AQA Biology AS Unit 2

7.1 Investigating VariationInterspecific Variation: One species differs from another

Intraspecific Variation: Members of the same species differ• Sampling Bias: Investigators might deliberately or unwittingly choose their area

of work• Chance: Pure chance can ruin representative results• The best way to remove sampling bias is reduce human involvement as much as

possible

Why Sampling Might not be

Representative:

• 1. Divide study area into a grid of numbered lines• 2. Use random numbers from a number generator to obtain coordinates• 3. Take samples at the intersection of each pair of coordinates

Random Sampling

• Large sample size: Reduces the probability of chance influencing results increasing reliability

• Analyse data: Use statistical tests to see how much chance influenced the data

How to Minimise Chance

• Mutations: Cause sudden changes to genes/chromosomes or genes may not have passed on to the next generation

• Meiosis: Genetic information is passed to the gametes by meiosis which gives varied genetic information

• Fusion of Gametes: Characteristics inherited from both parents, it is a random process increasing variety

• Variety in asexual reproducing organisms occurs due to mutation

Genetic Differences Causes

• The environment can influence how genes are expressed• Genes set limits but the environment decides where in those limits an

organism is• Temperature, nutrients, chemicals can all affect the environment• Variation is usually a combination of genetic differences and environmental

influences

Environmental Influences

7.2 Types of Variation• No intermediate values: they are categorical• Characters like blood type are usually only controlled by a

single gene • Data can be represented in a bar chart or pie chart• Environmental factors have little influence to this type of

variation

Due to Genetic Factors

• Characters are not controlled by single gene but many (polygenes)

• This data usually forms a normal distribution curve (bell shaped curve)

• The values can be continuous

Due to Environmental Influences

Mean: measurement at the maximum height of the curve, it provides an average value but doesn't give use information about the range of values

Standard Deviation: measure of the width of the curve, gives indication of the range either side of the mean. 68% lies within +1 standard deviation, 95% is +2 and 99% is +3

• 1. Calculate mean value• 2. Subtract the mean from each measured value• 3. Square all numbers• 4. Add the squared numbers together• 5. Divide this number by the original• 6. Square root the results

Calculate Standard Deviation

8.1 Structure of DNADNA (Deoxyribonucleic acid) is the chemical that determines inherited characteristics and is made up of 3 components

that form a nucleotide

A nucleotide is made up of deoxyribose (sugar), a phosphate group and organic base

The two types of base are:• Single ring bases: Cytosine (C) and Thymine (T)• Double ring bases: Adenine (A) and Guanine (G)

The deoxyribose, phosphate group and organic base are combined due to condensation reactions

A dinucleotide is formed from two mononucleotides and if the mononucleotides continue linking they form a polynucleotide

DNA Structure• Made up of two strands of nucleotides that are joined together by hydrogen bonds between the bases• In a ladder shape the deoxyribose and phosphates for the poles and the bases for the steps

Pairing of Bases• The bases contain nitrogen and are complementary of each other• The double ring structured bases (A and G) have longer molecules than the single ring structured bases (C and T)• A and T are paired by 2 hydrogen bonds, C and G are paired by 3 hydrogen bonds• A and T will always have the same number as each other just like C and G to each other• The DNA forms a double helix in which every turn has 10 bases

Function of DNA• It passes genetic information from cell to cell and generation to generation• There is almost an infinite amount of sequences for bases along the DNA• DNA is stable and difficult to be changed as well as being large carrying much genetic information• The separate strands are able to split apart during protein synthesis and DNA replication due to being held by

hydrogen bonds• Bases protected by the deoxyribose-phosphate backbone from chemical/physical forces

8.2 The Triplet Code

• Genes are sections of DNA that contains information for polypeptide production

• The information has a specific sequence of bases along the DNA molecule• Polypeptides form proteins such as enzymes• Enzymes control chemical reactions so are responsible for organism

development and activities• A Polypeptide is basically a sequence of amino acids

What is a Gene?

• 20 amino acids occur regularly in proteins• Each amino acid has its own code of bases, some have more than one

code• The code is known as the degenerate code due to some amino acids

having more than one code• The code is also non overlapping

The Triplet Code

8.3 DNA and Chromosomes

In eukaryotic cells the DNA molecules is large, linear and occurs in association with proteins to form chromosomes

In prokaryotic cells the DNA molecules are smaller, form a circle and are not associated with protein molecules so they have no chromosomes

Chromosome Structure

• Only visible when a cell is dividing and appear as two threads (each known as a chromatid) joined at a single point (centromere)

• DNA in chromosomes is held in place by proteins• In DNA the helix is wound around proteins, this DNA-protein complex is then coiled• The coil is looped and further coiled to pack into the chromosome• The single molecule of DNA in a chromosome contains many genes that occupy specific positions on the

DNA molecule• The number of chromosomes in species varies however they are always in pairs called homologous pairs

DNA and Chromosomes Cont.

