurs when the thyroid produces too much thyroxine;

it increases metabolic rate, nervousness, body temperature, weight loss, and blood pressure.

Hypothyroidism occurs when the thyroid produces too little thyroxine; it lowers metabolic

rates, body temperature, results in a lack of energy and weight gain. In some cases it causes

a goiter (enlarged thyroid).

The thyroid gland has the major role of regulating the body’s metabolism.

Parathyroid Glands

There are four parathyroid glands on the back of the thyroid, which maintain

homeostasis of calcium in blood. They secrete parathyroid hormone (PTH) which increases

absorption of calcium by kidneys and increases uptake of calcium from the digestive system.

Hormones from the thyroid gland and the parathyroid glands act to maintain homeostasis

of calcium levels in the blood.

Adrenal Glands

There are two adrenal glands, one on each kidney. They release hormones that help

the body prepare for and deal with stress. The adrenal cortex produces more than two dozen

corticosteroids. Aldosterone is one which regulates Na+ (sodium) absorption and K

+

(potassium) excretion. Another is cortisol which helps control the rate of metabolism. The

adrenal medulla is regulated by the sympathetic nervous system, produces the “fight or

flight” response to stress, and makes epinephrine and norepinephrine.

The adrenal glands release hormones that help the body prepare for and deal with stress.

99

Pancreas

The pancreas is both an exocrine and endocrine gland. The islets of Langerhans,

clusters of cells in the pancreas, have beta cells which secrete insulin and alpha cells which

secrete glucagon; both work to regulate blood sugar levels.

Diabetes mellitus is a condition in which the pancreas fails to produce or properly use

insulin causing glucose in the blood to be so high that the kidneys excrete sugar in the urine.

Type I is an autoimmune disorder in which there is little or no insulin secretion and requires

daily injections of insulin to control blood glucose levels. Type II commonly develops in

people over 40 years old. Low or normal amounts of insulin are produced but cells do not

respond to the hormone properly.

Insulin and glucagon help to keep the level of glucose in the blood stable.

Reproductive Glands

Reproductive glands are called gonads; they produce sex cells, called gametes, and

secrete sex hormones. Ovaries are female gonads which produce eggs called ova, and the

hormones estrogen and progesterone. Testes are male gonads which produce sperm and the

hormone testosterone.

The gonads serve two important functions: the production of gametes and the secretion of

sex hormones.

Other Endocrine Glands

The pineal gland (located in the brain) releases melatonin which is involved in

rhythmic cycles and activities. It controls Circadian rhythms, the sleep-wake cycles. The

thymus (located beneath the sternum) releases thymosin which stimulates T cell development

and proper immune responses in childhood.

Sexual Reproduction

Meiosis

Sexual reproduction occurs when two different parent cells produce offspring that are

genetically different from either parent. In simple organisms, there is just a transfer of

genetic material. In complex organisms, gametes (sex cells) fuse during fertilization to form

a single zygote. Gametes are formed by meiosis and have half the number of chromosomes

as the parent cell.

Diploid and Haploid Chromosome Number

Somatic cells (body cells) include all cells of the body except gametes. They have a

certain number of chromosomes that make up pairs called homologous chromosomes which

are similar in size, shape, and genetic content. Diploid (2n) cells have all homologous

chromosomes. Gametes (sex cells) have only one chromosome from each pair so they are

called haploid (1n). Meiosis ensures that gametes only get half the chromosomes; otherwise

the chromosome number would double each generation.

100

Reproductive Systems

Advantages of sexual reproduction include: new combinations of characteristics,

differences in structure or function, increased variation increases the possibility that some

organisms will survive better than others, and variations allow individuals to move into new

environments.

Conjugation occurs in simple organisms when a bridge of cytoplasm forms between

two cells and the transfer of nuclear material takes place. This occurs in bacteria, green

alage, and paramecium.

In complex organisms, there are two sexes, male and female. Gametogenesis, the

production of gametes, occurs in the ovaries of females to produce ova (egg cells) in a

process called oogenesis; it occurs in testes of males to produce sperm in a process called

spermatogenesis.

Fertilization

External fertilization occurs outside the females in animals that breed in water. The

problem is that there are many hazards in the environment: sperm may not meet eggs; eggs or

developing offspring may be eaten or die due to environmental conditions. Large numbers of

eggs and sperm are released to increase chances of survival. Almost all aquatic invertebrates,

most fish (not sharks), and many amphibians use external fertilization.

Internal fertilization occurs within the body of the female and is found in animals that

breed on land and in some aquatic animals. They have specialized sex organs to carry sperm

from the male to the female. After fertilization, the zygote is either enclosed in a shell to be

released from the body or it continues to develop within the female. Fewer eggs are needed

since it is less dangerous and the chances of survival are greater, but there are still large

numbers of sperm because they live only a short time.

Development

External development can occur in water or on land. In aquatic environments,

fertilization and development occur outside the female. Yolk provides nourishment, oxygen

from the water diffuses in, and wastes diffuse out. There is little to no care for the offspring

so survival depends on large numbers of eggs. In terrestrial environments, fertilization is

internal and development can occur outside the female within an egg. This occurs with birds,

most reptiles, and a few mammals. There is a lot of yolk within a protective shell that is

mostly waterproof but it allows oxygen in and carbon dioxide out. Egss with a shell survive

better so fewer young are produced; most reptiles leave the eggs but birds tend to the eggs

and their young so they lay fewer eggs than reptiles.

Internal development occurs with sharks, some reptiles, and most mammals.

Fertilization is internal and embryos develop within the uterus (womb) of the female. The

offspring are born underdeveloped and feed from the mother’s mammary glands. The young

are well protected so a high percentage of them survive. Placental mammals, such as

humans, develop a placenta through which food and wastes are exchanged. Nonplacental

mammals can be egg-laying, such as the platypus or spiny anteater, or marsupials (pouched),

such as kangaroos, which give birth to very immature offspring that crawl into the pouch,

attach to a mammary gland, and complete their development externally.

101

39-3 The Reproductive System

Sexual Development

During the first six weeks of development the male and female are identical, but

during the seventh week, reproductive organs start developing. Testes and ovaries are unable

to produce active reproductive cells until puberty, a period of rapid growth and sexual

maturation when the reproductive system becomes fully functional. Onset of puberty is

between ages 9 and 15, when the hypothalamus signals the pituitary to produce increased

levels of FSH and LH.

The Male Reproductive System

The main functions are to produce and deliver sperm. Structures include: testes,

scrotum, seminiferous tubules, epididymis, vas deferens, and penis. The testes are the male

gonads which make sperm cells and testosterone; they hang outside body in a sac of skin

called the scrotum. The scrotum keeps the temperature of testes slightly lower than the body

so that sperm can survive. Seminiferous tubules are 300 to 600 small coiled tubules that

make immature sperm. The epididymis is a storage area on upper rear part of each testis

where sperm mature (18 hours). The vas deferens is a tube that leads up from each testis into

the lower abdomen and empty into the urethra (passage for urine excretion). The seminal

vesicles, Cowper’s gland and prostate gland all secrete fluids into the urethra as sperm enter,

creating semen. Semen is a mixture of sperm and fluids.

Male reproductive system:

102

The main function of the male reproductive system is to produce and deliver sperm.

The Female Reproductive System

The main functions are to produce ova and prepare the female’s body to nourish a

developing embryo. The egg development structures are the ovaries and follicles. Egg

release involves two Fallopian tubes, the uterus, and vagina. The ovaries are the female

gonads which make eggs and estrogen. There are two ovaries found in lower abdomen,

which are 4 cm long, 2 cm wide, and contain 200,000 tiny follicles. A follicle is an egg sac

containing an immature egg. They are present from birth and no more than 500 mature in a

lifetime. Ovulation is the process by which an egg matures; the egg moves from a follicle to

the surface of an ovary. The follicle breaks and releases an egg; the egg can be fertilized for

24 hours after ovulation. Fallopian tubes (oviducts) are near each ovary and are lined with

cilia; a current draws an egg into the tube where the egg can be fertilized and then travels to

the uterus. The uterus is a thick walled, muscular, pear-shaped organ; if an egg is fertilized,

it finishes development here. The cervix is the narrow neck leading from the uterus to the

vagina. The vagina is the birth canal; the cervix opens into the vagina which leads to outside

of the body.

Female reproductive system:

The main function of the female reproductive system is to produce ova. In addition, the

female reproductive system prepares the female’s body to nourish a developing embryo.

The Menstrual Cycle

About every 28 days, a mature egg leaves one of the ovaries; if fertilized, it implants

in the uterus and embryonic development begins; if not fertilized, the built up part of uterine

103

wall and the unfertilized egg pass from the body, then another egg matures and the uterine

wall builds up again. The cycle begins at puberty (between ages 10-14), stops temporarily

when pregnant, and stops permanently in menopause (between ages 45-50).

Stages of the menstrual cycle:

1. During the follicle stage, the pituitary secretes FSH causing several follicles to start

developing. Only one matures, which secretes estrogen so that the uterine lining

thickens with mucus and blood vessels to prepare for possible pregnancy. This stage

lasts 10 to 14 days. 2. During ovulation, a high level of estrogen decreases FSH and starts LH secretion.

When LH gets to a certain level then ovulation occurs (egg is released). 3. During the corpus luteum stage, LH causes the broken follicle (where the egg

released from) to fill with cells forming the yellow corpus luteum which secretes

progesterone for continued growth of uterine lining and stops new follicles from

developing by inhibiting FSH. This stage lasts 10 to 14 days. 4. Menstruation occurs when LH decreases when an egg is not fertilized. The corpus

luteum breaks down so progesterone decreases and the thick lining breaks down.

Extra lining layers, the egg, and blood pass out of the body. This stage lasts 3 to 5

days. The menstrual cycle is found in humans and primates only. Other mammals have an

estrous cycle in which there are periodic changes in sex organs and the desire to mate.

Mating only occurs when the female is fertile (in heat).

The menstrual cycle has four phases: follicular phase, ovulation, luteal phase, and

menstruation.

Menstrual cycle:

104

Sexually Transmitted Diseases

Sexually transmitted diseases (STDs) are diseases that are spread from one person to

another during sexual contact. Chlamydia is caused by a bacterium and can result in

infertility due to the damage it causes to the reproductive system. Syphillis and gonorrhea

are other STDs caused by bacteria that spread easily. Viruses can also cause STDs; hepatitis

B, genital herpes, genital warts, and AIDS are caused by viruses. Human immunodeficiency

virus (HIV) can lead to acquired immune deficiency syndrome (AIDS).

39-4 Fertilization and Development

Fertilization

Hundreds of millions of sperm are released into the vagina and then go through cervix

and up the uterus into the oviducts. If the egg is passing down the oviducts at this time then

fertilization occurs. The egg secretes chemicals to attract sperm; one gets through the egg

and stops other sperm from entering. In vitro fertilization is fertilization in glass lab dish; the

egg fertilized by sperm and placed in the uterus two days later.

After a haploid (N) nucleus from an egg joins a haploid (N) nucleus from a sperm in

the process of fertilization, a single diploid (2N) nucleus forms. The fertilized egg, which

contains a set of chromosomes from each parent cell, is called a zygote.

The process of a sperm joining an egg is called fertilization.

Early Development

The zygote undergoes mitosis without growth to become a ball of cells called a

morula. The morula becomes a hollow ball of cells called a blastocyst (or blastula). About

105

six to seven days after fertilization, the blastocyst enters the uterus and attaches to the uterine

lining by secreting enzymes. Implantation is the fastening of an embryo to the wall of the

uterus and it marks the start of pregnancy. An ectopic pregnancy occurs when a zygote

implants in an oviduct or in the abdomen.

Differentiation is a specialization process during which cells of the blastocyst develop

into various types of tissues in the body. Gastrulation occurs after implantation and results in

the formation of three germ layers: ectoderm, mesoderm, and endoderm. Neuralation is the

development of the nervous system which occurs after gastrulation.

Humans have extraembryonic membranes, similar to those of birds and reptiles but

they have different roles. The chorion is the outermost membrane and surrounds the embryo;

chorionic villi form on the outer surface and extend into the uterine lining forming the

placenta. The placenta is the site of exchange of nutrients and wastes; it secretes hormones

to stop the menstrual cycle. The yolk sac and allantois becomes the umbilical cord which

connects the fetus to the placenta. The amnion is the innermost layer, surrounds the fetus,

and is filled with fluid to protect the fetus by absorbing shock and giving it a stable

environment. Nourishment from fertilization to implantation is from the yolk; from

implantation until birth, food comes from the mother across placenta through umbilical cord.

This early period of development is extremely important because a number of

external factors can disrupt development at this time. A developing human is called an

embryo until 8 weeks and is then called a fetus.

The stages of early development include implantation, gastrulation, and neuralation.

The placenta is the embryo’s organ of respiration, nourishment, and excretion.

Fertilization and development:

Birth

Gestation is the length of pregnancy which is a little over 9 months in humans. Labor

involves slow, rhythymic contractions of uterine muscles that occur when the fetus is ready

to be born. The cervix gets larger, from 1 or 2 centimeters to about 11 or 12 centimeters, and

when large enough the contractions will force the baby head first from the uterus into the

vagina and out of the body. The amniotic membrane bursts to release the fluid to ease the

passage through the birth canal.

106

The umbilical cord is cut which becomes the navel. Afterbirth involves contractions

that occur after the baby is born to expel the placenta and amnion. During pregnancy,

progesterone and estrogen prepare breasts for nuring. After birth, prolactin causes the breasts

to secrete milk.

Problems can occur such as premature birth which means the fetus is born before

ready and put in an incubator until fully developed. A Cesarian section involves making an

incision in the abdomen and uterus because delivery through the cervix and vagina is not

possible or not safe.

Multiple Births

Fraternal twins result from two eggs being fertilized by different sperm during the

same pregnancy. Identical twins involve a single egg being fertilized by a single sperm that

divides into two at the very early stages of development so they have the exact same

hereditary material. Three or more embryos may form which can be identical, fraternal, or a

mixture of both.

107

Chapter 40 – Immune System and Disease

40-1 Infectious Disease

A disease is any change that disrupts normal function of the body. It can be caused

by bacteria, viruses and fungi or by materials in the environment or they could be inherited or

it could be lack of immune response (allergies, autoimmune). A pathogen is any agent that

causes disease.

Some diseases are produced by agents, such as bacteria, viruses, and fungi. Others are

caused by materials in the environment, such as cigarette smoke. Still others, such as

hemophilia, are inherited.

