AQA Biology Unit 2 revision cards


Interspecific Variation: the variation that exists between different species.

Intraspecific: the variation that exists within species

Causes of Varitaion:


  • All members of a species have the same genes - but individuals within species can have different versions called alleles.
  • The alleles an organism has make up its genotype - displayed characteristics


  • Conditions we live in affect our appearnce (phenotype)
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Size and Surface Area

Organisms need to exchange substances with their enviroment

  • Cells take in oxygen (for aerobic respiration) and nutrients.
  • They also need to excrete waste products like CO2 and urea

Smaller animals have higher surface area : volume ratio

  • In single celled organisms, substances can diffuse into or out of the cell across cell surface membrane. Diffusion rate is quick becuase of the small distances the substances have to travel.
  • Multicellular organisms - diffussion slow - large distance between cell and outside enviroment.

Larger animals have low surface area : volume ratio

  • small surface area - large volume,heat loss hard.
  • large surface area - small volume, heat loss easier.
  • Animals with compact shape have a small surface area to their volume therfore minimisisng heat loss.
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Gas Exchange

Single celled organisms exchange gases across their body surface

  • Absorb and release gases by diffusion through their outer surface.
  • Large surface area, short diffusion pathway.

Fish use a counter current system for gas exchange

  • Water, containing O2 enters the fish through mouth - passed to the gills.
  • Each gill is made up of lots of thin plates called gill filaments which give a big surface area for exchange of gases. 
  • Gill filaments are covered in lots of tiny structures called lamellae, which increase the surface area even more.
  • Lamellae - lots of blood capilleries and a thin surface layer of cells to speed up diffusion.
  • Blood flows through the lamellae in one direction and water flows over in the opposite direction. This is called a counter current system. 
  • It maintains a large conc gradient between the water and the blood, so as much O2 as possible diffuses from the water into the blood.
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Gas Exchange

Dictyledonous plants exchange gases at the surface of the mesophyll cells

  • Plants need CO2 for photsynthesis, which produces O2 as a waste gas.
  • Main gas exchange surface = mesophyll cells
  • well adapted because they have a large surface area
  • Mesophyll cells are inside the leaf. Gases move in and out through specialised pores in the epidermis called stomata.
  • The stomata can open to allow gas exchange, or close of losing too much water. Controlled by guard cells.
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Circulatory System

Flow of blood through Heart

  • Deoxygenated blood back from body ---> Vena Cava ---> Right atrium ---> Right ventricle ---> Pulmonary artery ---> Lungs (gas exchange) ---> Oxgenated blood ---> Pulmonary vein ---> Left atrium ---> Left ventricle ---> aorta ---> blood to the body.
  • Liver receives blood from aorta via hepatic artery and blood from gut with dissolved digested food - glucose - amino acids via hepatic portal vein
  • Liver removes excess glucose and stores and glycogen. The blood leaves the liver via the hepatic vein and goes into the vena cava.

Blood Vessels


  • Thick walled - resist and maintain pressure
  • Elastic Tissue - stretch and recoil this smoothes out pressure changes due to ventricle contraction - blood flows smoothly in capilleries.
  • Muscle layer - contraction in arterioles can reduce blood flow. 

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Circulatory System

  • Epithelium - smooth - reduces friction - allows blood to flow smoothly.
  • No valves - (except for arteries leaving the heart), not needed as blood is at such a high pressure.
  • Lumen -  compared to a vein - is smaller as blood is at higher pressure.


  • muscle and elastic layers relatively thin - blood in veins flows slowly so thin walls do not resist flow.
  • Lumen is larger as the blood is flowing more slowly.
  • Epithelium - smooth - reduces friction - allows blood to flow smoothly.
  • Valves - prevent backflow as the blood is under low pressure.

