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Unit 4: Cell Theory

Discovery of cells Cells are far too small to see with the naked eye

Invention of the microscope 1590 – Zacharias and Hans Janssen made one of the

first compound microscopes

1660 – Robert Hooke made a compound microscope Hooke looked at cork and noticed room like

structures he called ‘cells’ He calculated that a square inch of cork contained

about 1 200 000 000 cells Drawing by Rita Greer - The original is a pencil drawing by Rita Greer, history painter, 2006. This was digitized by Rita and sent via email to the Department of Engineering Science, Oxford University, where it was subsequently uploaded to Wikimedia., FAL, https://commons.wikimedia.org/w/index.php?curid=7667256

1660 – Anton van Leeuwenhoek made a simple microscope (only one lens) that could magnify a specimen 266 times

He viewed many samples of water from lakes, gums and gutters finding many small organisms that moved in different ways and named these organisms animalcules (little animals)

Microscope picture: see page for author [CC BY 4.0 (http://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons

Development of cell theory 1838 – Matthias Schleiden

Concluded all plants are made of cells based on his own research 1839 – Theodor Schwann

Concluded all animals are made of cells, therefore all living things are made up of cells 1855 – Rudolf Virchow

Proposed that new cells are formed only from already existing cells

Cell theory

All living things are composed of cells

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

All cells are produced from other cells

Cells are usually very small It is very tricky to count the number of cells in the human body

Not all of us are the same size We contain microbes The number changes as we recycle cells

Best current estimate is 37.2 trillion cells in the human body

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Why are the cells so small? Very small cells are easier to replace without

disruption to the organism Very small cells can specialize which also makes

replacement much easier Mathematics

Surface to volume ratio Compare two cubic cells

Cell 1 – 10 μm (micrometers) on a side Surface area = 10 μm x 10 μm x 6 = 600 μm2

Volume= 10 μm x 10 μm x 10 μm = 1000 μm3 Surface area / Volume = 600 / 1000 = 0.6

Cell 2 – 20 μm on a side Surface area = 20 μm x 20 μm x 6 = 2400 μm2

Volume = 20 μm x 20 μm x 20 μm = 8000 μm3 Surface area / Volume = 2400 / 8000 = 0.3

By BallenaBlanca - https://commons.wikimedia.org/wiki/File:Esquema_del_epitelio_del_intestino_delgado.png, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=48093230

Notice that Cell 1 has twice the surface area to volume ratio and therefor can transport needed materials in the cell and wastes out of the cell twice as fast

Speed of diffusion – this limits cell size because large cells cannot transport things from one part of the cell to another part fast enough to survive

Limiting surface area / volume ratios Some cells that specialize in exchanging materials have many, tiny, finger-like extensions called

microvilli Microvilli (see picture inset of small intestine villi) greatly increase surface area without adding

much to the volume of a cell

Comparing eubacteria and archaebacteria Not too long ago, archaea were classified as bacteria (hence the name archaebacterial) but now we

know that they have a distinct evolutionary history and a much different biochemistry than bacteria

Similarities Both are prokaryotes (mostly single celled but never with a nucleus or membraned organelles) Both have cell walls Both have ribosomes Both have similar shapes – rods, cocci (spherical), and spirals Both have flagella to move Both reproduce asexually through binary fission, budding, and fragmentation

Differences Eubacteria

Cell membrane is ester linked lipid (peptidoglycan)

Flagella are like a hollow stalk subunits move up the central pore and add on at the tip

Can form spores that are dormant for years Are found almost everywhere in normal

environments

Archaebacteria Cell membrane is ether linked lipid (no

peptidoglycan) Flagella add on at the base Do not form spores Are found in harsh conditions like hot vents,

salty areas, and very acidic environments

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Comparing prokaryotes and eukaryotes

Prokaryotes

By This vector image is completely made by Ali Zifan - Own work; used

information from Biology 10e Textbook (chapter 4, Pg: 63) by: Peter Raven, Kenneth Mason, Jonathan Losos, Susan Singer · McGraw-Hill Education., CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=44194140