The Germ Theory of Disease

People believed diseases were caused by curses or evil spirits or because you were a

bad person. Observations of Pasteur and Koch showed that many infectious diseases were

caused by microorganisms commonly caused germs, which is called the germ theory of

disease.

Steere and Burgdorfer found spiral shaped bacteria in deer ticks and in patients with

symptoms resembling arthritis. Borelia burgdorferi is the bacteria found to be the cause of

Lyme disease. They used Koch’s postulates to test this theory.

Koch’s Postulates

Koch’s Postulates are a set of rules used to identify the microorganism that causes a

specific disease. Scientists follow Koch’s postulates to identify pathogen and to prevent or

cure the disease caused. The postulates are:

1. The pathogen should always be in the sick organism and not found in healthy ones.

2. The pathogen must be isolated and grown in lab culture.

3. A cultured pathogen placed or injected in a new host must cause the same disease as

the original host (where you took it from).

4. The injected pathogen must then be isolated from the new host and should be

identical to the original.

Agents of Disease

The human body is suitable for growth of pathogens due to our body temperature, it

being a watery environment, and the abundance of nutrients. Many microorganisms are

beneficial but some cause disease: they can destroy cells as they grow, release toxins,

produce sickness by blocking the flow of blood, remove nutrients, and disrupt normal bodily

functions (disrupt homeostasis).

Viruses are tiny particles that invade and replicate in living cells by attaching to the

surface of the cell or by entering the cell (inserting DNA or RNA) and taking over cell

functions. Examples: common cold, influenza (the flu), chickenpox, smallpox and warts.

Bacteria either break down tissues for food or release toxins. Examples: streptococcus

infections, anthrax, botulism. Protists can also cause diseases. Examples: Plasmodium

causes malaria, Trypanosoma is carried by insects in vertebrate bloodstreams feeding off

108

nutrients, and Entamoeba causes dysentery. Flatworms and roundworms also cause disease.

Examples: the flatworm Schistosoma enters freshwater streams and rice paddies; tapeworms

and hookworms can also infect humans. Fungi are also known to cause disease. Examples:

Tinea penetrates the outer layers of skin in athlete’s foot; ringworm can infect the scalp or

skin resulting in rough scaly patches.

Agents of disease:

How Diseases Are Spread

Some diseases spread from one person to another through coughing, sneezing, or

physical contact. Others spread through contaminated substances such as food and water,

and others spread by contact with infected animals. Animals that carry pathogens from

person to person are called vectors.

Some infectious diseases are spread from one person to another through coughing,

sneezing, or physical contact. Other infectious diseases are spread through contaminated

water or food. Still others are spread by infected animals.

Fighting Infectious Diseases

Prevention of disease is not always possible so antibiotics, infection fighting drugs,

can be used. They interfere with the normal functioning of certain microorganisms.

Alexander Fleming accidentally discovered penicillin in 1928 when his bacterial

culture dish was contaminated by a green mold called Penicillium notatum. Antibiotics have

no effect on viruses because they are not alive (acellular) so antiviral drugs might be needed.

Over the counter drugs treat only the symptoms of the disease such as coughing,

congestion, stuffy or runny nose, and fever but they do not treat the cause so the pathogen

continues to infect the body. The best treatment includes rest, a well balanced diet, and

plenty of fluids.

109

40-2 The Immune System

The function of the immune system is to fight infection through the production of

cells that inactivate foreign substances or cells; this is called immunity. There are two

general categories of defense: specific and nonspecific. Nonspecific defenses guards against

pathogens and infection by keeping most things out of the body. Specific defenses get to the

pathogen that gets in.

The function of the immune system is to fight infection through the production of cells

that inactivate foreign substances or cells.

Human immune system:

Nonspecific Defenses

The first line of defense involves the skin, mucus, sweat, and tears which keep

pathogens out of the body. The skin is most important because it prevents pathogens from

getting in and causes symptoms of infection. Symptoms of infection include redness,

swelling, heat and pain. Many secretions such as mucus, saliva, and tears have lysozyme to

break down bacterial cell walls. Oil and sweat are acidic. Mucus traps pathogens, cilia

moves them away from lungs, stomach acid and enzymes also destroy pathogens.

The second line of defense is the inflammatory response (redness, swelling, heat and

pain) which is triggered when pathogens enter the body and cause injury, infection, or tissue

damage. Millions of WBCs are produced in response, such as phagocytes which engulf and

destroy bacteria. An elevated body temperature (fever) occurs when the immune system

releases chemicals and this can slow down or kill pathogens. Fever and WBC count indicate

110

that the body is fighting infection. Interferos are proteins produced by cells infected with

virus to help other cells resist infection by inhibiting the synthesis of viral proteins and by

blocking viral replication.

Your body’s most important nonspecific defense is the skin.

The inflammatory response is a nonspecific defense reaction to tissue damage caused by

injury or infection.

Specific Defenses

Specific defenses that attack particular disease-causing agents make up the immune

response. Antigens (Ag) are found on the surfaces of viruses, bacteria, and other pathogens;

they can trigger an immune response.

Two types of lymphocytes recognize specific antigens: B cells and T cells. B cells

provide immunity against antigens and pathogens in body fluids in a process called humoral

immunity. T cells defend the body against abnormal cells and pathogens inside living cells

in a process called cell-mediated immunity.

In humoral immunity, B cells recognize antigens and produce plasma cells and

memory cells. Plasma cells release antibodies (Ab), proteins that recognize and bind to

antigens. Once exposed to an antigen, millions of memory B cells can make antibodies

specific to that antigen, reducing the chance that you will be infected by it again. If the

antigen enters the body a second time, a secondary response occurs where plasma cells make

antibodies (they remember the Ag), much faster than the initial infection so that no

symptoms appear. Ab structure looks like a “Y” with two identical Ag-binding sites which

are specific to each Ag. Healthy adults can produce over 100 million different types of Ab.

Cell-mediated immunity is the primary defense against cancer cells and cells infected

by virus, protist, and fungi; Ab by themselves cannot destroy these. T cells divide and split

into killer T cells, helper T cells, suppressor T cells, and memory T cells. Killer T cells track

down and destroy pathogens. Helper T cells produce memory T cells as well as the

secondary response (like B cells). Once pathogens are controlled, suppressor T cells release

substances that shut down the killer T cells.

Transplants can be complicated by killer T cells because the transplanted organ is not

recognized as “self” so the killer T cells try to destroy transplanted cells or organs.

Acquired Immunity

200 years ago, Edward Jenner discovered that it was possible to produce immunity

against diseases so he tried experimenting with cowpox.

The injection of a weakened form of pathogen to produce immunity is called

vaccination (vacca is Latin for cow). It works by stimulating the production of plasma cells.

We call this active immunity and it could result from natural exposure to an antigen, such as

when we fight an infection, or from deliberate exposure to a pathogen from a vaccine.

Antibodies made by other animals can be injected into the bloodstream to produce

passive immunity. It only lasts a short time because the body recognizes and destroys the

“foreign” antibodies; it is deliberate because it is injected. Natural passive immunity occurs

when antibodies from a mother are passed to her fetus across the placenta or when the baby

breast feeds.

111

40-3 Immune System Disorders

There are three different types of immune system disorders: allergies, autoimmune

diseases, and immunodeficiency diseases.

Allergies

Allergies are the overreaction of the immune system to antigens that are usually

harmless. An allergen is a substance that causes an allergic reaction. Examples of allergens

include: pollen, dust, mold, bee stings and certain foods. Allergy-causing antigens enter the

body, attach to mast cells which initiate the inflammatory response. Activated mast cells

release histamines which increase blood flow and fluids, and increase mucus production.

Asthma

Allergic reactions can cause a dangerous condition called asthma, which is a chronic

respiratory disease in which air passages are narrower than normal and leads to wheezing,

coughing, and difficulty breathing. It can be hereditary or from the environment. Triggers

for asthma attacks include: cold air, pollen, dust, tobacco smoke, pollution, molds, and pet

dander.

Autoimmune Diseases

The immune system can normally distinguish between self and nonself. When it

makes a mistake and attacks its own cells it is called an autoimmune disease. In type I

diabetes Ab attack insulin-making cells; in rheumatoid arthritis Ab attack connective tissues

around joints; in myasthenia gravis Ab attack neuromuscular junctions; and in multiple

sclerosis Ab destroy functions of the neurons in the brain and spinal cord.

When the immune system makes a mistake and attacks the body’s own cells, it produces

an autoimmune disease.

AIDS, an Immunodeficiency Disease

In one type of immunodeficiency disease, the immune system fails to develop

normally. In a second type, Acquired Immune Deficiency Syndrome (AIDS), a virus

destroys helper T cells and the immune response breaks down. People with AIDS suffer

from unusual illnesses, known as opportunistic infections, such as Pneumocystis carini (a

type of pneumonia), Kaposi’s sarcoma (a rare skin cancer), and severe fungal infections in

the mouth and throat.

In 1983 researchers found that AIDS was caused by human immunodeficiency virus

(HIV), a retrovirus with RNA. It is deadly and efficient because it evades the immune

system and attacks T cells. The lower the T cell count, the more the disease has progressed.

Opportunistic infections often occur.

Transmission of HIV occurs through the exchange of bodily fluids such as blood,

semen, vaginal secretions and breast milk. The four main ways it is transmitted:

Any form of sexual intercourse with infected person.

Sharing needles with infected person.

Contact with blood of infected person.

112

From mother to child through pregnancy, birth, and breastfeeding.

The only no-risk behavior is abstinence and avoiding drug use. Before 1985, HIV

was transmitted in blood transfusions because it was not screened for infections. Can AIDS

be cured? No! Drugs exist that can lower the infection and allow people to live for many

years, but the infection does not go away completely.

The only no-risk behavior with respect to HIV and AIDS is abstinence.

40-4 The Environment and Your Health

A risk factor has the potential to affect health in negative way. Both heredity and

environmental factors can affect your health.

Air Quality

Air quality refers to the number and concentrations of various gases present as well as

the nature and amount of tiny particles suspended in the air. Gases important to air quality

include carbon monoxide and ozone. Airborne particulates include dust, pollen, or

particulates produced by cars and trucks or from the burning of coal.

Water Quality

Water can carry biological and chemical pollution. Biological pollutants, such as

human or animal waste, can contain bacteria or viruses that can cause cramps, vomiting,

diarrhea, or diseases like hepatitis or cholera. Chemical pollutants can cause organ damage

or interfere with development of organs and tissues.

Bioterrorism

Bioterrorism, the intentional use of biological agents to disable or kill individuals, is a

new health threat. It can involve the release of infectious agents like viruses (smallpox) or

bacteria (anthrax) or the spread of toxic compounds (botulinus toxin) extracted from living

organisms.

Cancer

Cancer is a life-threatening disease in which cells multiply uncontrollably and destroy

healthy tissue. A cell or group of cells that begin to grow and divide uncontrollably can

result in the formation of a mass of tissue called a tumor. A benign tumor does not spread to

healthy tissue or other parts of the body. Malignant tumors can invade and destroy healthy

tissue. Carcinogens are chemical compounds that are known to cause cancer by triggering

mutations in the DNA of normal cells. Examples of carcinogens include: aflatoxin which is

produced by mold on peanuts, synthetic compounds such as benzene and chloroform,

radiation such as ultraviolet radiation from the sun, X-rays, and nuclear radiation.

Environmental factors that can affect your health include air and water quality, poisonous

wastes in landfills, and exposure to solar radiation.

Maintaining Health

There are many behaviors that help keep your immune system working properly.

Eating a balanced diet helps keep your body systems working their best. Regular exercise

113

helps move blood throughout the body and maintains cardiovascular fitness. You can keep

yourself healthy by avoiding harmful activities such as doing drugs, drinking alcohol, or

smoking. Having regular checkups with your doctor can also help maintain health.

Healthful behaviors include eating a healthful diet, getting plenty of exercise and rest,

abstaining from harmful activities, and having regular checkups.

114

Chapter 11 – Introduction to Genetics

11-1 The Work of Gregor Mendel

Genetics is the scientific study of heredity.

Gregor Mendel’s Peas

Gregor Mendel (born in 1822) became a priest and was put in charge of the

monastery garden where he conducted his work with peas. He knew that part of a flower

produces pollen (male reproductive cells) and part produces egg cells. During sexual

reproduction, the male and female reproductive cells join in a process called fertilization.

The stocks of pea plants were true-breeding which meant they would produce offspring

identical to themselves if they self-pollinated.

Genes and Dominance

A trait is a specific characteristic that varies from one individual to another. Mendel

studied seven different pea plant traits: seed shape (round versus wrinkled), seed color

(yellow versus green), seed coat color (gray versus white), pod shape (smooth versus

constricted), pod color (green versus yellow), flower position (axial versus terminal), and

plant height (tall versus short).

Pea traits studied by Mendel:

The P (parental) generation consists of the original plants. F1 (first filial) generation

are the offspring. Hybrids are offspring between parents with different traits. From these

experiments Mendel drew two conclusions. The first is that inheritance is determined by

115

factors passed from one generation to the next. Today, scientists call the chemical factors

that determine traits genes. Different forms of a gene are called alleles. The second

conclusion was the principle of dominance which states that some alleles are dominant and

others are recessive.

P: TT × tt F1: Tt and Tt

The principle of dominance states that some alleles are dominant and others are recessive.

Segregation

Mendel wanted to know if the recessive alleles disappeared or were still present in the

F1 so he allowed the F1 hybrids to produce an F2 generation by self-pollination. The F1 cross

showed that recessives reappear. He explained that when F1 produces gametes, the two

alleles segregate so each gamete has only one copy of each gene. Each F1 has two types of

gametes (sex cells).

Allele segregation:

When each F1 plant flowers and produces gametes, the two alleles segregate from each

other so that each gamete carries only a single copy of each gene. Therefore, each F1 plant

produces two types of gametes – those with the allele for tallness and those with the allele for

shortness.

11-2 Probability and Punnett Squares

Genetics and Probability

Probability is the likelihood that a particular event will occur. Principles of

probability can be used to predict genetic cross outcomes.

The principles of probability can be used to predict the outcomes of genetic crosses.

Punnett Squares

A Punnett square can be used to predict and compare genetic variations that will

result from a cross (mating). Letters in the Punnett square represent alleles. Homozygous

organisms have two identical alleles for a trait such as TT or tt. Heterozygous organisms

have two different alleles for a trait such as Tt (we do not write tT). The phenotype is the

116

physical characteristic (what you see) for example: tall or short. The genotype is the genetic

makeup (what is in the genes) for example: TT, Tt, or tt.