Capillary structure and function

  • These are vessels through which materials are exchanged between blood and tissues
  • walls - single layer of flattened epithelium - short diffusion pathway
  • narrower then red blood cells
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Flow of Water and Ions

  • Root Pressure - due to active ion uptake by root hairs (large SA), difference in water pressure between root and soil causes water to flow in by osmosis into the xylem, up the plant creating a positive pressure in xylem.
  • Transpiration - loss of water from large leaf SA via the stomata generates tension on the water in the xylem. Because water molecules cohere the water is hauled up the xylem in chains. This creates a negative pressure inside the xylem which is sufficient to haul water up to aheight of 100m +.
  • Xylem - wide hollow tubes - less resistence to water flow.
  • Molecular Cohesion - water molecules stock/cohere together becasue they are charged.
  • So they can resist pulling forces/tension, so they can be pulled up the xylem.
  • Transpiration is is the evaporation from the large internal SA of all of the leaves which sets up tension in the chains of water molecules in the Apoplast and Symplast of the leaves which exerts a tension on the water molecules in the xylem. This tension is sufficient to generate a force which can pull water molecules up narrow tubes to a height of 100m +
  • Large leaf Internal SA - lots of evaporation = large force
  • xylem has thick walls to withstand negative pressure.
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Species Diversity

Biodiversity - variety of living things in an ecosystem/ the world. Generally human activity is reducing bidiveristy. 

Species Diversity - the number of different species in a community.

Community - all the different species in an ecosystem.

Ecosystem - has a particular set of enviromental conditions and therefroe has a particular community of adapted plants and animals.

Habitat - a particular set of enviromental conditions to which an organism is adapted.

Genetic diversity - refers to the variety of genes within the members of a population of the smae species.

Ecosytems with a great species diveristy have; a wide variety of which creates many habitats and lots of different food sources for herbivours and therefore lots of different foods for carnivores.

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Genetic Diversity

Variation in DNA can lead to Genetic Diversity

  • All the members of a species will have the same genes but different alleles.
  • The more alleles in a population the more genetically diverse it is.
  • GD is increased by - mutations in the DNA - forming new alleles and the itroduction of new alleles into the population.

Genetic Bottlenecks reduce genetic diversity:

  • A genetic bootleneck is an event that causes a big reduction in a poulation, e.g. when a large number of arganisms within a poultaion die before reproducing. Reduces the number of different alleles in the gene pool and so reduces genetic diversity.

Founder effect:

  • The founder effect descibes what happens when just a few organisms from a population start a new colony.
  • Only a small number of organisms have contributed theri alleles to the gene poo. There's more inbreeding in the new population which can lead to a higher incidence of genetic diseases. 
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DNA structure

  • Nucleotide - deoxyribose sugar + phosphate + base 
  • They are the monomers of DNA
  • DNA is made up of two strands of nucleotides twisted around each other forming a double helix.
  • The sugars and phosphates form the backbone of the molecule holding the bases
  • 4 bases - Adenine, Thymine, Cytosine, Guanine
  • Bases on one strand bond complementarily with the bases on the other strand. T with A and C with G

Evidence of DNA being the genetic material:

  • Heat kills - harmful pneumonia bacteria - protein destroyed by heat, DNA not damaged by heat.
  • Extract from harmful heat killed bacteria was mixed with licing safe bacteria.
  • the safe bacteria transformed into harmful bacteria. The protein had been destroyed by heat, the DNA was not destroyed by heat, so the DNA had to be genetic material.
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DNA and the Genetic Code

  • Proteins are the molecules which make cells work, enzymes/ protein control reactions.
  • Different proteins have a differet primary structure, different sequence of amino acids.
  • A gene has a code for a particular protein - the code specifies the sequence of amino acids.
  • Code on the DNA is made by the order of the bases.
  • Each amino acid is coded for by a code of 3 bases (Triplet Code)
  • Most amino acids have more than one code - degenerate code
  • Some triplets dont code for amino acids they are start and stop codons.
  • Some parts of the genes have DNA which is non coding called Introns and coding sections called exons.