Similarities Cells Cell membrane Ribosomes (smaller)

Differences No true nucleus (have a nucleoid) No membrane bound organelles Circular DNA – multiple proteins act together

to fold and condense DNA Chlorophyll scattered in the cytoplasm Cell wall is usually chemically complexed

(peptidoglycan) Cell size – usually 1-10 μm

Eukaryotes

By Zaldua I., Equisoain J.J., Zabalza A., Gonzalez E.M., Marzo A., Public University of Navarre - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=46386894

Cells Cell membrane Ribosomes (larger)

True membrane bound nucleus present Membrane bound organelles present

Endoplasmic reticulum Golgi apparatus Lysosomes Peroxisomes Mitochondria

Linear DNA – DNA wraps around proteins called histones

Chlorophyll contained in chloroplasts (only plants have chlorophyll)

Cell wall is chemically simpler (only in plants and fungi – most eukaryotes have no cell wall)

Cell size – usually 10-100 μm

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Parts of a eukaryotic cell Ribosomes – make proteins Endoplasmic reticulum

Rough – associated with ribosomes Smooth – makes lipids

Nucleus Nucleolus – condensed region where

ribosomes are formed Chromatin – DNA plus associated proteins Nuclear envelope – membrane around the

nucleus that has pores that allow materials to move in and out

Golgi body (or apparatus) – modifies proteins Centriole – important part of centrosomes

which are involved in organizing microtubules in the cytoplasm

Lysosomes – digest food Peroxisomes – metabolize wastes (not shown) Mitochondria – produce energy

By Mediran (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

Comparing plant and animal cells

Animal cells Plant cells

Animal cells have centrioles, centrosomes, and lysosomes that plant cells do not have

Plant cells have cell walls, chloroplasts, plasmodesmata, plastids, and a large central vacuole that animal cells do not have

Source: Boundless. “Characteristics of Eukaryotic Cells.” Boundless Biology. Boundless, 26 May. 2016. Retrieved 26 Nov. 2016 from https://www.boundless.com/biology/textbooks/boundless-biology-textbook/cell-structure-4/eukaryotic-cells-60/characteristics-of-eukaryotic-cells-313-11446/

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Multicellular organisms

Advantages of multicellular organisms Longer lifespan An overall larger body size (usually fewer predators and better ability to maintain homeostasis) Cell differentiation (allows more structures, functions, and complexity)

Levels of organization in multicellular organisms Organelles – “little organs” typically, membrane bound structures or compartments in cells that

perform a specific function Examples: nucleus carries instructions for cells, peroxisomes metabolize wastes, and

mitochondria provide energy Cell – the basic structural, functional, and biological unit of living organisms

Often called the building blocks of life Examples: neurons (nerve cells), erythrocytes (red blood cells), epidermal cells (skin cells)

Tissue – groups of cells with a similar structure that work together for a specific function The four types of human tissues are: epithelial, connective, muscular, and nervous

Organ – collection of tissues joined in a structural unit to server a common function Examples: skeleton, muscles, teeth, stomach, intestines, liver, kidneys, lungs, brain, veins,

arteries, spleen, pancreas, spinal nerves, eye, ear, and skin System – groups of structures that perform the broadest functions in an animal

The eleven main types of human systems are: cardiovascular / circulatory, digestive / excretory, endocrine, integumentary / exocrine, lymphatic / immune, muscular / skeletal, nervous, reproductive, renal / urinary, respiratory, and vestibular

Structure vs Function Structure describes what something looks like or its makeup Function describes what a structure does or the job it performs

Example: mitochondrion Structure: a roughly ovoid organelle encased by an outer membrane and having an inner

membrane with many folds called cristae that contains its own DNA Function: energy production

Transport in cells

Diffusion

By JrPol - Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=4586487

Diffusion – the random mixing of substances due to the natural movement of particles

This process will always carry substances from areas of higher concentration to areas of lower concentration