Punnett squares can be used to predict and compare the genetic variations that will result

from a cross.

Probability and Segregation

Examples of Punnett squares:

T T T t

t Tt Tt T TT Tt

t Tt Tt t Tt tt

Phenotype:

Genotype:

100% tall plants

100% Tt

Phenotype:

Genotype:

3:1 ratio tall:short plants

1:2:1 ratio TT:Tt:tt

11-3 Exploring Mendelian Genetics

Independent Assortment

The principle of independent assortment states that genes for different traits can

segregate independently during gamete formation. It helps account for the many variations

seen in organisms. Genes that segregate independently do not influence each other’s

inheritance. A two-factor cross such as RRYY × rryy yields an F1 that is 100% RrYy. A

two-factor cross such as RrYy × RrYy yields an F2 that has a 9:3:3:1 ratio.

Example of a two-factor cross between RrYy × RrYy:

RY Ry rY ry

RY RRYY RRYy RrYY RrYy

Ry RRYy RRyy RrYy Rryy

rY RrYY RrYy rrYY rrYy

ry RrYy Rryy rrYy rryy

The principle of independent assortment states that genes for different traits can segregate

independently during the formation of gametes. Independent assortment helps account for

the many genetic varations observed in plants, animals, and other organisms.

Beyond Dominant and Recessive Alleles

In incomplete dominance, one allele is not completely dominant over another; the

heterozygous phenotype is somewhere in between the two homozygous phenotypes like

mixing paint. For example, four o’clock plants with red (RR) and white (WW) flowers

produce plants with pink (RW) flowers. Example:

R R R W

W RW RW R RR RW

W RW RW W RW WW

Phenotype:

Genotype:

100% pink

100% RW

Phenotype:

Genotype:

1:2:1 red:pink:white

1:2:1 ratio RR:RW:WW

117

In codominance, both alleles contribute to the phenotype. For example, white cattle

(CW

CW

) can mate with red cattle (CRC

R) to produce roan cattle (C

RC

W), which have both red

and white hairs on their body. Example:

CR C

R

CW

CRC

W C

RC

W

CW

CRC

W C

RC

W

Phenotype:

Genotype:

100% roan

100% CRC

W

Many genes have more than two alleles possible for a trait so this would be called

multiple alleles. Individuals can only have two alleles but more than two exist in the

population. For example, rabbits have four possible alleles for fur color: C for full color

which is dominant to all other alleles; cch

for chinchilla, a partial defect in pigmentation,

which is dominant to ch and c; c

h for Himalayan, color found on certain parts of the body,

which is dominant to c; or c for albino, no color, which is recessive to all other alleles.

Traits controlled by two or more genes are said to be polygenic traits. For example,

human skin color is controlled by more than four different genes; different combinations of

these genes produce very different skin color.

Some traits, such as human blood type, have a mixture of principles. Human blood

type has characteristics of both codominance and multiple alleles. The I allele, which can be

IA or I

B, is dominant to i. Example:

IA i

IB I

AIB I

Bi

i IAi ii

Phenotype:

Genotype:

1:1:1:1 type AB:type A:type B:type O

1:1:1:1 ratio IAIB: I

Ai : I

Bi : ii

Some alleles are neither dominant nor recessive, and many traits are controlled by

multiple alleles or multiple genes.

Applying Mendel’s Principles

The basic principles of Mendelian genetics can be used to study the inheritance of

traits of many organisms and to calculate the probability of certain traits appearing in future

generations.

Genetics and the Environment

Characteristics are not determined by genetics alone. Interactions between genes and

the environment determine the traits that appear in an organism. The environment influences

the expression of genes. For example, human skin color is determined by genes but

ultraviolet radiation from the sun (or tanning booths) can alter skin tone such as when a

person becomes tan or sunburned.

11-4 Meiosis

Gregor Mendel did not know about genes but knew there was something inside cells

that was involved in heredity. In order for his principles to work, each organism has to get a

118

copy of each gene from its parents and when an organism produces its own gametes, the sets

of genes mut be separated so that each gamete has one set.

Chromosome Number

Homologous chromosomes each have a corresponding chromosome from the

opposite-sex parent; they are similar in size, shape, and genetic content. Diploid, which

means “two sets”, refers to cells that have both sets of homologous chromosomes. Somatic

cells (all body cells except gametes) are diploid. Haploid, which means “one set”, refers to

cells that have only one set of chromosomes. Gametes (sex cells) are haploid, containing

only half the number of chromosomes of the parent.

Phases of Meiosis

Meiosis is the production of haploid (N) gamete cells from diploid (2n) cells.

Meiosis is reduction division because the number of chromosomes is cut in half; it involves

one chromosome replication and two distinct divisions called meiosis I and meiosis II. Each

original diploid cell produces four haploid daughter cells.

Meiosis:

During interphase I, before meiosis I, each chromosome is replicated. During

prophase I, each chromosome pairs with its homologous chromosome to form a tetrad (4

chromatids) in a process called synapsis. The strands can exchange parts in a process called

crossing-over; alleles are exchanged between homologous chromosomes producing new

combinations. The nuclear membrane disappears and the spindle forms. Near the end,

tetrads move toward the equator (middle). In metaphase I, tetrads are lined up on the equator

and attach to spindle fibers. In anaphase I, the tetrads are pulled to opposite poles (ends) in a

process called disjunction. Telophase I, which marks the end of the first meiotic division,

occurs at the same time as cytokinesis, the division of the cytoplasm. At this point there are

two daughter cells; no further replication occurs and the second division begins.

119

Meiosis II is like mitosis. In prophase II, each daughter cell forms a spindle and

chromosomes begin to move towards the equator. In metaphase II, chromosomes line up in

the center. In anaphase II, paired chromatids separate and move to opposite ends. Telophase

II and cytokinesis produce four haploid daughter cells.

Meiosis is a process of reduction division in which the number of chromosomes per cell is

cut in half through the separation of homologous chromosomes in a diploid cell.

Gamete Formation

Gametogenesis is the formation of gametes. Oogenesis, the production of egss, takes

place in the ovaries. The cell divisions are uneven so that a single cell becomes an egg and

the other three become polar bodies, which are not involved in reproduction.

Spermatogenesis, the production of sperm, takes place in the testes. This results in four

equal-sized gametes.

Comparing Mitosis and Meiosis

Mitosis is the division of a diploid cell (2N) into two diploid (2N) daughter cells that

are genetically identical to the parent cell. Mitosis is a form of asexual reproduction and is

involved in cell growth and repair. Meiosis is the division of a diploid cell (2N) into four

haploid (N) daughter cells which are genetically different from the parent cell and from each

other. Meiosis is responsible for producing gametes which are involved in sexual

reproduction. Meiosis cuts the chromosome number in half and fertilization (joining of

gametes) restores the chromosome number in the offspring.

Mitosis results in the production of two genetically identical diploid cells, whereas

meiosis produces four genetically different haploid cells.

11-5 Linkage and Gene Maps

Gene Linkage

Thomas Morgan worked with Drosophila genes in fruit flies in 1910 and found that

many appearded to be “linked” together because certain genes were almost always inherited

together. He discovered that there were linkage groups in fruit flies; the groups assorted

independently but all genes in one group were inherited together.

It is the chromosomes, however, that assort independently, not individual genes.

Gene Maps

Alfred Sturtevant, a student in Morgan’s lab, hypothesized that the rate at which

linked genes were separated and recombined can be used to produce a “map” of distances

between genes. A gene map shows the relative locations of each known gene. This method

of measuring the frequencies of crossing-over between genes has been used to construct

genetic maps, including maps of the human genome, ever since.

120

Chapter 12 – DNA and RNA

12-1 DNA

Griffith and Transformation

In 1928, Frederick Griffith wanted to see how certain bacteria caused pneumonia. He

isolated two strains of bacteria: smooth (type S) and rough (type R). When injected, live type

S killed mice and live type R was harmless. Heat-killed type S was harmless when injected

but when heat-killed type S was mixed with live type R and injected, the mice died. This

meant that something changed or transformed one strain into the other. Griffith called this

process transformation because the harmless strain had been permanently changed into the

disease-causing strain.

Griffith experiment:

Avery and DNA

In 1944, scientists Oswald Avery, Colin MacLeod, and Maclyn McCarty studied

Griffith’s experiment and treated an extract of heat-killed type S bacteria with enzymes to

figure out which molecule caused the transformation to occur. When they used an enzyme

that would break down the nucleic acid DNA, transformation did not occur which led them to

conclude the transforming material was DNA.

Avery and other scientists discovered that the nucleic acid DNA stores and transmits the

genetic information from one generation of an organism to the next.

The Hershey-Chase Experiment

The most important experiment that convinced other scientists of the chemical nature

of the gene was conducted by Alfred Hershey and Martha Chase in 1952. They used a

bacteriophage, a virus that infects bacteria, and radioactive isotopes to determine if protein or

DNA was the hereditary material. The radioactive marker phosphorus-32 (P-32) was used to

tag DNA and sulfur-35 (S-35) was used to tag protein; DNA has no sulfur and proteins have

almost no phosphorus. Radioactivity was present in bacteria infected by bacteriophages

contained P-32 in their DNA but not in those containing S-35 in their protein coat. This led

to the conclusion that DNA was in fact the genetic material.

121

Hershey and Chase concluded that the genetic material of the bacteriophage was DNA,

not protein.

Hershey and Chase experiment:

The Components and Structure of DNA

DNA is a long molecule made of subunits called nucleotides. A nucleotide consists

of the 5-carbon sugar deoxyribose, a phosphate group, and a nitrogenous base. There are

four types of nitrogenous bases in DNA: adenine (A) and guanine (G) are called purines

which have two rings; thymine (T) and cytosine (C) are called pyrimidines which have one

ring. The backbone of DNA is made of sugar and phosphate groups.

Erwin Chargaff discovered that the percent of G and C were almost equal in any

DNA sample. Chargaff’s rule became: [A] = [T] and [G] = [C]. In the early 1950s, Rosalind

Franklin used X-ray diffraction to get information about DNA structure. She found that

DNA strands are twisted into a helix (coil) shape.

James Watson and Francis Crick were tryng to understand DNA. In 1953 they built

3-D models of DNA using clues from Franklin’s X-rays to figure out that DNA was a double

helix. They found that hydrogen bonds could only form between A and T and between C and

G. This principle, called base pairing, explained Chargaff’s rules.

The double helix resembles a twisted rope ladder; sugar and phosphate make the

length of the ladder and the hydrogen bonded bases make up the rungs.

Watson and Crick’s model of DNA was a double helix, in which two strands were wound

around each other.

122

X-ray diffraction:

Double helix:

12-2 Chromosomes and DNA Replication

DNA and Chromosomes

Prokaryotes have a single, circular DNA molecule in the cytoplasm containing almost

all of the genetic material. Eukaryotes have DNA in the form of chromosomes in the nucleus

of every cell. The number of chromosomes varies; humans have 46 chromosomes and fruit

flies have only 8 chromosomes.

DNA is a very long molecule, made of millions or billions of base pairs. The

prokaryote E. coli has over 4 million base pairs, making it 1.6 mm long; to fit inside the

bacterium, the DNA molecule is folded over and over. Humans have over 3 billion base

pairs; this means each nucleus has over 1 meter of DNA in it, folded into chromosomes.

Chromatin consists of DNA tightly coiled around histones (proteins) forming a beadlike

structure called a nucleosome; DNA is in this form during most of the cell cycle. During

mitosis, chromosomes take form.

123

DNA and chromosomes:

DNA Replication

Before cells divide, they duplicate their DNA in a process called replication which

involves a series of enzymes. The DNA molecule is “unzipped” as hydrogen bonds are

broken and each strand serves as a template for the attachment of complementary bases; A

goes with T, G goes with C, T goes with A, and C goes with G. DNA polymerase is the

enzyme that joins individual nucleotides to produce the DNA polymer and proofreads each

new strand to make sure it is an exact copy.

During DNA replication, the DNA molecule separates into two strands, then produces two

new complementary strands following the rules of base pairing. Each strand of the double

helix of DNA serves as a template, or model, for the new strand.

124

DNA replication:

12-3 RNA and Protein Synthesis

Genes are coded DNA instructions that control the production of proteins within

cells. The first part of decoding genetic messages is to copy DNA into RNA.

The Structure of RNA

RNA is made of a long chain of nucleotides, like DNA, with three major differences:

RNA is a single-stranded molecule, the 5-carbon sugar is ribose, and the nitrogenous base

uracil (U) is found instead of thymine.

RNA structure:

Types of RNA

RNA molecules have many functions but most are involved in protein synthesis.

There are three types of RNA: messenger RNA (mRNA), ribosomal RNA (rRNA), and

transfer RNA (tRNA); mRNA carries instructions for assembling proteins and is the

125

messenger from DNA to the rest of the cell; rRNA combines with proteins and makes up the

ribosome; tRNA transfers amino acids to the ribosomes.

There are three main types of RNA: messenger RNA, ribosomal RNA, and transfer RNA.

Transcription

Copying DNA into complementary RNA is called transcription. This process

requires RNA polymerase which binds to DNA, separates DNA strands, and uses one strand

of DNA as a template for RNA. RNA polymerase binds in regions called promoters that

have a specific DNA sequence and act as signals to indicate where the enzyme should bind to

make RNA.

During transcription, RNA polymerase binds to DNA and separates the DNA strands.

RNA polymerase then uses one strand of DNA as a template from which nucleotides are

assembled into a strand of RNA.

RNA Editing

When RNA is formed from DNA, both introns and exons are copied. Introns are

sequences that are not involved in coding for proteins; exons are sequences expressed in

protein synthesis. Introns are cut out of the RNA and exons are then joined together.

RNA processing:

126

The Genetic Code

Proteins are made by joining amino acids into long chains called polypeptides. The

genetic code is the “language” of mRNA instructions. Each “word” of the message, called a

codon, is three bases long. Each codon consists of three nucleotides that specify a single

amino acid. There are 64 possible combinations of A, U, C, and G that make up the 20

amino acids. AUG is a start codon; UGA, UAA, and UAG are stop codons.

Codon chart:

Translation

Translation is decoding an mRNA message into a protein and it takes place on

ribosomes. Before translation, the mRNA is transcribed in the nucleus and released into the

cytoplasm; translation begins when mRNA attaches to the ribosome starting with AUG.