Adaptations Of DNA:

  • large molecule and therefore carries alot of genetic information
  • Very stable - can pass from generation to generation without change.
  • Two seperate strands are joined together by hydrogen bonds - means they can seperate during DNA replication and protein synthesis
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DNA Replication

  • Takes place in Interphase of mitosis and meiosis.
  • The DNA strand is copied so that the new cell prdouced has a copy of the genes form the parent cells.
  • Replication is semi conservative - DNA unzips - DNA helicase breaks the H bonds between the strands.
  • Free A,T,C,G nucleotides line up and complementary pair with the bases on the individual strands. 
  • DNA polymerase joins the nucleotides together. 
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DNA Replication


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Mitosis - produces two daughter nuclei that have the same number of chromososmes as the parent cell and each other.

4 stages -

  • Prophase - chromosomes become visible as two chromatids one chromatid is the identical copy or the original chromosome made during DNA replication - nuclear envelope disappears
  • Metphase - chromosomes lin up on the cells equator.
  • Anaphase - spindle threads pull the chromatids apart.
  • Telophase - cell divides into two - nuclear envelope reforms

Importance of Mitosis:

  • Growth - divides to give a group of identical cells
  • Differentiation - specialised cells divide by mitosis to give tissues made up of identical cells which perform a particular function.
  • Repair - If cells are damaged or die it is important that the new cells produced have an identical structure and function to the ones that have been lost.
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The Cell Cycle

3 stages;

  • Interphase - occupies most of cell cycle, - resting phase - no division takes place.
  • a) first growth phase (G1) proteins are produced
  • b) Synthesis (S) phase, DNA is replicated
  • c) Second growth phase (G2) organelles grow and divide and energy stores are increased
  • Nuclear Division - when the nucleus divides either into two (mitosis) or four (meiosis)
  • Cell Division - follows nuclear division - whole cell divides into two (mitosis) or four (meiosis)
  • Mammalian Cells take about 24 hours to complete cell cycle - 90% interphase
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Meiosis - produces four daughter nuclei, each with half the number of chromosomes as the parent cells.

  • Meiosis 1 first division:
  • the homologous chromosomes pair up and their chromatids wrap around each other. Cromatids exchanged in a process called crossing over.
  • Homologous pairs are seperated, with one chromosome from each pair going into one of the two daughter cells.
  • Meiosis 2 second division:
  • Chromatids move apart - four cells have been formed - in humans each cell contains 23 chromosomes.
  • Gene - a section of DNA that codes for a polypeptide
  • locus - the position of a gene on a chromosome or DNA molecule
  • Allele - one of the different forms of a particular gene
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Meiosis - genetic variation

Independant Segregation: during meiosis 1 each chromosome lines up alongsie its homologous partner randomly. Since the pairs the line up randomly, the combination of chromosomes that goes into the daughter cell at meiosis 1 is also random.

Variety from new genetic combinations: each member of chromosomes has exactly the same genes and therefore determines the same characteristics. However the alleles of these genes may differ. The random distribution and consequent independant assortment, of these chromosomes therefore produces new genetic combinations. 

Crossing Over: 

  • chromatids of each pair become twiste around one another.
  • During twisting tensions are created and portions of the chromatidsbreak off.
  • Broken portions then rejoin with the chromatids of its homologous partner.
  • New genetic combinations are produced
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Antibiotic Action and Resistence

Antibiotics are used to treat bacterial diseases

  • Kill or inhibit the growth of bacteria.
  • Some prevent growing bacteria cells from forming the bacterial cell wall, which usually gives the cell structure and support - leads to osmotic lysis
  • inhibit enzymes that are needed to make chemical bonds in the cell wall - weakens cell wall
  • water enters cell by osmosis - cell wall cant withstand pressure increase an bursts (lysis) 

Mutations in Bacteria DNA can cause Antibiotic Resistence

  • genetic material in bacteria similar to most organisms
  • DNA contains genes that carry the instructions for different proteins - determine characteristics
  • Mutations are changes in the base sequence of an organisms DNA
  • If a mutation occurs in the DNA of a gene it could change the protein and cause a different characteristic.
  • Some mutations in bacterial DNA mean that the bacteria are not affected by a particular antibiotic any more, they have developed antibiotic resistence.
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Vertical and horizontal gene transmission