Homologous Chromosomes• In sexually produced organisms half the DNA comes from each parent• One of each pair of chromosomes comes from each parent, they have the same gene loci

and are known as homologous pairs• Cells that contain two sets of chromosomes in the nucleus are known as diploids• A homologous pair possess information for the same thing e.g. eye colour but the

chromosomes may carry different alleles e.g. brown colour or blue colour• During meiosis the halving of chromosomes ensures that each daughter cell receives one

chromosome from each homologous pair• When the haploid cells combine they form a diploid

What is an Allele?• Allele: One of the forms of a gene• You receive one allele from each parent, different alleles code for different polypeptides• Any differences in base sequence of an allele can result in different sequence of amino acids

being coded causing different polypeptide production

8.4 Meiosis and Genetic Variation

Why is Meiosis Necessary?• Meiosis produces four daughter nuclei each with half the number

of chromosomes as the parent cell• In sexual reproduction two gametes fuse to create the offspring

with the full amount of chromosomes needed• Meiosis is needed as by halving the number of chromosomes it

makes sure the offspring with have the right amount of chromosome and it won't have doubled

• During meiosis the chromosome pairs separate so only once chromosome enters the gamete (haploid)

• When two haploids gametes fuse the diploid number is restored

The Process of Meiosis• Meiosis 1: Homologous chromosomes pair up and their chromatids wrap around

each other. Equivalent portions of the chromatids exchanged in crossing over. By the end the homologous pairs have separated with one chromosome from each pair going into one of two daughter cells

• Meiosis 2: Chromatids move apart, 4 cells are formed each with a single chromatid• Meiosis produces genetic variation allowing offspring that can adapt and survive

It involves two nuclear divisions

that occur straight after each other:

• Independent segregation of homologous pairs• Recombination of homologous pairs by crossing over

• Gene: section of DNA that codes for a polypeptide• Locus: the position of a gene on a chromosome/ DNA molecule• Allele: one of the different forms of a particular gene

Meiosis brings Genetic Variation

by:

• When homologous pairs arrange themselves it is done randomly• The combination of chromosomes into the daughter cell is completely random

Independent Segregation

• The independent segregation of the chromosomes produces new genetic combinations

• Gametes produced from meiosis will be genetically different due to the different combination of maternal and paternal chromosomes

• Each gamete had a different make up and due to random fusion variety is produced

Variety from New Genetic

Combinations

• The chromatids of each pair become twisted around each other• During this tensions are created and portions of chromatids break off• These broken portions then re-join with the chromatid of its homologous pair• New genetic combinations are produced• Recombination: broken off portions of chromatid recombine with another

chromatid• Crossing over increases genetic variety as it produces 4 cells with different genetic

composition

Genetic Recombination by

Crossing Over

Meiosis Process

9 DiversitySimilarities and differences between organisms can be defined in terms of variation of

DNA

DNA difference leads to genetic diversity

• Also known as artificial selection• It uses the desired characteristics from one animal• Offspring are produced with the desired characteristic• If the offspring doesn't have the characteristic it can be killed or stopped from

breeding• Alleles for unwanted characteristics are bred out• Selective breeding reduces genetic diversity• It is carried out to produce high yield plants or animals

Selective Breeding

• When few individuals from a population colonise a new region• The individuals have few alleles• When the colony repopulate they have less genetic diversity• In time the population may develop into a separate species• They are less adapt to changing conditions since they have fewer alleles

The Founder Effect

• Natural disasters can cause this• The survivors will possess smaller variety of alleles then original population• When repopulating the genetic diversity will remain restricted• The fewer alleles mean they are less adapt to change in conditions

Genetic Bottleneck

10.1 HaemoglobinPrimary structure of a protein is the sequence of amino acids determined by DNA that

makes up a polypeptide chain

It is this sequence that determines how the polypeptide chain is shaped into its tertiary structure

Haemoglobins: group of protein molecules that have a quaternary structure

• Haemoglobins are a group of chemically similar molecules found in organisms• The structure is made up of:• Primary Structure: consisting of four polypeptide chains• Secondary Structure: Each polypeptide chain is coiled into a helix• Tertiary Structure: Each polypeptide chain is folded into a precise shape allowing

ability to carry oxygen• Quaternary Structure: All four polypeptides are linked together. Each polypeptide

is associated with a haem group which contains a ferrous (Fe2+) ion. Each Fe2+ ion can combine with an O2 molecule

Haemoglobin Molecules

The Role of HaemoglobinTransport oxygen efficiently

• It can readily associate with oxygen at the surface where gas exchange occurs• It can readily dissociate from oxygen at tissues requiring it

These two requirements are achieved by haemoglobin being able to change its affinity for oxygen under different conditions

• Haemoglobin can change shape in the presence of certain substances such as CO2• With CO2 present haemoglobin molecule binds more loosely to oxygen

Why have Different Haemoglobin

• Haemoglobin with high affinity for oxygen: take up O2 easily but release it less easily• Haemoglobin with low affinity for oxygen: take up O2 less easily but release it easily• Organism living in low O2 area requires high affinity haemoglobin• Organism with high metabolic rate needs low affinity haemoglobin