Osmosis

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Osmosis – the process by which solvent molecules tend to pass through a semipermeable membrane from a less concentrated solution to a more concentrated solution

In cells, this process will always carry solvent (water) from areas of higher concentration to areas of lower concentration across the plasma (or cell) membrane

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Conditions for osmosis Osmosis is, basically, diffusion across a semipermeable membrane and requires:

A semipermeable membrane A concentration gradient

Cell walls are phospholipid bilayers that form a liposome

Water (and any charged particles that dissolve in water) cannot pass through the plasma membrane

except through pores (or holes in the membrane

Passive transport The movement of substances across cell membranes by osmosis

Passive transport always moves ions from higher concentrations to lower concentrations (with or down the concentration gradient)

Requires no energy input from the cell Typically, passive transport occurs through ion channels which are

composed of four proteins that form a pore (or hole) through the plasma (or cell) membrane

Ion channels are usually very fast (often a million ions per second or more)

Active transport Forcing the movement of substances across cell membranes against osmosis

Active transport always moves ions from lower concentrations to higher concentrations or up the concentration gradient

Typically, active transport occurs through ion transporters (or ion pumps like the Na+/K+ pump)

Ion pumps require cellular energy from some source (often ATP)

Example: the Na+/K+ pump Moves 3 Na+ ions out of the cell and

2 K+ ions into the cell In a typical cell, 1/5 the metabolic

energy is required for ion pumps

In a neuron, up to 2/3 the metabolic energy is required for ion pumps

By BruceBlaus. Blausen.com staff. "Blausen gallery 2014". Wikiversity Journal of Medicine.

DOI:10.15347/wjm/2014.010. ISSN 20018762. Derivative by Mikael Häggström - File:Blausen_0211_CellMembra

Schematic diagram of an ion channel. 1 - channel

domains (typically four per channel), 2 - outer

vestibule, 3 - selectivity filter, 4 - diameter of

selectivity filter, 5 - phosphorylation site, 6 - cell

membrane.

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Similarities and Differences between Active and Passive Transport

Similarities

Both involve movement of ions across a plasma (or cell) membrane Both require a pore to move ions through the plasma membrane Both are required to maintain proper cell functioning

Differences Passive Active

Moves from high to low concentration Moves from low to high concentration Requires no metabolic energy Requires metabolic energy from ATP Makes use of osmosis through ion channels Uses Na+/K+ pumps (or some ion pump)

Moving large particles in and out of cells Endocytosis – a form of active transport in which cells transport large molecules into the cell by

engulfing them The cell walls expand outward (or inward) and engulf the large particles The substances that enter the cell will be surrounded by membranes (vesicles or vacuoles) There are three types of endocytosis

Phagocytosis transports solid particles Pinocytosis transports particles in liquids Receptor-mediated endocytosis transports particles with specific sites that can bind to a

receptor in the cell membrane The vacuoles that form in this manner will be protein coated vacuoles

Exocytosis – a form of active transport in which cells transport large molecules out of the cell

Proteins that are to be transported out of the cell are surrounded by membranes and are called secretory vesicles

The membrane of the secretory vesicle fuses with the cell membrane then opens to the outside of the cell to push the protein out

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Basal Metabolic Rate (BMR) – the minimal amount of energy the body requires to function at rest BMR is expressed in food calories (kcal) Some resting body processes include breathing, blood circulation, maintaining body temperature,

brain function, nerve function, and muscle contraction (especially the heart), and cell metabolism Cell metabolism refers to all the processes a cell requires for functioning

Catabolism – processes which break larger molecules into smaller ones generally for body use to release energy

Anabolism – processes which assemble smaller molecules into larger ones whereby energy is stored in chemical bonds for later use

The BMR is most accurately determined by experimental measurement for an individual Usually, BMR is estimated by use of a mathematical formula

The Mifflin St Jeor Equation:

BMR = (10.0 𝑚

1 kg+

6.25 ℎ

1 𝑐𝑚−

5.0 𝑎

1 𝑦𝑟+ 𝑠)