Each tRNA carries only one kind of amino acid; it has unpaired bases, called the anitcodon,

that are complementary to one mRNA codon. For example, tRNA has bases UAC to pair

with the codon AUG. The ribosome forms peptide bonds between amino acids to join them

and the tRNA breaks off. The chain continues to grow until a stop codon is reached.

During translation, the cell uses information from messenger RNA to produce proteins.

The Roles of RNA and DNA

DNA and RNA play important roles in protein synthesis. DNA is like a “master

plan” used by a builder and RNA is like the “blueprint”.

127

Genes and Proteins

Most genes have nothing but coded instructions to make proteins. Many proteins are

enzymes which catalyze and regulate reactions. Proteins are used in everything

Transcription and translation:

Gene expression:

128

12-4 Mutations

Mutations are changes in the genetic material.

Kinds of Mutations

Gene mutations produce changes in a single gene. A point mutation involves changes

in one or a few nucleotides and occurs at a single point in the DNA sequence. For example,

a substitution would change one base to another. A frameshift mutation involves a shift in

the reading frame of the genetic message and could change every amino acid that follows the

point of mutation. An insertion or deletion adds or removes a base which changes the

groupings of codons.

Chromosomal mutations produce changes in the number or structure of

chromosomes. The four types are deletion, duplication, inversion, and translocation.

Chromosomal mutations:

Significance of Mutations

Many mutations are neutral or have no effect. Some can be detrimental and cause

genetic disorders. Others are beneficial and can be a source of variation in a species.

Polyploidy is a condition in which an organism has extra sets of chromosomes; breeders take

advantage of this because polyploidy plants and crops are often larger and stronger than

diploids.

12-5 Gene Regulation

Regulatory sites next to the promoter are places where other proteins can regulate

transcription and determine whether a gene is turned on or off.

Gene Regulation: An Example

An operon is a group of genes that operate together. There are two regulatory

regions: promoter and operator. In the promoter, RNA polymerase binds and then starts

129

transcription. The operator is the region of a chromosome in an operon to which the

repressor binds when the operon is “turned off”.

In the lac operon of E. coli, when lactose is not present the repressor binds to the

operator region which prevents RNA polymerase frm starting transcription. Lactose causes

the repressor to be released from the operator region.

The lac genes are turned off by repressors and turned on by the presence of lactose.

Lac operon:

Eukaryotic Gene Regulation

Operons are not usually found in eukaryotes. Typical eukaryotic genes have a short

region of about 30 base pairs containing the sequence TATATA or TATAAA before the start

of transcription. This sequence has been given the name “TATA box” because it is found

before so many eukaryotic genes and seems to help position RNA polymerase.

Genes are regulated in many ways by enhancer sequences located before the point

where transcription begins. Many proteins can bind to enhancer sequences making

eukaryotic gene regulation quite complex.

Most eukaryotic genes are controlled individually and have regulatory sequences that are

much more complex than those of the lac operon.

Development and Differentiation

During embryonic development cells grow, divide, and undergo differentiation which

means cells become specialized in structure and function. Hox genes, known as “master

control genes”, control the differentiation of cells and tissues in the embryo. A mutation in a

hox gene can completely change the organs that develop in specific parts of the body.

130

Chapter 13 – Genetic Engineering

13-1 Changing the Living World

Selective Breeding

Selective breeding is a method of breeding that allows only those individual

organisms with desired characteristics to produce the next generation. Hybridization is the

crossing of dissimilar individuals to bring together the best of both organisms. Hybrids are

often hardier than either of the parents. Inbreeding is the continued breeding of individuals

with similar characteristics that can be used by breeders to maintain the desired

characteristics of a line of organisms. Although useful, inbreeding can pose serious risks for

the organisms because there is a greater chance of bringing together recessive alleles that can

cause genetic defects.

Humans use selective breeding, which takes advantage of naturally occurring genetic

variation in plants, animals, and other organisms, to pass desired traits on to the next

generation of organisms.

Increasing Variation

Although mutations occur spontaneously, breeders can increase the rate of mutation

with chemicals and radiation. Scientists have been able to develop useful bacterial strains,

such as those that digest oil to help clean oil spills, and polyploid plants which are often

larger and stronger than their diploid relatives.

Breeders can increase the genetic variation in a population by inducing mutations, which

are the ultimate source of genetic variability.

13-2 Manipulating DNA

Scientists use their knowledge of the structure of DNA and its chemical properties to

study and change DNA molecules. Different techniques are used to extract DNA from cells,

to cut DNA into smaller pieces, to identify the sequence of bases in a DNA molecule, and to

make unlimited copies of DNA.

The Tools of Molecular Biology

Genetic engineering is the process of making change in the DNA code of living

organisms. DNA can be extracted from cells by a simple chemical procedure which opens

cells and separates DNA from other cell parts. Restriction enzymes are used to precisely cut

DNA molecules into smaller fragments ay a specific nucleotide sequence.

Gel electrophoresis is a procedure in which a mixture of DNA fragments is placed on

a porous gel; when electric voltage is applied, the negatively charged DNA molecules move

towards the positive end of the gel creating bands that can be compared. The smaller the

fragment, the faster and farther it moves.

131

Gel electrophoresis:

Using the DNA Sequence

In DNA sequencing, a complementary strand of DNA can be made using

fluorescently labeled nucleotides; as each nucleotide is added, replication stops which

produces a short color-coded fragment. The fragments can be separated on a gel and the

sequence can be read from the gel.

Recombinant DNA molecules are produced by combining DNA from different

sources. The polymerase chain reaction (PCR) allows biologists to make many copies of a

particular gene.

PCR:

132

13-3 Cell Transformation

During transformation, a cell takes in DNA from outside the cell. This external DNA

becomes a component of the cell’s DNA.

Transforming Bacteria

Recombinant DNA can be used to transform bacteria. Foreign DNA is joined to a

plasmid, a small circular DNA molecule found naturally in some bacteria. Plasmid DNA has

a sequence that promotes plasmid replication and the plasmid has a genetic marker, a gene

that makes it possible to distinguish bacteria that carry the plasmid from those that don’t.

Transforming bacteria using plasmids:

Transforming Plant Cells

The bacterium Agrobacterium tumefaciens can be used to introduce foreign DNA

into plant cells. The tumor-producing gene in the plasmid is inactivated and foreign DNA is

inserted into the plasmid; the recombinant DNA can then infect plant cells. When cell walls

are removed, plant cells in culture will take up DNA on their own. DNA can also be injected

directly into some cells.

If transformation is successful, the recombinant DNA is integrated into one of the

chromosomes of the cell.

Transforming Animal Cells

Animal cells can be transformed like plant cells. DNA can be directly injected.

Recombinant DNA can replace a gene in an animal’s genome; ends of the recombinant DNA

recombine with sequences in the host cell DNA. When recombinant DNA is inserted, the

host cell’s original gene is lost or knocked out of place.

133

13-4 Applications of Genetic Engineering

Transgenic Organisms

A transgenic organism contains genes from another species. Transgenic bacteria

produce many important substances for health and industry. For example, bacteria can be

inserted with human genes for insulin, growth hormone, and blood clotting factors.

Transgenic animals have been used to study genes and improve the food supply. For

example, mice have been injected with human genes to study how the immune system

responds to various diseases and some livestock have extra growth hormone genes so they

grow faster and produce leaner meat. Transgenic plants are an important part of our food

supply and many have genes that make them produce their own insecticide so that we do not

have to use chemical pesticides. These organisms are labeled as genetically modified (GM)

Genetic engineering has spurred the growth of biotechnology, which is a new industry

that is changing the way we interact with the living world.

Transgenic mammal:

134

Cloning

A clone is a member of a population of genetically identical cells produced from a

single cell. Asexual reproduction and vegetative propagation produce clones naturally. The

purpose of cloning is to produce large numbers of genetically identical offspring. Known or

controlled heredity is useful in the study of cancer, aging, birth defects, and regeneration of

damaged parts.

In 1997, Ian Wilmut cloned the first mammal, a sheep named Dolly. To clone an

organism, a donor somatic (body) cell is taken from the organism to be cloned (organism A)

and its nucleus is removed. An egg cell is taken from a female (organism B) and its nucleus

is removed. The donor nucleus is then inserted into the empty egg cell. After fusing the cells

with electric shock, it begins to divide normally and the embryo is then implanted in the

uterus of a foster mother (organism C). The embryo develops into a fully grown organism

(organism D). In this example, organism A and D are genetically identical clones; there is no

relatedness among any other organisms in this example.

Gene cloning:

135

Chapter 14 – The Human Genome

14-1 Human Heredity

Human Chromosomes

A karyotype is a picture of chromosomes grouped in pairs. Biologists photograph

cells during mitosis and then cut the chromosomes out to arrange them. Human egg or sperm

cells have 23 chromosomes and diploid body cells have 46 chromosomes. Two of the 46

chromosomes are sex chromosomes which determine gender. Females have two copies of a

large X chromosome; males have one X and one small Y chromosome. The other 44

chromosomes are called autosomes.

All human egg cells carry a single X chromosome (23, X). However, half of all sperm

cells carry an X chromosome (23, X) and half carry a Y chromosome (23, Y). This ensures

that just about half of the zygotes will be 46, XX and half will be 46, XY.

Constructing a karyotype:

Human Traits

136

A pedigree chart shows the relationships within a family and can be used to help

study how a trait is passed from one generation to the next. Genetic counselors can analyze

pedigree charts to infer genotypes of family members. Circles represent females and squares

represent males.

Some of the most obvious human traits are polygenic, which means they are

controlled by many genes. Many of these traits are also influenced by environmental factors

such as nutrition and exercise. Environmental effects on genes are not inherited; only genes

are inherited.

Example of a pedigree chart:

Human Genes

The human genome is our complete set of genetic information which includes tens of

thousands of genes. By 2000, the DNA sequence of the human genome was almost

complete. A number of genes are responsible for human blood groups; the most well known

are the ABO blood groups and the Rh blood groups. The Rh group is determined by one

gene with two alleles – positive and negative. Rh stands for “rhesus monkey”, the organism

in which the factor was discovered. Rh+ is dominant and Rh

– is negative.

In many cases of genetic disorders, the presence of a normal, functioning gene is

revealed only when an abnormal or nonfunctioning allele affects the phenotype. One of the

first disorders to be understood this way was phenylketonuria (PKU). People with PKU are

missing an enzyme to break down phenylalanine which can build up in tissues and lead to

brain damage and mental retardation. It can be diagnosed at birth with a urine test and can be

avoided with diet low in phenylalanine but there is no cure. PKU is caused by an autosomal

recessive allele on chromosome 12.

137

Tay Sachs is an autosomal recessive allele found in central and eastern European

Jewish families. It results in nervous system breakdown and death in the first few years of

life due to a lack of the enzyme needed to break down lipids in the brain; lipids build up and

destroy brain cells. There is no treatment or cure but parents can be tested for the presence of

the allele to see if they are at risk of having children with the disorder.

Some genetic disorders are caused by autosomal dominant alleles such as

achondroplasia, a form of dwarfism. Another example is Huntington’s disease which is a

fatal nervous system disorder whih causes the progressive loss of muscle control and mental

function. People with this disease usually have no symptoms until their thirties or later and

because it is controlled by a dominant allele, the child of parent with disease has a 50%

chance of inheriting it.

From Gene to Molecule

For many genetic disorders, scientists are still trying to figure out how DNA

sequences affect phenotype and they are hoping to discover the link between DNA bases in

the allele for a genetic disorder and the disorder itself.

Cystic fibrosis is the most common fatal inherited disorder among Caucasians in the

U.S. It is caused by a recessive allele on chromosome 7. Glands make too much mucus

which clogs and damages the lungs making it hard to breathe.

Sickle cell disease is a common disorder found in African Americans. The RBCs

have a sickle shape, which causes clots and clumping, low oxygen levels, pain, and

weakness. It is caused by the change of one base in a gene that controls the production of

one polypeptide chain (protein) in hemoglobin. Normal hemoglobin has GAA for glutamic

acid; abnormal hemoglobin has is GUA for valine. Carriers of the sickle cell allele are

unaffected by the disease and are more resistant to malaria.

In both cystic fibrosis and sickle cell disease, a small change in the DNA of a single gene

affects the structure of a protein, causing a serious genetic disorder.

14-2 Human Chromosomes

Human Genes and Chromosomes

Human chromosomes 21 and 22 are the smallest autosomes and so they were the first

two to be sequenced. Both have impotant genes for health as well as long regions of

repetitive DNA that do not code for proteins. Some genetic disorders can be caused by

alleles on these chromosomes. Chromosome 22 has an allele for a form of leukemia and an

allele for neurofibromatosis, a tumor-causing disease of the nervous system. Chromosome

21 has the allele for amyotropic lateral sclerosis (ALS), also known as Lou Gehrig’s disease.

Genes located close together on the same chromosomes are linked and tend to be inherited

together.

Sex-Linked Genes

Genes located on the X and Y chromosomes are called sex-linked genes. There are

many genes and alleles for over 100 disorders on the X chromosome. The Y chromosome

only has a few genes because it is much smaller.

138

Colorblindness is the inability to see certain or all colors and is caused by a recessive

gene on the X chromosome. Since males only have one X chromosome, they are more likely

to be affected; about 1 in 10 males in the United States has a form of colorblindness. To be

expressed in females, both X chromosomes must contain the allele for colorblindness so it is

rare for females to inherit this disorder; only about 1 in 100 females has colorblindness.

Hemophilia is a disorder in which the blood is unable to clot because it lacks a certain

protein. It is caused by a recessive gene on the X chromosome; About 1 in 10,000 males is

born with a form of hemophilia. Female carriers show no signs of the illness.

Duchenne muscular dystrophy results in the progressive weakening and loss of

skeletal muscle tissue and is caused by a recessive gene on the X chromosome. About 1 out

of every 3000 males is born with this condition. Affected individuals have an inactive

muscle protein caused by a defective gene.

Males have just one X chromosome. Thus, all X-linked alleles are expressed in males,

even if they are recessive.

X-Chromosome Inactivation

Since females have two X chromosomes, they have to adjust to having an extra one.

Mary Lyon discovered that one X chromosome is randomly switched off and forms a dense

region in the nucleus called a Barr body.