Why different Haemoglobins have different Affinities

• Due to slightly different amino acid sequences

Loading and Unloading Oxygen

• Loading/Associating: process haemoglobin combines with oxygen• Unloading/dissociating: process haemoglobin releases oxygen

10.2 Oxygen Dissociation CurvesWhen haemoglobin is exposed to different partial pressures of oxygen it does not absorb O2

evenly

Low Concentrations: Four polypeptides of haemoglobin molecule are closely united so difficult to absorb first O2 molecules

Once loaded the O2 causes the polypeptides to load remaining O2 easily

•The further to the left the curve the greater the affinity of haemoglobin for oxygen (it takes up oxygen readily but releases it less easily)•The further to the right the curve the lower the affinity of haemoglobin for oxygen

Graph Reading

•Haemoglobin has a reduced affinity for O2 in the presence of CO2•The Bohr Effect: the greater the concentration of CO2 the more readily O2 is released•At gas exchange surfaces CO2 level is low due to it being expelled. The affinity of haemoglobin for oxygen is increased meaning O2 is readily loaded by haemoglobin. The reduced CO2 level shifts the graph to the left

•In rapidly respiring tissues the CO2 level is high, the affinity for oxygen is reduced causing the oxygen to be readily unloaded from the haemoglobin. The increased CO2 level shifts the graph to the right

•Carbon Dioxide is acidic so lowers pH causing haemoglobin to change shape

Effects of CO2 Concentration

•The higher the rate of respiration->the more carbon dioxide the tissue produces->the lower the pH->the greater the haemoglobin shape change->the more readily oxygen is unloaded-> the more oxygen available for respiration

•In humans haemoglobin becomes saturated in the lungs so they carry the 4 oxygen molecules

•When the haemoglobin reaches a low respiratory rate tissue one of the oxygen molecules is released

•At a very active tissue 3 oxygen molecules will be unloaded from each haemoglobin

Loading, Transport and

Unloading Oxygen

10.3 Starch, Glycogen and Cellulose

• Found in plants: seeds and storage organs• Forms important part of diets as an energy source• Made up of alpha-glucose monosaccharides linked by

glycosidic bonds• Glycosidic bonds formed by condensation reactions• Unbranched chain wound into a coil so compact

Starch

• Insoluble so doesn't draw in water by osmosis• Doesn't diffuse out of cells easily as insoluble• Compact: lots can store in small space• Hydrolysed into alpha glucose used in respiration

Suited as a energy

store:

• Similar structure to starch• Short chain and highly branched• Major carbohydrate storage in animals• Stored as small granules in muscles and liver• Readily hydrolysed due to small chains• Not found in plant cells

Glycogen

• Made of Beta-Glucose monomers: position of -H group and -OH group on single carbon atom are reversed

• -OH group is above the ring so to form glycosidic bonds the monomers are rotated 180 degrees

• The -CH2OH group on each Beta-Glucose molecule alternates• Forms straight unbranched chain that runs parallel allowing

hydrogen bonds to form cross links between adjacent chains• Cellulose is strengthened by the multiply hydrogen bonds• The cellulose molecules group to form microfibrils which form

fibres• Major component of plant cell walls as provides rigidity• Prevents cell bursting when water enters as it exerts an

inward pressure• Herbaceous parts of plants are semi-rigid as the cells push

against each other• Important in stems and leaves• Provide maximum surface area for photosynthesis

Cellulose

10.4 Plant Cell StructurePlant cells are eukaryotic cells: have distinct nucleus and membrane bound organelles like

mitochondria and chloroplasts

• Function: Photosynthesis• Features:• Long, thin cells form continuous layer (Absorb Sunlight)• Chloroplast arrange themselves for maximum light• Chloroplast carries out the photosynthesis• Large vacuole pushes cytoplasm and chloroplast to edge of cell

Leaf Palisade Cell

• Typically disc shaped• Features:• Chloroplast Envelope: double plasma membrane surrounds organelle, selects

what enters and leaves the cell• Grana: Stacks of discs called thylakoids, thylakoids contain chlorophyll. The first

stage of photosynthesis occurs here• Stroma: fluid filled matrix that contains other structures such as starch grains.

The second stage of photosynthesis occurs here• Adaptations for Photosynthesis:• Granal membranes: large surface area for chlorophyll attachment, electron

carriers and enzymes that carry out 1st stage of photosynthesis• Fluid of stroma possess enzymes needed for 2nd stage of photosynthesis• Chloroplast contains DNA and ribosomes that allow manufacture of proteins

for photosynthesis

Chloroplasts

• Consists of microfibrils of cellulose embedded in a matrix• Features:• Consists of many polysaccharides• Middle Lamella marks boundary between adjacent cell walls

that sticks them together• Function:• Provide mechanical strength to stop cell bursting• Mechanical strength to whole plant• Allow water to pass along it