𝑘𝑐𝑎𝑙

𝑑𝑎𝑦

Where m = mass in kg = wt lbs x 2.2

h = height in cm = h in x 2.54 a = age in years s = +5 for males and –161 for females

Photosynthesis Photo means “light” and synthesis means “putting together”

During photosynthesis, carbon dioxide (CO2) and water (H2O) are put together by plants using light energy from the sun to form sugars

The chemical that allows plants to do this is called chlorophyll In plant cells, chlorophyll is found in the chloroplasts

Plants break down some of the sugars they make into smaller molecules in order to release the energy they need for their cells to function

Some of the sugars are used to build cellulose Some of the sugars are stored for later use

Organisms that eat plants are using these stored sugars as food Nearly all living things obtain energy either directly or indirectly from the energy of sunlight captured

during photosynthesis

Chemical equations Scientists use chemical symbols and chemical equations as a kind of a shortcut to represent

processes such as photosynthesis Substances that are used in the reaction are called the reactants

Reactants are listed on the left of the equation Substances that are produced by the reaction are called the products

Products are listed on the right side of the equation

The photosynthesis equation:

Reactants Products

6 (CO2) + 6 (H2O) + sunlight → C6H12O6 + 6 O2 carbon dioxide + water + sunlight → glucose + oxygen

where the reactants are carbon dioxide and water and the products are glucose (a sugar) and oxygen as a waste product for the plant

The arrow in the equation is a yields sign and can be read as ‘yields’ or as ‘reacts to produce’ The entire equation would be read as:

Carbon dioxide plus water react to produce glucose plus oxygen.

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Steps of photosynthesis 1. Stage 1 – Capturing the sun's energy

Chlorophyll in the chloroplasts captures sunlight in two systems Photosystem I (PSI)

Sunlight causes chlorophyll to lose an electron (e–) The electron moves down the chloroplast electron transport chain This happens twice producing 2 e–

Photosystem II (PSII) Sunlight causes the splitting of water producing 2 e–, 2 H+ ions, and ½ O2 molecule These 2 e– replace the e– lost by chlorophyll in PSI

2. Stage 2 – Using energy to make food

Calvin cycle CO2 enters the stoma on the underside of leaves and H2O enters from the roots A complex set of reactions uses energy and the electrons from Stage 1, the H2O, and the CO2 to

produce the sugar glucose (C6H12O6) Plants throw out waste O2 formed in PSII through the stoma and use the glucose as food

Cellular respiration Although cellular respiration is often referred to as respiration it should not be confused with

breathing (which is also called respiration) Respiration is the process by which cells obtain energy from glucose

During respiration cells break down simple food molecules such as sugars to release the energy they contain

Respiration occurs in two stages Stage 1 occurs in the cytoplasm of the cells

Glucose is broken down into smaller molecules No oxygen is involved Only a small amount of energy is released

Stage 2 occurs in the mitochondria The small molecules from Stage 1 are broken down into even smaller molecules The chemical reactions in the mitochondria require oxygen A large amount of energy is released (explaining why mitochondria are called powerhouses) Carbon dioxide (CO2) and water (H2O) are also released in respiration

When humans breathe out they release CO2 and H2O

The respiration equation:

C6H12O6 + 6 O2 → 6 (CO2) + 6 (H2O) + energy glucose + oxygen → carbon dioxide + water + energy

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Note that the respiration equation is the reverse of the photosynthesis reaction Because these two reactions are opposite, they form a cycle that helps keep the CO2 and O2 levels

nearly constant on Earth

Fermentation Some single-celled organisms live deep in the ocean, in mud, or in other places where there is no

oxygen Such organisms use fermentation to obtain energy instead of respiration Fermentation – a process that produces energy for cells without using oxygen

The amount of energy released during fermentation is much less than during respiration

Alcoholic fermentation Yeast is an example of an organism that uses alcoholic fermentation

Bakers and brewers use alcoholic fermentation because this method also produces CO2 The CO2 bubbles cause bread to rise and form the bubbles in beer