Chromosomal Disorders

Nondisjunction is an error in meiosis in which homologous chromosomes fail to

separate. If two copies of an autosome fail to separate during meiosis an individual could be

born with three copies; this condition is called trisomy. Trisomy of chromosome 21 is called

Down syndrome; it causes mild to severe mental retardation and physical abnormalities.

Disorders can also occur with sex chromosomes. In females, nondisjunction can lead

to Turner’s syndrome, which is caused by presence of only one X chromosome and results in

a sterile female with underdeveloped sexual characteristics. In males, nondisjunction can

lead to Klinefelter’s syndrome, which is caused by an extra X chromosome (XXY) and

results in a male with underdeveloped sex organs. There are no instances of babies born

without an X chromosome because it contains genes that are vital to the survival and

development of the embryo. The Y chromosome is responsible for sex determination in

humans.

If nondisjunction occurs, abnormal numbers of chromosomes may find their way into

gametes, and a disorder of chromosome numbers may result.

14-3 Human Molecular Genetics

Human DNA Analysis

Genetic testing can be done to determine if parents have alleles for genetic disorders.

Sometimes these tests use labeled DNA probes that detect the complementary base sequences

found in disease-causing alleles. Other tests search for changes in restriction enzyme cutting

sites.

139

DNA fingerprinting is the anaylsis of sections of DNA that have little or no known

function, but vary widely from one individual to another, in order to indentify individuals.

Restriction enzymes cut DNA into small fragments which are separated by size using gel

electrophoresis. Fragments with repeats are labeled with radioactive probes to produce a

series of bands, called the DNA fingerprint. DNA fingerprinting has been used in the United

States since the late 1980s to help convict criminals and determine paternity.

The Human Genome Project

DNA sequencing technologies have made it possible to sequence entire genomes. At

first, scientists worked with small organisms such as the bacterium Escherichia coli which

has 4,639,221 base pairs. In 1990, the United States and other countries began the Human

Genome Project; it was announced that a working copy of the human genome was essentially

complete in June 2000.

Scientists used “shotgun sequencing” which involves cutting DNA into random

fragments and then determining the sequence of bases in each fragment. Computers found

areas of overlap and put fragments together by linking the overlapping areas.

Only a small part of a human DNA molecule is made up of genes. Some say as little

as 25,000 genes whereas others believe it to be more like 100,000.

The Human Genome Project is an ongoing effort to analyze the human DNA sequence.

Gene Therapy

Gene therapy is the process of changing the gene that causes a genetic disorder.

Genes can be replaced by a normal gene so that the body can make the correct protein or

enzyme it needs, eliminating the cause of the disorder. Viruses, modified so they can no

longer cause disease, are often used in gene therapy because of their ability to enter a cell’s

DNA. A replacement gene on a DNA fragment is then spliced to viral DNA and the patient

is infected with the modified virus particles.

In gene therapy, an absent or faulty gene is replaced by a normal, working gene.

Gene therapy:

140

141

Chapter 15 – Darwin’s Theory of Evolution

15-1 The Puzzle of Life’s Diversity

Evolution, or change over time, is the process by which modern organisms have

descended from ancient organisms. A scientific theory is a well-supported testable

explanation of phenomena that have occurred in the natural world.

Voyage of the Beagle

Charles Darwin is considered the father of evolutionary theory. He joined the crew of

the H.M.S. Beagle in 1831, which set sail for a five-year voyage around the world. Darwin

collected plant and animal specimens; he studied them and filled notebooks with his

observations. He came to view every new finding as a piece of a puzzle which would

provide a scientific explanation for the diversity of life on Earth.

During his travels, Darwin made numerous observations and collected evidence that led

him to propose a revolutionary hypothesis about the way life changes over time.

Darwin’s voyage:

Darwin’s Observations

Darwin realized that many plants and animals are well suited to the environment they

inhabit and was impressed by the ways they survive and reproduce. He was puzzled by

where different species live and do not live. Darwin collected many fossils, the preserved

remains of ancient organisms, and found that some resembled living organisms and others

were unlike any creature he had ever seen.

The Galápagos Islands, 1000 km west of South America, influenced Darwin the most.

Although the islands were close to one another, they had very different climates and the

organisms inhabiting the islands had notable differences. The shape of a tortoise’s shell

could help identify which island it lived on; birds on the islands had differently shaped beaks.

142

The Journey Home

While heading home, Darwin thought a lot about his findings but he could not

understand the reason for these patterns of diversity. He wondered if animals on the different

islands had once been part of the same species that would have evolved from an original

ancestor after becoming islolated from one another.

Darwin observed that the charactertistics of many animals and plants varied noticeably

among the different islands of the Galápagos.

15-2 Ideas That Shaped Darwin’s Thinking

An Ancient, Changing Earth

In the eighteenth and nineteenth centuries, scientists examined the Earth and gathered

information that suggested the Earth was very old and changed slowly over time. James

Hutton and Charles Lyell formed important theories based on this evidence.

In 1795, Hutton published a hypothesis about the geological forces that have shaped

Earth; he proposed that layers of rock form slowly and are moved by forces beneath Earth’s

surface. Lyell’s book, Principles of Geology, explained how geological features could be

built up or torn down over long periods of time. Darwin was influenced by this

understanding of geology; he thought that if Earth could change, then life could change; he

also realized that it would have taken a very long time for this to happen.

Hutton and Lyell helped scientists recognize that Earth is many millions of years old, and

the processes that changed Earth in the past are the same processes that operate in the

present.

Lamarck’s Evolution Hypotheses

Jean-Baptiste Lamarck was among the first scientistst to recognize that living things

change over time, that all species were descended from other species, and that organisms

were somehow adapted to their environments. He published his hypotheses in 1809, the year

Darwin was born.

Lamarck proposed that organisms have an innante tendency toward complexity and

perfection. He proposed that organisms could alter size or shape of parts by using their

bodies in new ways. He thought acquired characteristics could be inherited. Although his

hypotheses are incorrect in many ways, he was one of the first to develop a scientific

hypothesis of evolution.

Lamarck proposed that by selective use or disuse of organs, organisms acquired or lost

certain traits during their lifetime. These traits could then be passed on to their offspring.

Over time, this process led to change in a species.

Population Growth

Thomas Malthus was an economist; in 1798 he published a book in which he noted

that babies were being born faster than people were dying and that if this trend continued

143

there would not be enough food or living space. When Darwin read Malthus’ work he

realized that this reasoning applied to plants and animals more so than to humans.

Malthus reasoned that if the human population continued to grow unchecked, sooner or

later there would be insufficient living space and food for everyone.

15-3 Darwin Presents His Case

Publication of On the Origin of Species

When Darwin returned home in 1836, he filled notebooks with his ideas about species

diversity and the process that would be called evolution. He shelved his ideas until 1858

when he received an essay from Alfred Wallace; the essay summarized Darwin’s ideas about

evolutionary change so he decided to publish his work. In 1859, he published his results and

proposed a mechanism for evolution that he called natural selection in his book, On the

Origin of Species.

Inherited Variation and Artifical Selection

Members of each species vary from one another in important ways. Darwin noted

that plant and animal breeders used heritable variation, now called genetic variation, to

improve crops and livestock. He called this process artificial selection, the selection by

humans for breeding of useful traits from the natural variation among different organisms.

In artificial selection, nature provided the variation, and humans selected those variations

that they found useful.

Evolution by Natural Selection

Darwin compared processes in nature to artificial selection which allowed him to

develop a hypothesis to explain how evolution occurs. He realized that high birth rates and a

shortage of life’s basic needs would eventually force organisms into a competition for

resources. The struggle for existence means that members of each species compete regularly

to obtain food, living space, and other resources. Darwin called the ability of an individual to

survive and reproduce in its specific environment fitness. He proposed that fitness is the

result of adaptations, any inherited characteristic that increases an organism’s chance of

survival.

Individuals with characteristics that are not well suited to their environment (low

fitness) either die or leave few offspring whereas those better suited to their environment

(with adaptations that enable fitness) survive and reproduce most successfully. Darwin

called this survival of the fittest. Darwin referred to the survival of the fittest as natural

selection.

Darwin proposed that natural selection produces organisms with differences making

species today look different from their ancestors. He referred to this as descent with

modification which is the principle that each living species has descended, with changes,

from other species over time. Descent with modification implies all living organisms are

related; common descent is the principle that all living things were derived from common

ancestors.

144

Over time, natural selection results in changes in the inherited characteristics of a

population. These changes increase a species’ fitness in its environment.

Evidence of Evolution

Darwin saw fossils as a record of the history of life on Earth. Transitional fossils

document various intermediate satges in the evolution of modern species from organisms that

are now extinct.

Fossils:

Observing the geographic distribution of organisms is important. The finches of the

Galápagos Islands were similar yet distinctly different from one another because they could

have descended with modification from a common mainland ancestor. Species on different

continents, each with different ancestors, could have ended up evolving common features if

they were living under similar ecological conditions and were exposed to similar pressures of

natural selection.

Biogeography:

Homologous structures have different mature forms but develop from the same

embryonic tissue; they are constructed from the same basic bones but have different

functions to enable survival in different environments. Some structures are so reduced in

145

size that they are just traces of homologous organs in other species. They are called vestigial

organs and serve no useful functions.

Homologous structures:

The embryos of many animals with backbones are similar and can be compared. The

same groups of embryonic cells develop in the same order and in similar patterns to produce

tissues and organs that grow in similar ways to produce homologous structures.

Comparative embryology:

Darwin argued that living things have been evolving on Earth for millions of years.

Evidence for this process could be found in the fossil record, the geographical distribution of

living species, homologous structures of living organisms, and similarities in early

development, or embryology.

146

Chapter 16 – Evolution of Populations

16-1 Genes and Variation

How Common is Genetic Variation?

We know that many genes have at least two forms, or alleles, and that organisms have

additional genetic variation that is “invisible” because it involves small differences in

biochemical processes. In addition, individuals are heterozygous for many genes.

Variation and Gene Pools

Genetic variation is studied in populations, a group of individuals of the same specie

that interbreed. A gene pool consists of all genes, including all the different alleles, that are

present in a population. Gene pools are important to evolutionary theory because evolution

involves changes in populations over time.

In genetic terms, evolution is any change in the relative frequency of alleles in a

population.

Sources of Genetic Variation

A mutation is any change in a sequence of DNA. They can be caused by mistakes in

DNA replication, or as a result of exposure to radiation or chemicals in the environment.

Some mutations do not affect phenotype but many do and can affect an organism’s fitness.

Most heritable differences are due to gene shuffling that occurs during the production

of gametes; homologous pairs move independently during meiosis. Crossing-over, which

also occurs during meiosis, increases the number of different genotypes that can appear in

offspring. Sexual reproduction can produce many different phenotypes ut it does not change

the relative frequency of alleles in a population.

The two main sources of genetic variation are mutations and the genetic shuffling that

results from sexual reproduction.

Single-Gene and Polygenic Traits

A single-gene trait is controlled by a single gene that has two alleles. As a result,

variation in this gene leads to only two distinct phenotypes. A polygenic trait is controlled

by two or more genes. As a result, one polygenic trait can have many possible genotypes and

phenotypes.

The number of phenotypes produced for a given trait depends on how many genes control

the trait.

16-2 Evolution as Genetic Change

Natural selection never acts directly on genes because it is the entire organism that

surivives and reproduces or dies without reproducing. It is populations, not individual

organisms, that can evolve over time.

147

Natural Selection on Single-Gene Traits

Natural selection on single-gene traits can lead to changes in allele frequencies. For

example, organisms of one color may produce fewer offspring than organisms of other

colors.

Natural selection on single-gene traits can lead to changes in allele frequencies and thus to

evolution.

Natural Selection on Polygenic Traits

When traits are controlled by more than one gene, the effects of natural selection are

more complex and the range of phenotypes often fits a bell curve. When fitness varies,

natural selection can affect phenotypes in three ways: directional, stabilizing, and disruptive

selection.

Directional selection takes place when individuals at one end of the curve have a

higher fitness than individuals in the middle or at the other end. The range of phenotypes

shifts. For example, in a population of seed-eating birds, when the supply of small seeds

runs low, the beak size of the population shifts towards larger beaks since they can easily eat

larger seeds.

Stabilizing selection takes place when individuals near the center of the curve have

higher fitness than individuals at either end of the curve. This keeps the center of the curve

at its current position but narrows the overall curve. For example, human babies born at an

average weight are more likely to survive than babies born either much smaller or much

larger than average.

Disruptive selection takes place when individuals at the upper and lower ends of the

curve have higher fitness than individuals near the middle. In such situations, selection acts

against the intermediate and can cause the curve to split into two distinct phenotypes. For

example, in a population of seed-eating birds, when the supply of average-size seeds runs

low and larger and smaller seeds become more common, the population splits into two

subgroups that eat different-size seeds.

Modes of selection:

148

Natural selection can affect the distributions of phenotypes in any of three ways:

directional selection, stabilizing selection, or disruptive selection.

Genetic Drift

An allele can become more or less common by chance. The smaller a population is,

the farther the results may be from what laws of probability predict. A random change in

allele frequency is called genetic drift. It may occur when a small group of individuals, who

may have alleles in different frequencies than the larger original population, colonizes a new

habitat. A situation in which allele frequencies change as a result of the migration of a small

subgroup of a population is known as the founder effect.

In small populations, individuals that carry a particular allele may leave more descendants

than other individuals, just by chance. Over time, a series of chance occurrences of this type

can cause an allele to become common in a population.

Bottleneck effect:

Evolution Versus Genetic Equilibrium

The Hardy-Weinberg principle states that allel frequencies in a population will

remain constant unless one or more factors cause those frequencies to change. Genetic

equilibrium is the situation in which allele frequencies remain constant; if this occurs the

population will not evolve.

Five conditions are required to maintain equilibrium. Random mating ensures that

each individual has an equal chance to mate. A large population is important because genetic

drift has less effect. The gene pool must be kept together so individuals cannot leave and no

new alleles can be brought in. If genes mutate, new alleles could be introduced. No

phenotype can have a selective advantage over another so there can be no natural selection

operating on the population.

Five conditions are required to maintain genetic equilibrium from generation to

generation: (1) There must be random mating; (2) the population must be very large; and (3)

there can be no movement into or out of the population, (4) no mutations, and (5) no natural

selection.

149

16-3 The Process of Speciation

Speciation is the formation of a new species. A species is defined as a group of

organisms that breed with one another and produce fertile offspring.

Isolating Mechanisms

The gene pools of two populations must become separated for them to become new

species. When members of two populations cannot interbreed and produce fertile offspring,

reproductive isolation has occurred.