Cell Wall

• Absorb water and mineral ions• Water absorbed by osmosis• Roots have high concentration of ions and sugar compared to

soil• Uptake of mineral ions is against the concentration gradient so

used active transport• Special carrier proteins use ATP to absorb mineral ions

Root Hair Cell

• Transport water• Thick cell walls• Formed from dead cells• Lignin often forms rings around the vessel

Xylem Vessels

11 Replication of DNACell division occurs in two main stages:

• Nuclear Division: nucleus divides either in mitosis or meiosis• Cell Division: follows nuclear division, it is where the whole cell divides• Before a nucleus divides its DNA must be replicated• This makes sure daughter cells have genetic information to produce enzymes and other needed proteins

Semi-Conservative Replication• Has four requirements:• Four types of nucleotide must be present with their bases adenine, guanine, cytosine and thymine• Both DNA molecule strands must act as a template for the attachment of nucleotides• The enzyme DNA polymerase is needed to catalyze the reaction• A source of energy is required

Semi-Conservative Replication Process• DNA helicase breaks hydrogen bonds linking base pairs• The double helix separates into two strands• Each exposed polynucleotide strand acts as a template to which complementary nucleotides are attracted• Energy used to activate the nucleotides• Activated nucleotides are joined together by DNA polymerase to form missing polynucleotide strand• Each new DNA molecule has one original DNA strand and one new DNA strand

MitosisDivision of the nucleus of a cell that results in each daughter cell having exact copy of the DNA of the parent cell

Genetic make up of the two daughter nuclei is identical to the parent unless mutation

The period when the cell is not dividing is called interphase

Interphase: cell is actively synthesising proteins, chromosomes are visible and DNA replicates

Mitosis is divided into four stages:

• Prophase: Chromosomes are visible, nuclear envelope disappears• Metaphase: Chromosomes arrange themselves at the equator of the cell, spindles form• Anaphase: The chromatids are pulled towards poles as the spindles contract• Telophase: Nuclear envelope reforms, spindles disappear and cell division commences

The Importance of Mitosis

• It makes daughter cells identical to the parent cells• Growth: When two haploid cells fuse to form a diploid cell the diploid has all genetic information needed to

from the new organism. All the cells must have same set of genetic information so it resembles its parents• Differentiation: Cells change to give specialised cells, the cells divide by mitosis to give tissues made of

identical cells which perform particular functions. Essential for efficient functioning of cells• Repair: If cell is damaged or dies it needs to be replaced with a cell identical in structure and function.

Without mitosis an identical cell would not be formed

The Cell CycleCells do not divide continuously, they have a cycle of

division separated by periods of cell growth.

• Interphase: most of cell cycle, no division occurs, divided into 3 parts:• G1 (first growth): Proteins from which cell organelles are synthesized are

produced• S (Synthesis): DNA is replicated• G2 (second growth): Organelles grow and divide, energy stores are increased

• Nuclear Division: Nucleus divides by mitosis or meiosis• Cell Division: Whole cell divides into two (Mitosis) or four (meiosis)

The Cell Cycle which has 3 stages:

• Caused by growth disorder of cells• Result of damage to the genes that regulate mitosis and the cell cycle• Leads to uncontrolled growth of cells, forming abnormal cells known as a

tumour• The tumour can expand

Cancer

12 Cellular DifferentiationThe process by which cells in Multicellular organisms become specialised for different functions.

Cells organised into tissues and tissues organise organs which organise organ systems

All cells in an organism are initially identical. As a cell matures, it takes on its own individual characteristics that suit it to the function that it will perform, when it has matured.

Each cell is specialised in structure to suit the role it will carry out.

Every cell contains the same genes for its development, the cell differentiates due to the number of genes switched on (Expressed) or off.

Tissues:

• Collection of similar cells aggregated together to perform a specific function• For them to work efficiently cells are Aggregated together.

• E.g epithelial cells and xylem.

Organs:

• Aggregation of tissues performing physiological functions.• Combination of tissues that are co-ordinated to perform a variety of functions, although they do have one

predominant function• Lungs, heart, stomach, leaf

• Artery and vein : made up of many tissues- epithelial, muscle and connective. Have one predominant function- to carry blood (transportation of blood)

• These systems may be grouped together to perform particular functions more efficiently.• Digestive system, respiratory system, circulatory system .

13 Exchange between Organisms and EnvironmentSubstances need to be interchanged:

• Respiratory gases (Oxygen and Carbon Dioxide)• Nutrients (fatty acids, glucose, amino acids, vitamins and minerals)• Excretory products (urea and Carbon Dioxide)• Heat

The exchange takes place:• Passively (no energy required): diffusion and osmosis• Actively (energy required): active transport

Surface Area: Volume Ratio• Small organisms have surface area large enough for efficient exchange across their body• Larger organisms cannot do this so have adaptations:• Flattened shape so cells close to surface• Specialised exchange surface with large area to increase surface area: volume ratio

Features of Specialised Exchange Surfaces• Large surface area to volume ratio: increases rate of exchange• Thin: diffusion distance is short so exchange is rapid• Partially permeable: allow selected materials to cross without obstruction• Movement of environmental medium e.g. air• Movement of internal medium e.g. blood