  • Bacteria reproduce asexually, so each daughter cell is an exact copy of the parent.
  • This means that each daughter cell has an exact copy of parent cells genes, including any that give antibiotic resistence.
  • genes for antibiotic resistence can be found in the bacterial chromosome or in plasmids (small rings of DNA)
  • The chromosome and any plasmids are passed onto the daughter cells during reproduction.


  • Genes for resistence can also be past on horizontally
  • Two bacteria join together in a process called conjugation and a copy of a plasmid is passed from one cell to another.
  • Plasmids can be passed on to a member of the same species or a totally different species.
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  • Starch is a polysaccharide
  • major energy source
  • made up of chains of alpha glucose linked by glycosidic bonds, that are formed by condensation reactions
  • unbranched chain is wound into a tight coil, which makes the molecule very compact.
  • Main role is energy storage - suited because: 
  • insoluble, does not draw water into cells by osmosis.
  • insoluble, does not easily diffuce out of cells
  • compact, a lot stored in a small space
  • hydrolysed to from alpha glucose easily transported and readily used in respiration.
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  • Glycogen has a similar shape to starch but shorter chains and more branched
  • major carb storage product in animals
  • stored as small granules mainly in the muscles and the liver
  • Good storage - compact
  • However because its made of smaller chains, even more readily hydrolysed to alpha glucose
  • never found in plant cells
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  • Made of monomers of beta glucose
  • In the beta glucose units, position of the H group and the OH group are reversed;
  • OH group is above, rather than below the ring. This means that to form glycosidic bonds each Beta glucose molecule must be rotated 
  • Result is that the CH2OH group alternates between being above and below the chain.
  • Straight unbranched chain
  • Run parallel, allowing H bonds to form cross linkages between adjacent chains.
  • Strengthens the cellulose, sheer number of H bonds
  • Cellulose molecules are grouped together to form microfibrils
  • plant cell walls - provide rigidity
  • prevents it from bursting as water enters by osmosis
  • leaves and stem remain turgid, maximising photosynthesis.
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Oxygen Dissociation Curves


  • HB is a quaternary protein as it is made of more than two polypeptides.
  • It is made of four polypeptide chains each of which is associated with an iron containing Haem group. 
  • HB with a high affinity of O2 picks it up easily ( at a low partial pressure of O2 ) unloads it less easily ( at even lower partial pressures )

O2 Curves

  • shape of curve shows that small changes in partial pressure of O2 in the tissues make large changes in the amount of haemoglobin dissociation i.e. if the amount of O2 in the respiring tissues is low then a lot of haemoglobin will dissociate to give tissues more O2 for respiration.
  • Respiring tissues produce CO2, more respiration = more CO2
  • CO2 reduces HB affinity for O2 i.e. it gives off its O2 more easily so that rapidly respiring tissues are supplied with O2, This shifts the dissociation curve to the right. Bohr shift
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O2 curves

Different Animals have variations of HB to suit different enviroments

  • relatively inactive animlas living in low O2 enviromemts have HB with a high affinity for O2. The dissociation curve is to the left of normal. This allows them to pick up O2 at low partial pressures and unload it at the very low partial pressures of O2, found in their respiring tissues.
  • Active Animals living in high O2 enviroments, have HB with a low affinity, the O2 curve is to the right of normal. So they pick up O2 at high partial pressures and unload it at relatively high partial pressures. This maintains a high level of O2 in the tissues so that they can respire faster and be more active.
  • Small Active animals - like mice - have a large SA to volume ratio and so lose heat rapidly, so they have a high metabolic rate/respire rapidly to generate heat, so they can have a dissociation  curve to the right of larger animals. This means that they can unload O2 more easily to their tissues and maintain a high level of respiration.
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