Behavioral isolation occurs when two populations are capable of interbreeding but

have differences in courtship rituals or other reproductive strategies that involve behavior.

With geographic isolation, two populations are separated by geographic barriers such as

rivers, mountains, or bodies of water. Geographic barriers do not guarantee speciation; if

two formerly separated populations can still interbreed, they remain a single species.

Temporal isolation is a form of isolation in which two or more species reproduce at different

times.

As new species evolve, populations become reproductively isolated from each other.

Barriers:

Testing Natural Selection in Nature

Darwin hypothesized that all of the finches he observed on the Galápagos Islands had

descended from a common ancestor. Peter and Rosemary Grant worked with the finches to

test Darwin’s hypothesis which relied on two testable assumptions: in order for beak size and

shape to evolvem there must be enough heritabe variation in those traits to provide raw

material for natural selection; differences in beak size and shape must produce differences in

fitness that cause natural selection to occur. Their work shows that natural selection takes

place frequently and sometimes very rapidly.

Speciation in Darwin’s Finches

We can devise a hypothetical scenario for the evolution of all Galápagos finches from

a single group of founding birds. First, founders arrived at one of the Galápagos Islands from

the mainland (species A). Geographic isolation occurred when some of these birds went to

another island and no longer shared a common gene pool. Over time, the populations

became adapted to their own environments and there were changes in the gene pool forming

a separate species (species B). If birds from the second island return to the first island and

are no longer able to interbreed, reproductive isolation has occurred; the gene pools of each

150

population remain isolated. As the two species live together on the same island, ecological

competition may occur causing the species to evolve in way that increases differences

between them (species B evolves into species C). Over many generations, this process of

isolation on different islands, genetic change, and reproductive isolation probably repeated

itself across the entire chain of islands which eventually led to the 13 different species found

there today.

Speciation in the Galápagos finches occurred by founding of a new population,

geographic isolation, changes in the new population’s gene pool, reproductive isolation, and

ecological competition.

Adaptive radiation:

Studying Evolution Since Darwin

Many new discoveries have led to new hypotheses that refine and expand Darwin’s

original ideas and many unanswered questions remain. Understanding evolution is important

to understand changes in the living world such as drug resistance in bacteria and viruses, and

pesticide resistance in insects.

151

Chapter 17 – The History of Life

17-1 The Fossil Record

Fossils and Ancient Life

Paleontologists are scientists who study fossils. The fossil record contains

information about past life, including the structure of organisms, what they ate, what ate

them, in what environment they lived, and the order in which they lived. The fossil record

reveals that fossils occur in a particular order and that life on Earth has changed over time.

More than 99 percent of all species that have every lived on Earth have become extinct,

which means the species died out.

The fossil record provides evidence about the history of life on Earth. It also shows how

different groups of organisms, including species, have changed over time.

Fossils:

How Fossils Form

For a fossil to form, either the remains of the organism or some trace of its presence

must be preserved; this depends on a precise combination of conditions which means the

fossil record provides an incomplete history of life. Most fossils form in sedimentary rock.

Interpreting Fossil Evidence

When paleontologists study a fossil, they look for anatomical similarities and

differences between the fossil and living organisms. They can determine the age of fossils

using two techniques: relative dating and radioactive dating.

In relative dating, the age of a fossil is determined by comparing its placement with

that of fossils in other layers of rock. Rock layers form in order by age with the oldest layers

on the bottom, and more recent layers on top. An index fossil is a distinctive fossil used to

compare the relative ages of fossils; it must be easily recognized and must have existed for a

short period of time but have a wide geographic range.

Radioactive decay is used to find the absolute age, or age in years, of rocks using a

half-life. A half-life is the length of time required for half of the radioactive atoms in a

152

sample to decay. Radioactive dating is the use of half-lives to determine the age of a sample.

Different elements have different half-lives.

Relative dating allows paleontologists to estimate a fossil’s age compared with that of

other fossils.

In radioactive dating, scientists calculate the age of a sample based on the amount of

remaining radioactive isotopes it contains.

Relative dating:

Absolute (radioactive) dating:

153

Geologic record:

154

Geologic Time Scale

The geologic time scale is used to represent evolutionary time. Studying the fossil

record revealed major changes in the fossil animals and plants at specific layers in rock; these

times were used to mark where one segment ends and the other begins. Radioactive dating

showed that geologic divisions vary in duration by many millions of years. Geologic time

begins with the Precambrian which covers 88 percent of Earth’s history.

The time between the Precambrian and the present is divided into three eras: the

Paleozoic, Mesozoic, and Cenozoic. The Paleozoic began 544 million years ago (mya) and

lasted close to 300 million years; many vertebrates and invertebrates lived during this time.

The Mesozoic began 245 mya and lasted about 180 million years; it is often called the Age of

Dinosaurs. The Cenozoic began 65 mya and continues today; it is often called the Age of

155

Mammals. Eras are subdivided into periods which could be tens of millions of years to less

than two million years in length.

After Precambrian Time, the basic divisions of the geologic time scale are eras and

periods.

17-2 Earth’s Early History

Formation of Earth

Geologic evidence shows the Earth is about 4.6 billion years old. Early Earth was

much hotter than it is now and there was little or no oxygen present. About 4 billion years

ago the Earth cooled enough to allow the first solid rocks to form on the surface and for

millions of years after, violent volcanic activity shook the Earth. About 3.8 billion years ago,

Earth was cool enough for water to remain in liquid form.

Earth’s early atmosphere probably contained hydrogen cyanide, carbon dioxide, carbon

monoxide, nitrogen, hydrogen sulfide, and water.

The First Organic Molecules

In the 1950s, Stanley Miller and Harold Urey tried to figure out if organic molecules

could have evolved under the conditions of early Earth. They filled a flask with a mixture of

hydrogen, methane, ammonia, and water to represent the early atmosphere and shocked it

with electricity to stimulate lightning. Within a few days, amino acids began to accumulate.

Miller and Urey’s experiments suggested how mixtures of the organic compounds

necessary for life could have arisen from simpler compounds present on a primitive Earth.

Miller and Urey’s experiment:

The Puzzle of Life’s Origins

156

The leap from nonlife to life is the greatest gap in scientific hypotheses of Earth’s

early history. Geological evidence suggests that 200 to 300 million years after Earth cooled,

the first cells came about.

Large organic molecules can form tiny bubbles called proteinoid microspheres which

are not cells but share some characteristics with living things. They have selectively

permeable membranes and simple means of storing and releasing energy. Scientists still

have not been able to figure out the origin of DNA and RNA. Some experiments have

suggested that small sequences of RNA could have formed and replicated on their own.

From the simple RNA-based form of life, several steps could have led to the DNA-directed

protein synthesis that exists now.

Origin of the first cells:

Free Oxygen

Microfossils are microscopic fossils of single-celled prokaryotes that resemble

modern bacteria; they have been found in rock 3.5 billion years old, at a time when there was

little oxygen present. By 2.2 billion years ago (bya), photosynthetic organisms were

producing oxygen; this caused oceans to rust and the atmospheric concentrations of oxygen

to rise.

157

The rise of oxygen in the atmosphere drove some life forms to extinction, while other life

forms evolved new, more efficient metabolic pathways that used oxygen for respiration.

Origin of Eukaryotic Cells

About 2 bya, prokaryotic cells began evolving internal cell membranes; the result was

the ancestor of eukaryotic cells. Other prokaryotic organisms entered this ancestral

eukaryote but instead of infecting the cell or becoming prey to the cell, it lived inside the

larger cell. According to endosymbiotic theory, eukaryotic cells formed from a symbiosis

among several prokaryotic organisms.

Although this hypothesis was proposed over a century ago when scientists realized

that membranes of mitochondria and chloroplasts resembled plasma membranes of free-

living prokaryotes, it was not given much support until the 1960s when Lynn Margulis and

her supporters provided several pieces of evidence. They found that mitochondria and

chloroplasts have DNA similar to bacterial DNA, ribosomes that resemble those of bacteria,

and reproduce by binary fission like bacteria do.

The endosymbiotic theory proposes that eukaryotic cells arose from living communities

formed by prokaryotic organisms.

Sexual Reproduction and Multicellularity

Most prokaryotes reproduce asexually which is efficient but produces identical

daughter cells. This form of reproduction restricts variation to mutations in DNA whereas

sexual reproduction increases variability and increases the chances of evolutionary change.

A few hundred million years after the evolution of sexual reproduction, multicellular

life forms began to develop from single-celled organisms.

17-3 Evolution of Multicellular Life

Precambrian Time

Almost 90 percent of Earth’s history occurred during the Precambrian. Simple

anaerobic organisms were followed by photosynthetic forms of life that added oxygen to the

atmosphere. Aerobic forms of life evolved and then eukaryotes appeared. Some of these

gave rise to multicellular forms.

Paleozoic Era

Life was highly diverse by the first part of the Paleozoic Era, the Cambrian Period; it

is referred to as the “Cambrian Explosion”. The first known representatives of most animal

phyla evolved. Invertebrates, brachiopods, and trilobites were common. During the

Ordovician and Silurian, ancestors of octopi and squid appeared. Arthropods became the

first animals to live on land. The first vertebrates were jawless fishes and the first land plants

evolved. During the Devonian, often called the Age of Fishes, invertebrates and vertebrates

thrived. Some early four-legged vertebrates evolved into the first amphibians. During the

Carboniferous, remains of ancient plants formed thick deposits of sediment that changed into

coal over millions of years. At the end of the Paleozoic, many organisms died out in the

Permian mass extinction.

158

Rich fossil evidence shows that early in the Paleozoic Era, there was a diversity of marine

life.

During the Devonian, vertebrates began to invade the land.

The mass extinction at the end of the Paleozoic affected both plants and animals on land

and in the seas. As much as 95 percent of the complex life in the oceans disappeared.

Mesozoic Era

The Mesozoic Era, called the Age of Reptiles, lasted about 180 million years.

Organisms that survived the Permian mass extinction became the main forms of life in the

Triassic Period. The first dinosaurs and early mammals appeared. During the Jurassic

Period, dinosaurs ruled the Earth. One of the first birds, Archaeopteryx, appeared. Reptiles

were still dominant during the Cretaceous Period which also brought new forms of life such

as leafy trees, shrubs, and small flowering plants. At the end of the Cretaceous, another mass

extinction occurred during which half of all plant and animal groups, and all dinosaurs

became extinct.

Events during the Mesozoic include the increasing dominance of dinosaurs. The

Mesozoic is marked by the appearance of flowering plants.

Cenozoic Era

The extinction of dinosaurs created a different world in which mammals evolved.

The Cenozoic Era is called the Age of Mammals. During the Tertiary Period, the climate

was warm and mild. Insects and flowering plants flourished and grazing mammals evolved.

During the Quaternary Period, Earth’s climate cooled producing a series of ice ages and

causing ocean levels to drop. Over thousands of years the Earth warmed, glaciers melted,

and sea levels rose. The fossil record suggests that the early ancestors of our species

appeared 4.5 mya; the first fossils of our species, Homo sapiens, appeared as early as

200,000 years ago in Africa.

During the Cenozoic, mammals evolved adaptations that allowed them to live in various

environments – on land, in water, and even in the air.

17-4 Patterns of Evolution

Macroevolution refers to large-scale evolutionary patterns and processes that occur

over long periods of time.

Extinction

More than 99 percent of all species that have ever lived are now extinct. Several

times in Earth’s history, mass extinctions wiped out entire ecosystems. Many paleontologists

believe that mass extinctions were caused by several factors such as volcanic eruptions,

changing sea levels, continental movement, and asteroid impacts.

159

Adaptive Radiation

Adaptive radiation is the process by which a single species or small group of species

evolves into several different forms that live in different ways. For example, with Darwin’s

finches more than a dozen species evolved from one species. Dinosaurs underwent an

adaptive radiation and ruled the Earth for about 150 million years; the extinction of dinosaurs

allowed mammals to undergo adaptive radiation.

Convergent Evolution

Convergent evolution is the process by which unrelated organisms independently

evolve similarities when adapting to similar environments. Groups of different organisms

can undergo adaptive radiation in different places or at different times but in ecologically

similar environments; since they face the same demands, natural selection works on them in

similar ways. For example, fish, aquatic mammals, and simming bords have streamlined

bodies and swimming appendages that look alike. Analgus structures look and function

similarly but are made of parts that do not share common ancestry. For example, dolphin

flukes resemble fish tail fins.

Coevolution

Coevolution is the process by which two species evolve in response to changes in

each other over time. Some organisms that are closely connected to one another by

ecological interactions evolve together; an evolutionary change in one may be followed by a

corresponding change in the other. For example, many plants have evolved poisons that

prevent insects from feeding on them.

Punctuated Equilibrium

Darwin felt that biological change needed to be slow and steady, an idea known as

gradualism. The fossil record confirms that populations did change gradually but there is

evidence that this pattern does not always hold. Some groups of organisms evolved rapidly

following mass extinctions. Punctuated equilibrium is a pattern of evolution in which long

stable periods are interrupted by brief periods of more rapid change.

Developmental Genes and Body Plans

Small changes in the activity of control genes can affect many other genes to produce

large changes in adult animals.

Six important topics in macroevolution are extinction, adaptive radiation, convergent

evolution, coevolution, punctuated equilibrium, and changes in developmental genes.

160

Chapter 3 – The Biosphere

3-1 What is Ecology?

Interactions and Interdependence

Ecology is the study of interactions among organisms and between organisms and

their environment, or surroundings. The biosphere is the part of Earth in which life exists

including land, water, and air or atmosphere. It extends about 8 kilometers above Earth’s

surface to 11 kilometers below the surface of the ocean.

Levels of Organization

The study of ecology ranges from the study of an individual organism to populations,

communities, ecosystems, biome, and the biosphere. A species is a group of organisms so

similar to one another that they can breed and produce fertile offspring. Populations are

groups of individuals of the same species that live in the same area. Communities are

assemblages of different populations that live together in a define area. An ecosystem is a

collection of all the organisms that live in a particular place, together with their nonliving,

physical environment. A biome is a group of ecosystems that have the same climate and

similar dominant communities. The highest level of organization is the biosphere.

To understand relationships within the biosphere, ecologists ask questions about the

events and organisms that range in complexity from a single individual to the entire

biosphere.