Fick's Law:• Diffusion = Surface Area X Difference in Concentration

iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiLength of Diffusion Path

Gas Exchange in Single Celled Organisms/InsectsSingle Celled Organisms

• Are small so have large surface area: volume ratio• Oxygen is absorbed by diffusion across the body which is covered by a cell surface membrane• Carbon dioxide diffuses out the body surface• Living cells with cell walls are completely permeable so no barrier for diffusion

Gas Exchange in Insects• Insects are terrestrial so suffer from water evaporation easily making them dehydrated• To reduce water lose terrestrial organisms have:• Waterproof covering• Small surface area to volume ratio: minimize area for water loss• Insects have tracheae (internal network of tubes supported by strengthened rings)• Tracheoles (smaller tubes tracheae divide into): Extend through the body so air is brought directly to respiring

tissues• Respiratory gases move in and out:• Along a diffusion gradient: Cell respiration uses oxygen so concentration towards Tracheoles end falls creating a

diffusion gradient. Gradient causes oxygen to diffuse from the atmosphere along tracheae. Cells respiring produce CO2 is removed into the atmosphere. It is quick as diffusion in air is more rapid than in water

• Ventilation: Movement of muscles creates mass movements of air in and out of tracheae speeding up exchange of gases

• Gas enters and leaves through spiracles (tiny pore) on the body surface, the spiracles open and close by a valve• Spiracles are usually kept closed to reduce water lose• The tracheal system relies on diffusion, which needs a short pathway, this limits the size of an insect

Gas Exchange in Fish• Fish have waterproof, air tight outer covering• Have relatively small surface area to volume ratio• They have specialised internal gas exchange surface: Gills

Structure of the Gills• Made up of gill filaments stacked up in a pile• Right angles to the filaments are gill lamellae which increase surface area• Water taken in through the mouth and forced over the gills, goes out opening sides of body• The flow of blood and the flow of water are in opposite directions: countercurrent flow

The Countercurrent Exchange Principle• Blood well loaded with oxygen meets water, water has maximum concentration of oxygen so

diffusion to the blood occurs• Blood with little oxygen meets water that has most oxygen removed so diffusion occurs• If the flow was parallel only 50% of oxygen from the water would diffuse into the blood

• When photosynthesis occurs, some CO2 comes from respiring cells, most CO2 comes from external air. Oxygen from photosynthesis is used in respiration but most is diffused out

• When photosynthesis isn't occurring oxygen diffuses into the leaf as needed by the cells during respiration. Carbon dioxide produced during respiration diffuses out

Gas Exchange in Leaves

• Plant leaves have large surface area and short diffusion path allowing quick diffusion

• Leaves have many stomata in lower epidermis and air spaces throughout mesophyll

Structure of a Leaf

• Surrounded by guard cells to prevent water loss but also allow gases in and outStomata

Circulatory System of a MammalHow Large Organisms move Substances around their Bodies

• Specialist exchange surface absorbs nutrients, respiratory gases and removes waste• Transport system required to take materials from cells to exchange surfaces and from exchange surfaces to cells• Tissues and organs have been specialised for their job• The lower the surface area: volume ratio and the more active an organism the greater the need for specialised

transport system with a pumpFeatures of Transport Systems

• Suitable medium to carry materials e.g. blood (normally liquid based in water as it readily dissolves substances and moves easily)

• Form of mass transport• Closed system of vessels that form a branching network• A mechanism to move the medium which requires pressure (Animals have the heart)• Mechanism to maintain the mass flow movement in one direction e.g. valves• Means of controlling the flow of medium to suit changes

How Blood is Circulated in Mammals• There is a closed blood system• A muscular pump (heart) circulates the blood around the body• Mammals have a double circulatory system• Blood passes through the heart twice since:• After passing through the lungs its pressure is reduced, this would make circulation slow if passed around rest of

the body• Blood is returned to the heart to boost pressure and then passed through the body• This is needed to keep the body temperature and metabolism in safe conditions• The blood is passed into the cells be diffusion over a large surface area with a short distance and steep diffusion

gradient

Blood Vessels Structure

Arteries: Carry blood away from the heart into arterioles

Arterioles: Smaller arteries that control brood flow from arteries to capillaries

Capillaries: Tiny vessels that link arterioles to veins

Veins: Carry blood from capillaries back to the heart

Arteries, arterioles and veins have same basic structure:• Tough outer layer: Resist pressure changes from both within and outside• Muscle layer: Can contract and control blood flow• Elastic layer: Help maintain blood pressure by stretching and springing back• Thin inner lining (Endothelium): Smooth to prevent friction and thin to allow diffusion• Lumen: Central cavity of the blood vessel

The vein's lumen is larger than the arteries

Structure and Function: ArteryFunction: Transport blood rapidly under high pressure from heart to tissues

Thick muscle layer: Smaller arteries can constrict and dilate to control volume of blood passing through them

Thick elastic layer: To keep blood pressure high to reach body extremities. It can then stretch and recoil according to when the heart beats allowing it to maintain a high pressure