Lactic acid fermentation When cells in the human body use oxygen faster than it can be replaced, the cells can generate

some energy by a fermentation process that also produces lactic acid Lactic acid causes the painful sensation that results in muscles that feel sore and weak

Similarities and Differences between Respiration and Fermentation

Stages of Respiration and Fermentation Respiration Fermentation

Stage 1: in the cytoplasm Stage 1: in the cytoplasm Glycolysis produces 2 ATP molecules and

2 pyruvate molecules Glycolysis produces 2 ATP molecules and

2 pyruvate molecules Stage 2: in the mitochondria Stage 2: no oxygen, can’t use mitochondria

Krebs cycle makes CO2 and 2 ATP Pyruvate converted to lactic acid Electron transport chain – about 34 ATP Repeat from Stage 1

Total: 38 ATP Total: 2 ATP

Similarities

Both methods produce energy for cells Both produce energy by creating adenosine triphosphate (ATP) Both use glycolysis in the cytoplasm

Differences Respiration Fermentation

Starts in cytoplasm moves to mitochondria Process never moves out of cytoplasm Requires oxygen Proceeds without use of oxygen Produces much more energy (38 ATP) Produces much less energy (2 ATP)

Life cycle of a cell Why cells reproduce

For single celled organisms, reproduction carries on the species For multicellular organisms:

Growth – increase the number of cells (growth of the organism) Maintenance – replace cells that grow old and die Repair – replace damaged cells

Stage 1: Interphase – the period before cell division Growth – cell reaches full size and copies of chloroplasts (plants) and mitochondria are made Replication – DNA is copied so that the cell has two identical sets of DNA Preparation for cell division – structures are made that are used for division

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Stage 2: Mitosis – the period when the cell nucleus divides into two new nuclei Prophase

The threadlike chromatin in the nucleus condenses to form chromosomes Chromosomes are double-rod structures

Chromatid – one single rod in a chromosome The two chromatids are identical because the cell DNA has replicated The chromatids are held together by a structure called a centromere

The pairs of centrioles move to opposite ends of the cell Spindle fibers form a bridge between the opposite ends of the cell The nuclear envelope breaks down

Metaphase Chromosomes line up across the center of the cell

This prepares the chromosomes so they can split with one daughter chromatid to each end Each chromosome attaches to a spindle fiber at its centromere

Anaphase The centromeres split One chromatid is drawn by its spindle fiber to one end of the cell and the other chromatid moves

to the opposite end of the cell drawn by its spindle fiber The cell stretches out as the opposite ends are pushed apart

Telophase The chromosomes unwind, stretch out, and lose their rod-like appearance A new nuclear envelope forms around each region of chromosomes

Stage 3: Cytokinesis – the period when the cell completes the process of division Cytokinesis starts at about the same time as the telophase

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Cytokinesis specifically for animal and plant cells: Animal cells

The cell membrane pinches in around the middle of the cell The cell splits in two Each daughter cell ends up with an identical set of chromosomes and about half the organelles

Plant cells The cell wall cannot pinch together in the middle of the cell Instead, the cell makes a structure called a cell plate which eventually forms into new cell

membranes that separate the two daughter cells A new cell wall forms around the two cell membranes

Time required for the cell cycle to occur in human liver cells (time measured in hours)

Binary fission in prokaryotic cells 1. The DNA replicates 2. Each copy of the DNA attaches to the cell membrane at opposite ends of the cell 3. The cell splits and each end pulls its copy of the DNA into its part of the new cell

Similarities in mitosis and binary fission:

Both are asexual forms of cell reproduction Both replicate DNA into two exact copies Both split cells into two exact copy daughter cells

9

10

2

0.83 0.17

Length of the Cell Cycle

Interphase Growth

Interphase DNA Replication

Interphase Division Prep

Mitosis

Cytokinesis

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Differences in mitosis and binary fission Mitosis Binary Fission

Occurs in eukaryotic cells Occurs in prokaryotic cells Many chromosomes are involved Only one single strand of DNA is involved Proceeds in five complex steps Proceeds in three simple steps