Levels of biological organization:

161

Ecological Methods

Observing is often the first step in asking ecological questions. Experiments can be

used to test hypotheses. Many ecological phenomena occur over long periods of time or on

such large spatial scales that they are difficult to study so ecologists make models, many of

which consist of mathematical formulas.

Regardless of the tools they use, scientists conduct modern ecological research using three

basic approaches: observing, experimenting, and modeling. All of these approaches rely on

the application of scientific methods to guide ecological inquiry.

3-2 Energy Flow

Producers

The sun is the ultimate source of energy for life on Earth. Of all the energy that

reaches Earth’s surface, less than 1 percent is used by living things. In some ecosystems,

organisms obtain energy from a source other than sunlight; some use energy stored in

inorganic compounds such as in mineral water underground, hot springs, or undersea vents.

An autotroph is an organism that can capture energy from sunlight or chemicals and

use it to produce its own food from inorganic compounds. Autotrophs can produce complex

organic substances from inorganic compounds which is why they are also called producers.

Examples: plants, algae, and certain types of bacteria.

During photosynthesis, autotrophs use light energy to power chemical reactions that

convert carbon dioxide and water into oxygen and carbohydrates. Chemosynthesis is the

process by which organisms use chemical energy to produce carbohydrates.

Sunlight is the main energy source for life on Earth.

Some types of organisms rely on the energy stored in inorganic chemical compounds.

Consumers

A heterotroph is an organism that obtains energy from the foods it consumes so they

are also called consumers. Herbivores eat only plants. Examples: caterpillars and deer.

Carnivores eat only animals. Examples: snakes and dogs. Omnivores eat both plants and

animals. Examples: humans and bears. Detrivores feed on plant and animal remains and

other dead matter, collectively called detritus. Examples: earthworms and snails.

Decomposers break down organic matter. Examples: bacteria and fungi.

Feeding Relationships

Energy moves along a one-way path. Energy stored by producers can be passed

through an ecosystem along a food chain, a series of steps in which organisms transfer

energy by eating and being eaten. A food web is a network of complex interactions formed

by the feeding relationships among the various organisms in an ecosystem. Each step in a

food chain or food web is called a trophic level.

162

Energy flows through an ecosystem in one direction, from the sun or inorganic

compounds to autotrophs (producers) and then to various heterotrophs (consumers).

Food chains:

Food web:

Ecological Pyramids

163

An ecological pyramid is a diagram that shows the relative amounts of energy or

matter contained within each trophic level. There are three different types of ecological

pyramids: energy pyramid, biomass pyramid, and pyramid of numbers.

An energy pyramid shows the relative amount of energy available at each trophic

level. Only part of the energy stored in one level is passed to the next because organisms use

energy for life processes and release some as heat. A biomass pyramid represents the amount

of living organic matter at each trophic level. Biomass is the total amount of living tissue

within a given trophic level. A pyramid of numbers shows the relative number of individual

organisms at each level.

Only about 10 percent of the energy available within one trophic level is transferred to

organisms at the next trophic level.

Energy balances:

Ecological pyramid:

164

3-3 Cycle of Matter

Recycling in the Biosphere

Energy and matter move through the biosphere differently. Although energy has a

one-way flow, matter can be recycled within and between ecosystems. A biogeochemical

cycle is a process in which elements, chemical compounds, and othe forms of matter are

passed from one organism to another and from one part of the biosphere to another.

Unlike the one-way flow of energy, matter is recycled within and between ecosystems.

The Water Cycle

All living things need water to survive. Many processes are involved in the water

cycle. Scientists estimate that it takes 4000 years for a single water molecule to complete

one cycle. Evaporation is the process by which water changes from liquid to atmospheric

gas. Transpiration is the process by which water enters the atmosphere by evaporating from

leaves of plants. Water vapor condenses into droplets to form clouds. When droplets get

large enough, water returns to Earth’s surface through precipitation as rain, snow, sleet, or

hail. Precipitation can be carried to oceans or lakes in runoff or it can seep into the ground to

become groundwater or be taken up by roots of plants.

Water cycle:

165

Nutrient Cycles

Nutrients are all the chemical substances that an organism needs to sustain life. The

carbon cycle, nitrogen cycle, and phosphorus cycle are especially important.

Carbon is found in the atmosphere as carbon dioxide gas; in the oceans as dissolved

carbon dioxide; on land in organisms, rocks, and soil; and underground as coal, petroleum,

and calcium carbonate rock. Carbon dioxide is taken in by plants during photosynthesis and

released by animals and plants during respiration. Four main processes move carbon through

its cycle: biological processes, such as photosynthesis, respiration, and decomposition, take

up and release carbon and oxygen; geochemical processes, such as erosion and volcanic

activity, release carbon dioxide to the atmosphere and oceans; mixed biogeochemical

processes, such as burial and decomposition of dead organisms and conversion into fossil

fuels, store carbon underground; human activities, such as mining, cutting and burning

forests, and burning fossil fuels, release carbon dioxide into the atmosphere.

Nitrogen is needed to make amino acids and proteins. Nitrogen fixation is the

process in which organisms such as bacteria convert nitrogen gas into ammonia.

Denitrification is the process in which organisms like soil bacteria convert nitrates into

nitrogen gas.

Phosphorus is essential to living things because it is needed for DNA and RNA. It is

not very common in the biosphere; it never enters the atmosphere and it mostly found on land

in rock and soil minerals and in ocean sediments.

Carbon cycle:

166

167

Nitrogen cycle:

Phosphorus cycle:

168

Every living organism needs nutrients to build tissues and carry out essential life

functions. Life water, nutrients are passed between organisms and the environment through

biogeochemical cycles.

Nutrient Limitation

Primary productivity is the rate at which organic matter is created by producers.

When an ecosystem is limted by a single nutrient that is scarce or cycles very slowly it is

called a limiting nutrient. When aquatic ecosystems receive large inputs of a limiting

nutrient, there is often an immediate increase in algae and other producers, called an algal

bloom.

169

Chapter 4 – Ecosystems and Communities

4-1 The Role of Climate

What is Climate?

Weather is the day-to-day condition of Earth’s atmosphere at a particular time and

place. Climate refers to the average, year-after-year conditions of temperature and

precipitation in a particular region.

The Greenhouse Effect

The atmosphere is like an insulating blanket due to the greenhouse effect, which is a

natural situation in which heat is retained in Earth’s atmosphere by carbon dioxide, methane,

water vapor, and other gases. These gases allow solar radiation to enter the biosphere but

slow down the loss of heat to space.

Carbon dioxide, methane, water vapor, and a few other atmospheric gases trap heat

energy and maintain Earth’s temperature range.

Greenhouse effect:

The Effect of Latitude on Climate

Solar radiation strike different parts of Earth’s surface at an angle that varies

throughout the year because Earth is a sphere tilted on its axis. Polar zones are cold areas

where the sun’s rays strike at very low angles; they are near the poles between 66.5° and 90°

North and South. Temperate zones sit between the polar zones and the tropics. They are

more affected by the changing angle of the sun over the year so climate ranges from hot to

cold based on the season. Tropical zones, or the tropics, receive direct or nearly direct

sunlight all year; they are near the equator between 23.5° North and 23.5° South.

170

As a result of differences in latitude and thus the angle of heating, Earth has three main

climate zones: polar, temperate, and tropical.

Distribution of solar energy:

Heat Transport in the Biosphere

Unequal heating of Earth’s surface drives winds and ocean currents which transport

heat throughout the biosphere. Warm air tends to rise and cold air tends to sink; upward

movement of warm air and downward movement of cold air creates air currents. Prevailing

winds bring warm or cold air to a region, affecting its climate. Surface ocean currents warm

or cool the air above them which affects the weather and climate of nearby landmasses.

4-2 What Shapes an Ecosystem?

Biotic and Abiotic Factors

Biotic factors are influences on organisms within an ecosystem; they include the

entire living set of organisms. Abiotic factors are the physical, or nonliving, factors that

shape an ecosystem, such as temperature, precipitation, and humidity. A habitat is the area

where an organism lives and includes both biotic and abiotic factors.

Together, biotic and abiotic factors determine the survival and growth of an organism and

the productivity of the ecosystem in which the organism lives.

The Niche

A niche is the full range of physical and biological conditions in which an organism

lives and the way the organism uses those conditions. A niche includes the type of food the

organism eats, how it obtains this food, and which other species use the organisms as food; it

includes the physical conditions as well as when and how the organism reproduces. No two

species can share the same niche in the same habitat.

171

Community Interactions

When organisms live together in communities, they interact constantly; these

interactions shape the ecosystem.

Competition occurs when organisms of the same or different species attempt to use an

ecological resource in the same place at the same time. A resource is any necessity of life

such as water, nutrients, light, food, or space. The competitive exclusion principle states that

no two species can occupy the same niche in the same habitat at the same time.

Predation is an interaction in which one organism captures and feeds on another

organism. The predator does the killing; the prey is the food organism. For example, a hawk

would be a predator and a rabbit could be the prey.

Symbiosis is any relationship in which two species live closely together. In

mutualism, both species benefit from the relationship. For example, insects help flowers

reproduce and the flowers provide food for the insects. In commensalism, one member

benefits and the other is neither helped nore harmed. For example, lions and vultures have a

commensalistic relationship; the lion kills and eats its prey and the vulture swoops in to

finish what was leftover, without having to hunt for prey. In parasitism, one organism lives

on or inside another and harms it. The parasite obtains its nutrients from the host, weakening

but not killing it. For example, fleas, ticks, and lice are parasites.

Community interactions, such as competition, predation, and various forms of symbiosis,

can powerfully affect an ecosystem.

Ecological Succession

Ecological succession is a series of predictable changes that occurs in a community

over time. It can be the result of slow changes in the physical environment or a sudden

disturbance in nature or from human activity.

Primary succession occurs on surfaces where no soil exists such as after a volcanic

eruption covers land or when glacier melting exposes bare rock. The first species to populate

the area are called pioneer species.

Secondary succession is a succession following a disturbance that destroys a

community without destroying the soil. For example, wildfires can burn forests but

community interactions can restore the ecosystem.

Secondary succession:

172

Ecosystems are constantly changing in response to natural and human disturbances. As

an ecosystem changes, older inhabitants gradually die out and new organisms move in,

causing further changes in the community.

4-3 Biomes

A biome is a complex of terrestrial communities that covers a large area and is

characterized by certain soil and climate conditions and particular assemblages of plants and

animals. Tolerance is the ability to survive and reproduce under conditions that differ from

their optimal conditions.

Biomes and Climate

Climate is an important factor in determining which organisms can survive in a

region. A microclimate is climate in a small area that differs from the climate around it.

Climate and biomes:

173

The Major Biomes

Ecologists recognize at least 10 different biomes.

Tropical rain forests have more species than all other biomes combined. Leafy tops

of tall trees form a dense covering called a canopy. The understory is in the shade

below the canopy. It is hot and wet year round and the soil is nutrient-poor.

Tropical dry forests have deciduous trees that shed their leaves to conserve water in

the dry season. It is generally warm with alternating wet and dry seasons.

Tropical savannas, also called grasslands, are characterized by a cover of grasses.

They have warm temperatures, seasonal rainfall, and frequent fires.

Deserts are dry with less than 25 centimeters of rain a year. They have variable

temperatures and soils rich in minerals but poor in organic matter.

Temperate grasslands have a rich mix of grasses and the most fertile soil. They have

warm to hot summers, cold winters, and moderate, seasonal precipitation.

Temperate woodland and shrubland has a semiarid climate and a mix of shrub

communities. They have hot, dry summers and cool, moist winters.

Temperate forests have a mixture of deciduous and coniferous trees, which are cone-

bearing with leaves shaped like needles. They have precipitation year round, warm

summers, and cold to moderate winters.

Northwestern coniferous forests have mild temperatures, abundant precipitation in the

fall, winter, and spring, and a cool, dry summer. They have a variety of conifers and

hemlock trees and rocky, acidic soil.

Boreal forests, or taigas, are dense evergreen forests of coniferous trees with long,

cold winters and short, mild summers. They have moderate precipitation and high

humidity.

Tundra is characterized by permafrost, a layer or permanently frozen subsoil. They

have low precipitation, short and soggy summers, and long, cold, dark winters.

174

The world’s major biomes include tropical rain forest, tropical dry forest, tropical

savanna, desert, temperate grassland, temperate woodland and shrubland, temperate forest,

northwestern coniferous forest, boreal forest, and tundra. Each of these biomes is defined by

a unique set of abiotic factors – particularly climate – and a characteristic assemblage of

plants and animals.

Biomes:

Tropical rain forest Tropical dry forest

Tropical savannah/grassland Desert

175

Temperate grassland Temperate woodland and shrubland

Temperate forest Northwestern coniferous forest

Boreal forest/taiga Tundra

176

Other Land Areas

Mountain ranges can be found on all continents. Abiotic and biotic conditions vary

with elevation; as you move up the mountain temperatures become colder and precipitation

increases. Polar ice caps, which border the tundra, are cold year-round. There are few plants

such as mosses and lichens.

Distribution of terrestrial biomes:

4-4 Aquatic Ecosystems

Nearly three fourth of Earth’s surface is covered with water so many organisms make

their homes in aquatic habitats such as in oceans, streams, lakes, and marshes. Aquatic

ecosystems are often grouped by the abiotic factors that affect them.

Aquatic ecosystems are determined primarily by the depth, flow, temperature, and

chemistry of the overlying water.

Freshwater Ecosystems

Only 3 percent of the surface water on Earth is fresh water. Flowing-water

ecosystems such as rivers, streams, creeks, and brooks are all freshwater ecosystems that

flow over land. They often originate in mountains or hills, often springing from an

underground water source.

Lakes and ponds are the most common standing-water ecosystems; water flows in

and out and is distributed within them. Plankton is the general term for tiny, free-floating

177

organisms that live in both freshwater and saltwater environments. Phytoplankton are

unicellular algae supported by nutrients in the water. Zooplankton are planktonic animals

that feed on phytoplankton.

A wetland is an ecosystem in which water either covers the soil or is present at or

near the surface of the soil for at least part of the year. Freshwater wetlands may be flowing

or standing and fresh, salty, or brackish (a mixture of fresh and salt water). The three main

types of freshwater wetlands are bogs, marshes, and swamps.

Freshwater ecosystems can be divided into two main types: flowing-water ecosystems and

standing-water ecosystems.

Estuaries

Estuaries are wetlands formed where rivers meet the sea which means they have a

mixture of fresh water and salt water, and are affected by the rise and fall of ocean tides.