Thickness of wall is large: Resists the vessel bursting under pressure

No Valves: Blood under constant high pressure so doesn't flow backwards

• Function: Carry blood under lower pressure than arteries to the capillaries

• Thicker muscle layer: Contractions of the muscle layer allow constriction of the lumen, this restricts the flow of blood and controls movement

• Thinner elastic layer: Blood is under lower pressure

Arteriole Structure and Function

Structure and Function: VeinsFunction: Transport blood under low pressure to the heart

Thin muscle layer: Veins carry blood away from tissues so don't need to control flow to them

Thin elastic layer: Low pressure stops the bursting

Small thickness of wall: No need for thick wall as the pressure of the blood is too low to burst the vein, it also allows them to flatten easily

Valves: Ensure blood flows in the right way, when body muscles contract they compress the veins and pressurize them, the valves stop backwards flow

Function: Exchange metabolic materials like oxygen, carbon dioxide and glucose

Walls with lining layer: Allow short diffusion distance for rapid diffusion of materials between blood and cells

Branched: Many of them providing large surface area

Narrow diameter: Permeate tissues

Narrow Lumen: Red blood cells flattened so closer to cells

Spaces between endothelial cells: Allows white blood cells to escape and help infection

Structure and Function: Capillary

Tissue FluidTissue Fluid

• Capillaries cannot reach every cell directly so tissue fluid bathes the tissues• It contains glucose, amino acids, fatty acids, salts and oxygen• It supplies these substances to the tissues in return for CO2 and waste materials• It is formed from blood plasma and is controlled by homeostatic systems

Formation• Blood pumped by the heart passes along arteries, then the arterioles and then the capillaries which

causes hydrostatic pressure at the arterial end of the capillaries• This pressure forces tissue fluid out of the blood plasma• The outward pressure is opposed by two forces:• Hydrostatic pressure of tissue fluid outside the capillaries prevents outward movement of liquid• The lower water potential of the blood due to the plasma proteins pulls water back into the blood

within the capillaries• Ultrafiltration occurs as the pressure is only enough to force small molecules out of the capillaries

leaving cells and proteins in the blood

Tissue Fluid Return to the Circulatory SystemOnce it exchanges metabolic materials it must return to the circulatory system

Most tissue fluid returns to the blood plasma directly via capillaries:

• The loss of tissue fluid from the capillaries reduces hydrostatic pressure inside them• By the time blood reaches the venous end of the capillary network its hydrostatic pressure is less than

tissue fluid outside of it• Tissue fluid is then forced back into the capillaries by high hydrostatic pressure outside them• Osmotic forces resulting from the proteins in the blood plasma pulls water back into the capillaries

The remainder tissue fluid is carried back via the lymphatic system; a system of vessels than begin in the tissues that resemble capillaries but then merge into larger vessels

The larger vessels drain their contents back to the bloodstream via two ducts that join veins close to the heart

The contents of the lymphatic system are moved by;

• Hydrostatic pressure of the tissue fluid that left the capillaries• Contraction of body muscles squeeze the lymph vessels, valves in the lymph vessels ensure fluid inside

them moves away from the tissues in the direction of the heart

Movement of Water through Roots

Uptake of Water by Root Hairs• Plants lose water by transpiration so the water is replaced through the root hairs• Root hairs are long, thin extensions of a root epidermal cell

Efficient surface for exchange of water and minerals:• Provide large surface area• Thin surface layer

The soil the root hairs go into is mostly water so has a high water potential

The root hair cells have sugars and amino acids dissolved in them

This causes water to move by osmosis into the root hair cells

The water travels via:• The Apoplastic pathway• The Symplastic pathway

• Water drawn into the endodermal cells• It pulls more water in due to its cohesive property• This creates tension that draws water along the cell walls of the cells of

the root cortex• The mesh like structure of the cellulose cell walls contains many water

filled spaces• These water filled spaces provide little resistance to the pull of water

along the cell walls

The Apoplastic Pathway

• Takes place across the cytoplasm of the cells of the cortex due to osmosis

• Water passes through the cell walls along tiny openings called plasmodesmata

• Each plasmodesmata is filled with thin strand of cytoplasm• There is a continuous column of cytoplasm extending through the

root hair cell to the xylem at the center of the root• Water moves along the column:• Water enters by osmosis increasing the water potential of the root

hair cell• The root hair cell has a higher water potential than the first cell in the

cortex• Water then movers from the root hair cell to the first cell in the

cortex• This is then repeated through the cortex• Water is then pulled in as the water potential of the first cortical cell

is lowered again making the water travel by osmosis from the root hair cell to the first cell of the cortex

• A water potential gradient is set up along the cells

The Symplastic Pathway: Through the

cells

Passage of Water into the XylemThe waterproof band that makes up the casparian strip prevents water from passing further through the cell

wall due to the apoplastic pathway

Water is forced into the living protoplast of the cell

It joins the water from the symplastic pathway

Active transport of mineral salts can take the water into the xylem

Endodermal cells actively transport salts into the xylem

As the process requires energy it can only occur in living cells

It takes place along carrier proteins in the cell surface membrane

The active transport of mineral ions into the xylem by the endodermal cells creates a lower water potential so water can move into the xylem by osmosis along a water potential gradient

The movement of the mineral ions creates a force that helps to move water up the plant: The is Root Pressure

Evidence root pressure is due to the pumping of salts into the xylem:

Pressure increases with a rise in temperature

Metabolic inhibitors e.g. cyanide prevent most energy release by respiration and stop root pressure

Decrease in availability of oxygen causes a reduction in root pressure

• The main force that pulls water up the stem is transpiration• Transpiration is the evaporation of water from the leaves

Movement of Water up Stems

• When stomata are open water vapour molecules diffuse out of the air spaces

• The water is replaced by water evaporating from the cell walls of the mesophyll cells

• Water from the mesophyll cells is then replaced by water in the xylem by the apoplastic or symplastic pathways

• In symplastic pathways it occurs:• Mesophyll cells lose water to air spaces• Cells now have lower water potential so water enters by osmosis• The neighboring cells lose water lowering their water potential• The neighbouring cells then take water from the cells next to them• This establishes a water potential gradient to pull the water from the

xylem

How Water moves through the Leaf

• The two main factors that cause water movement up the xylem are cohesion tension and root pressure

• Cohesion Tension operates:• Water evaporates from leaves due to transpiration• Water molecules form hydrogen bonds between them this is

cohesion• Water form continuous pathway across the mesophyll cells and down

the xylem• As water evaporates from the mesophyll cells in the cells in leaves

into the air spaces beneath the stomata molecules of water are brought up

• Water is pulled up the xylem due to transpiration pull• Transpiration pull put the xylem under tension

How Water moves up the Xylem

Evidence of the Cohesion Tension Theory

Change in diameter of tree trunks• During the day transpiration is at its greatest so more tension in the xylem• This causes the trunk to shrink• At night transpiration is at its lowest so little tension in the xylem• Diameter then increases at night

If Xylem vessel is broken and air enters it• The tree can no longer draw up water as no continuous column• If broken air is drawn in

Transpiration pull is a passive process so doesn't require metabolic energy

As the xylem is dead it can form series of continuous unbroken tubes from root to leaves

These tubes are essential to the cohesion tension theory

Energy is needed for transpiration and it comes from the heat of the sun

13.9 Transpiration

Factor How Factor Affects Increase in Transpiration caused by

Decrease in transpiration caused by

Light Stomata open in the light and close in the dark

Higher light intensity Lower light intensity

Temperature Alters the kinetic energy of the water molecules and the relative humidity of the air

Higher temperature Lower temperature

Humidity Affects the water potential gradient between the air spaces in the leaf and the atmosphere

Lower humidity Higher humidity

Air Movement Changes the water potential gradient by altering the rate at which moist air is removed from around the leaf

More air movement Less air movement

Why Transpiration Occurs•Leaves have a large surface area to absorb light and stomata to allow diffusion of CO2 into the plant•Both these features cause a huge lose in water •Mineral ions, sugars and hormones are moved around the plant in dissolved water by the transpiration pull•Without transpiration water wouldn't be plentiful and transport of materials would be slowFactors

13.10 Limiting water loss

14 Classification• Species: Similar to one another but different to members of

other species that are capable of breeding to produce living, fertile offspring

The Organisation of living organisms into groups

• Based on Greek or Latin• First name (generic name) denotes the genus • The second name (specific name) denotes the species

The Binomial System:

• Taxonomy: Theory and practice of biological classification• Artificial classification: divides organisms due to

differences useful at the time. Features such as number of legs are analogous characteristics that have the same function but not evolutionary origins

• Natural Classification: based on evolutionary relationships, classifies species in groups using shared features and arranges the group in hierarchy

• Natural classification is based upon homologous characteristic so have similar evolutionary origins

Grouping Species Together:

• Evolutionary relationship between organisms• It reflects the evolutionary branch that led up to an

organism Phylogeny

RankKingdomPhylum

ClassOrderFamilyGenus

Species

15.1 Genetic ComparisonDNA determines proteins including enzymes and proteins determine features of an organism

When a species arises from another due to evolution the DNA will initially be similar

Due to mutations the sequences of nucleotide bases in the DNA will change

DNA Hybridisation• DNA from two species is extracted, purified and shortened• DNA from one species is labelled with radioactive/fluorescent marker so it can be identified• The mixture of DNA is heated to separate the strands• It is then cooled so the strands recombine that have complementary base sequence• Hybrid strands are formed when one strand of each species combines and can be separated by certain temperatures • If the species are closely related they will share many complementary nucleotide bases so there will be more

hydrogen bonds linking the strands• The greater the number of hydrogen bonds the higher the temperature needed. This means the higher the

temperature the more closely related Comparison of Amino Acid Sequences in Proteins

• DNA determines amino acids in proteins • Similarity in the amino acid sequence of the same protein in two species will reflect how closely related they are• Amino acid sequences for a certain protein can be compared