Most primary production is not consumed by herbivore but instead enters the food web as

detritus, tiny pieces of organic material that provide food for organisms at the base of the

estuary’s food web.

Salt marshes are temperate-zone estuaries dominated by salt-tolerant grasses above

the low-tide line, and by seagrasses under water. Mangrove swamps are coastal wetlands

that are widespread across tropical regions.

Marine Ecosystems

In oceans, photosynthesis is limited to a well-lit upper layer known as the photic zone

which is typically 200 meters deep. Below the photic zone is the aphotic zone which is

permanently dark. Chemosynthetic autotrophs are the only producers that can survive in the

aphotic zone. The benthic zone covers the ocean floor so it is not exclusive to any of the

other marine zones.

The intertidal zone is an area submerged in sea water once or twice a day and exposed

to air, sunlight, and temperature changes the rest of the time. Competition among organisms

leads to zonation, the prominent horizontal banding of organisms that live in a particular

habitat.

The coastal zone extends from the low-tide mark to the outer edge of the continental

shelf, the relatively shallow border that surrounds continents. Kelp forests, named for their

dominant organism, are one of the most productive coastal ocean communities.

Coral reefs are found in the warm, shallow water of tropical coastal oceans; they are

named for the coral animals whose hard, calcium carbonate skeletons make up their primary

structure.

Open ocean, referred to as the oceanic zone, begins at the edge of the continental

shelf and extends outward. It is the largest marine zone, covering more than 90 percent of

the surface area of the world’s oceans yet typically has very low nutrient levels.

The benthic zone is the ocean floor. Benthos are organisms such as sea stars,

anemones, and marine worms that live attached to or near the bottom of the ocean. This zone

extends horizontally from the coastal ocean through the open ocean.

178

In addition to the division between the photic and aphotic zones, marine biologists divide

the ocean into zones based on the depth and distance from shore: the intertidal zone, the

coastal ocean, and the open ocean.

179

Chapter 5 – Populations

5-1 How Populations Grow

Characteristics of Populations

A population can be described by its geographic distribution, density, growth rate,

and age structure. Geographic distribution, or range, describes the area inhabited by a

population. Population density is the number of individuals per unit area.

Three important characteristics of a population are its geographic distribution, density,

and growth rate.

Population Growth

A population will increase or decrease in size based on how many individuals are

added to it or removed from it; population size is affected by birth rate, death rate, and the

number of individuals that enter or leave. Immigration is the movement of individuals into

an area. Emigration is the movement of individuals out of an area.

Three factors can affect population size: the number of births, the number of deaths, and

the number of individuals that enter or leave the population.

Exponential Growth

Exponential growth occurs when the individuals in a population reproduce at a

constant rate. At first there is a slow increase but over time the population gets larger and

larger. In the presence of unlimited resources and in the absence of predation and disease, a

population will grow exponentially creating a J-shape curve.

Under ideal conditions with unlimited resources, a population will grow exponentially.

Exponential growth:

180

Logistic Growth

Logistic growth occurs when a population’s growth slows or stops following a period

of exponential growth as resources become less available creating an S-shape curve.

Carrying capacity is the largest number of individuals that a given environment can support.

As resources become less available, the growth of a population slows or stops.

Logistic growth:

5-2 Limits to Growth

Limiting Factors

A limiting factor is a factor that causes population growth to decrease. Such factors

include competition, predation, parasitism and disease, drought and other climate extremes,

and human disturbances.

Density-Dependent Factors

A density-dependent factor is a limiting factor that depends on population size. These

factors become limiting only when the population density reaches a certain level and operates

most strongly when a population is large and dense. Examples of density-dependent factors

include competition, predation, parasitism, and disease. A predator-prey relationship is a

mechanism of population control in which a population is regulated by predation.

Density-dependent limiting factors include competition, predation, parasitism, and

disease.

Density-Independent Factors

Density-independent factors affect all populations in similar ways, regardless of the

population size. Examples of density-independent factors include natural disasters, unusual

181

weather, and human activities. Many species show a crash in population size in response to

such factors.

Unusual weather, natural disasters, seasonal cycles, and certain human activities – such as

damming rivers and clear-cutting forests – are all examples of density-independent limiting

factors.

5-3 Human Population Growth

Historical Overview

For most of human existence, the population grew slowly; harsh conditions and

limiting factors such as incurable disease and food scarcity kept the population low. About

500 years ago, the human population began to grow more rapidly due to agriculture, industry,

more reliable food supply, improved sanitation, and medicine.

Like the populations of many other living organisms, the size of the human population

tends to increase with time.

Patterns of Population Growth

Demography is the scientific study of human populations; it examines characteristics

of human populations and attempts to explain how those populations will change over time.

Demographic transition is a dramatic change in birth and death rates. In Stage I, both the

birthrate and death rate are high. In Stage II, the death rate drops while birthrate remains

high. In stage III, birthrate also decreases. Age-structure diagrams, or population profiles,

are models used by demographers to predict future growth.

Birthrates, death rates, and the age structure of a population help predict why some

countries have high growth rates while other countries grow more slowly.

Future Population Growth

Current projections suggest that by 2050 the world population may reach more than 9

billion people. Most ecologists sugget that if the growth of the human population does not

slow down, there could be serious damage to the environment as well as the global economy.

182

Chapter 6 – Human in the Biosphere

6-1 A Changing Landscape

Earth as an Island

The Earth is like an island – all organisms share limited resources and rely on natural

ecological processes to sustain these resources.

Human Activities

Humans participate in food webs and chemical cycles like all organisms so we must

be aware that human activities can change local and global environments.

Among human activities that affect the biosphere are hunting and gathering, agriculture,

industry, and urban development.

Hunting and Gathering

For most of human history, our ancestors obtained food by hunting and gathering. It

is believed that the first humans in North America about 12,000 years ago caused a major

mass extinction of mammals.

Agriculture

Agriculture, the practice of farming, began about 11,000 years ago by the end of the

last ice age. The spread of agriculture was among the most important developments in

human history because it provides a dependable supply of food that can be produced in large

quantity and stored for later use.

Farming developed for thousands of years and farmers gradually acquired machinery

to help with cultivation. Agricultural scientists developed new varieties of crops that produce

higher yields and were often grown using monoculture, a practice in which large fields are

planted with a single crop year after year.

The green revolution is the development of highly productive crop strains and the use

of modern agricultural techniques to increase yields of food crops; it has provided many

people with better nutrition. While increasing world food supplies, modern agriculture has

created ecological challenges: large-scale monoculture can lead to problems with insect pests

and diseases; finding enough water for irrigation is also an issue.

Industrial Growth and Development

The Industrial Revolution added machines and factories to civilization during the

1800s. Although it led to many modern conveniences, we need energy to produce and power

these machines; the energy comes mostly from fossil fuels such as coal, oil, and natural gas.

Wastes were discarded into the air, water, and soil and urban centers became crowded

causing the spread of suburban communities across the American landscape.

183

6-2 Renewable and Nonrenewable Resources

Classifying Resources

Renewable resources can regenerate if they are alive or can be replenished by

biochemical cycles if they are nonliving. Nonrenewable resources cannot be replenished by

natural processes. Fossil fuels are nonrenewable resources.

Environmental goods and services may be classified as either renewable or nonrenewable.

Sustainable Development

Sustainable development is a way of using natural resources without depleting them

and of providing for human needs without causing longterm environmental harm. To work

well, sustainable development must take into account both the functioning of ecosystems and

the ways that human economic systems operate.

Human activities can affect the quality and supply of renewable resources such as land,

forests, fisheries, air, and fresh water.

Land Resources

Land is a resource that provides space for human communities and raw materials for

industry, and includes soils in which crops are grown. Plowing the land removes roots that

hold soil in place; it increases the rate of soil erosion, the wearing away of surface soil by

water and wind. Desertification is a process that occurs in dry climates when a combination

of farming, overgrazing, and drought turns once productive land into desert.

Forest Resources

Forests are an important resource for the products they provide and for the ecological

functions they perform. They are called the “lungs of the Earth” because they remove carbon

dioxide and produce oxygen. They also store nutrients, provide habitats and food for

organisms, moderate climate, limit soil erosion, and protect freshwater supplies.

Deforestation, or the loss of forests, can lead to severe soil erosion and permanent

changes to local soils and microclimates. There are a variety of sustainable-development

strategies for forest management such as planting, managing, harvesting, and replanting tree

farms.

Fishery Resources

Fishes and other animals that live in water are a valuable source of food for humanity.

Overfishing, or harvesting fish faster than they can be replaced by reproduction, greatly

reduce the amount of fish in parts of the world’s oceans. The U.S. National Marine Fisheries

Service created guidelines that are helping fish populations recover. Aquaculture, the raising

of aquatic animals for human consumption, is also helping to sustain fish resources.

Air Resources

Air is a commond resource that we use every time we breathe. The condition of air

affects people’s health so preservation of air quality is important. Smog is a mixture of

184

chemicals that occurs as a gray-brown haze in the atmosphere due to automobile exhausts

and industrial emissions. Smog is a pollutant, a harmful material that can enter the biosphere

through the land, air, or water. The burning of fossil fuels and release of potentially toxic

chemicals such as nitrates, sulfates, and particulates can be troublesome. Burning fossil fuels

can release nitrogen and sulfur compounds that fall as acid rain when these compounds

combine with water vapor in the air, forming drops of nitric acid and sulfuric acid.

Freshwater Resources

Americans use billions of liters of fresh water daily for drinking, watering crops, and

making steel. Water is a renewable resource but the total supply of freshwater is limited.

Pollution threatens water supplies in many ways. To ensure sustainable use of water

resources we need to protect the natural systems involved in the water cycle.

6-3 Biodiversity

The Value of Biodiversity

Biodiversity is the sum total of the genetically based variety of al organisms in the

biosphere. Ecosystem diversity includes the variety of habitats, communities, and ecological

processes in the living world. Species diversity refers to the number of different species in

the biosphere. Biologists have named 1.5 million different species but estimate there are

millions more to be discovered. Genetic diversity refers to the sum total of all the different

organisms living on Earth today.

Biodiversity is one of Earth’s greatest natural resources. Species of many kinds have

provided us with foods, industrial products, and medicines – including painkillers,

antibiotics, heart drugs, antidepressants, and anticancer drugs.

Levels of biodiversity:

185

Threats to Biodiversity

Extinction occurs when a species disappears from all or part of its range. An

endangered species is one whose population size is declining in a way that places it in danger

of extinction.

Human activity can reduce biodiversity by altering habitats, hunting species to extinction,

introducing toxic compounds into food webs, and introducing foreign species to new

environments.

Habitat Alteration

Natural habitats can be destroyed when land is developed. Development often splits

ecosystems into pieces in a process called habitat fragmentation, which creates biological

“islands” in which fewer species can live.

Habitat fragmentation:

Demand for Wildlife Products

Throughout history, humans have pushed some animal species to extinction by

hunting them for food or other products. Although endangered species are protected, hunting

in many other countries still threaten rare animals.

Pollution

Many forms of pollution can threaten biodiversity but the worst problems occur when

toxic compounds accumulate in the tissues of organisms. In biological magnification,

concentrations of a harmful substance increases in organisms at higher trophic levels in a

food chain or food web.

In 1962, Rachel Carson alerted people to the dangers of biological magnification in

her book Silent Spring. DDT, a widely used pesticide, is nonbiodegradable and when picked

up by organisms cannot be eliminated from the body. DDT threatened many animals, such

as the osprey, pelican, and bald eagle, with extinction because it made their eggs so fragile

that they could not survive. Although banned in the United States in the 1970s, DDT is still

used in some countries.

186

Types of pollution:

Introduced Species

One of the greatest threats to biodiversity comes from harmless plants and animals

that are either accidentally or intentionally introduced into new environments. These

organisms often become invasive species which reproduce rapidly because the new habitat

lacks parasites and predators that normally keep their population in check.

Kudzu, an introduced species:

Conserving Biodiversity

Conservation is the wise management of natural resources including the preservation

of habitats and wildlife. Many conservation efforts are aimed at managing individual species

to keep them from becoming extinct whereas others focus on protecting entire ecosystems.

Protecting resources for the future can require people to change the way they earn their living

today.

187

Today, conservation efforts focus on protecting entire ecosystems as well as single

species. Protecting an ecosystem will ensure that the natural habitats and the interactions of

many different species are preserved at the same time.

6-4 Charting a Course for the Future

Today, much of Earth’s land surface has been altered by human activity. Scientists’

investigations of the ozone layer and global climate system, and the actions taken as a result,

show how research can have a positive impact on the global environment.

Researchers are gathering data to monitor and evaluate the effects of human activities on

important systems in the biosphere. Two of these systems are the ozone layer high in the

atmosphere and the global climate system.

Ozone Depletion

The ozone layer, an atmospheric layer in which ozone gas is relatively concentrated,

is found between 20 and 50 kilometers above Earth’s surface. At the ground level ozone is a

pollutant but in the ozone layer it absorbs a good deal of harmful ultraviolet radiation from

sunlight.

In the 1970s scientists found a gap, or hole, in the ozone layer over Antarctica and

since its discovery the hole has grown larger and lasted longer. Gases such as

chlorofluorocarbons, or CFCs, could damage the ozone layer. Nations started reducing the

use of CFCs in the 1980s and today most CFCs are banned.

Depletion of the ozone shield:

Global Climate Change

Global warming is an increase in the average temperature on Earth. Since the late

nineteenth century, average atmospheric temperatures on Earth’s surface have risen about

0.6°C. The 1990s were the warmest decade ever recorded.

The geologic record shows that Earth’s climate has changed repeatedly throughout

history. Scientists hypothesize that the current warming of Earth is due in part to human

activities that are adding carbon dioxide and other greenhouse gases to the atmosphere faster

than natural cycles can remove them.

188

Computer models suggest that average global surface temperatures will increase by 1

to 2°C by the year 2050. Sea levels could rise, causing coastal areas to be flooded. Some

areas could experience more droughts during the summer growing season.

Temperature and carbon dioxide levels:

The Value of a Healthy Biosphere

Ecosystems provide many services such as solar energy, production of oxygen,

storage and recycling of nutrients, regulation of climate, purification of water and air, storage

and distribution of fresh water, food production, nursery habitats for wildlife, detoxification

of human and industrial waste, natural pest and disease control, and management of soil

erosion and runoff. The biosphere is strong and humans are clever; both humans and

ecosystems can adapt to change of different kinds.