Immunological Comparison of Proteins• Proteins of different species can be compared as antibodies of one species will respond to specific antigens on

proteins e.g. albumin in the blood serum of another• Serum albumin from species A is injected into species B• B produces antibodies specific to all antigen sites on albumin from A• Serum is extracted from B containing antibodies specific to the antigens on the albumin of A• Serum from B is mixed with serum from species C• Antibodies respond to their corresponding antigens on the albumin in serum C• The response forms a precipitate, the more precipitate the more similar the antigens and the more closely related

15.2 Courtship BehaviourPhysical and Chemical make-up of organisms help distinguish members of own species

Behaviour of members of the same species is more alike than other species

Behaviour is genetically determined and has evolved it influence chance of survival

Courtship is Necessary

• Reproduction is important so a species can survive• Courtship behaviour helps achieve chance of offspring by:• Allowing members for same species to recognise each other so fertile offspring can be produced• To identify mate capable of breeding• Form a pair bond to lead to successful mating and raising of offspring• Synchronise mating so it takes place when maximum chance of fertilisation• Males carry out a specific action which stimulates the female to respond and her action then causes him to

react• It is a stimulus-response chain where if the species is the same the chain of actions will be the same

16.1 Genetic Variation in BacteriaAdaptation can occur in natural selection

Long term reproductive success of a species is increased by adaptation

Bacteria is adaptable and able to develop resistance to antibiotics

Changes in DNA can occur by mutation or recombing DNA of two individuals

Bacteria can increase genetic diversity by mutations and conjugation

• Changes in DNA that cause different characteristics• Bases can be added, deleted or replaced during replication • Difference in bases can cause a change in amino acid which will then cause a

difference in polypeptide causing a different protein to be formed

Mutations

• When one bacterial cell transfers DNA to another bacterial cell• One cell produces thin projection that meets the other cell and forms a thin

conjugation tube between them• The donor cell replicates one of its circular DNA pieces (plasmid)• The DNA is broken to make it linear and is then passed along the tube to the

recipient cell• The contact is brief so only portion of the donor DNA is transferred • The recipient cell acquires new characteristics

Conjugation

Horizontal gene transmission: From one species to another

Vertical gene transmission: From one generation of a species to another

16.2 Antibiotics• Prevent bacteria forming a normal cell wall• Osmotic lysis causes the cell to burst when water enters the cell• Due to the cell wall surrounding the bacteria the content will

expand and push against the wall• The wall resists expansion stopping further water entry• Certain antibiotics kill bacteria by preventing cell wall formation • They inhibit synthesis and assembly of the peptide cross linkages in

the bacteria cell walls weakening the wall• The walls are then unable to withstand pressure and water enters

the bacteria causing it to burst

How Antibiotics Work

• Due to the chance mutation within bacteria• The mutation can result in certain bacteria being able to make new

proteins• E.g. resistance to penicillin as the bacteria mutate and can produce

penicillinase which breaks down penicillin• The resistance can be passed on by vertical gene transmission • When an antibiotic is used it kills the bacteria without the mutation

leaving the mutated bacteria able to multiply• Due to this the allele pool is reduced and the resistance bacteria

can increase in population • The allele is carried in the plasmid which can be transferred in

conjugation so other bacterial species can gain the resistance

Antibiotic Resistance

16.3 Antibiotic Use and Resistance

• Treatment involves antibiotics for 6-9 months • When people believe they have recovered they stop

using the antibiotics so only the least resistant strains of mycobacterium are killed

• The most resistant remain, survive and multiply • A cocktail of antibiotics is used

Tuberculosis

• Staphylococcus can be carried in the throat• Staphylococcus aureus can cause major health issues

if forms methicillin resistant staphylococcus aureus which is resistant to many antibiotics

• It is easily spread in hospitals due to weak immune systems and people living close together causing transmission

• It is very difficult to treat

MRSA

17.1 Species DiversityBiodiversity: The variety in the living world• Species diversity: Number of species and individuals of each species

within a community• Genetic diversity: Variety of genes possessed by individuals in the

species• Ecosystem diversity: Range of different habitats within particular area• Biodiversity can be measured by; the number of different species in a

given area and the proportion of the community that is made up of an individual species

Measuring Species Diversity• Using the Species Diversity Index• The higher the value of D the greater the species diversity

17.2 Species Diversity and Human Activity• Natural ecosystems develop to form complex communities with

many different species• Agricultural ecosystems are controlled by humans • Farmers select species for particular qualities that are productive• As a result genetic variety of alleles is reduced to ‘desired features’• Any area can only support a certain amount of biomass so the more

of one species the less room for another and so they have to compete

• Overall reduction in species diversity causes a decrease in species diversity index so there are low agricultural ecosystems

Impact of Agriculture

• Forests form many different habitats and species diversity is high • Deforestation e.g. by forest fires, acid rain or human intervention

permanently clear forests • The land is converted for grazing, housing or reservoirs • Biodiversity is decreased

Impact of Deforestation