AS Biology Unit 2

I made these revision cards from all the notes I've taken during lessons and free time for some of AQA AS Biology Unit 2. I hope you find these useful! :)

Please note. these cards were made in 2012/2013, and are NOT in any perticular order (I've ordered it how my lessons were taught). This is also only one half of Unit 2, another resource will be added later.

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Cell Cycle

  • Interphase:
    • G1 - Cellular contents (not chromosomes) are duplicated
    • S - Chromosomes are duplicated
    • G2 - Cell "double checks" chromosomes for errors, making any repairs needed
  • Mitosis - I.P.M.A.T
  • Cytokinesis - Cell division.


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Cell mass & DNA mass during cell cycle


Cell Mass

  • Cell mass increases steadily as organelles replicate (G1) and DNA replication (S).
  • Sudden large decrease to original level, due to cytokenesis (cell division).

DNA Mass

  • DNA mass increases later as organelles copy first.
  • Levels off as replicated DNA "goes through" mitosis.
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  • Mitosis creates two identical cells with a full compliment of chromosomes (Diploid). e.g.Humans, 46 chromosomes
    • I.P.M.A.T - Interphase, Prophase, Metaphase, Anaphase, Telophase.
  • Interphase - G1, S and G2
  • Prophase - Chromosomes become visible, nulear envelope disappears. ("poles" migrate)
  • Metaphase - Chromosomes arrange along the equator of the cell. (spindle fibres attach to the chromosomes)
  • Anaphase - Chromatids migrate to opposite poles of the cell. (Via spindle fibres)
  • Telophase - Nulear envelope reforms. (Cell begins to reform)
    • The cell then goes through Cytokinesis (Cell division).


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Meiosis creates four cells with half compliment of chromosomes (Haploid), then after fertilisation diploid is restored. e.g. Humans, 23 chromosomes.

  • Homolgus (same) chromosomes pair up.
  • Chromatids can wrap around each other and exchange genetic information. CROSSING OVER.
  • One chromosome from each pair separates into the daughter cells.
  • Chromatids move apart, creating 4 NON IDENTICAL haploid cells.


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Meiosis Variation

1) Independent segregation of homologous chromosomes.

    • Chromosomes line up randomly in meiosis 1
    • One pair passes to each daughter cell
    • Combinations of chromosomes varies with how they line up
      • How they line up is random and this is termed independent segregation of homologous chromosomes.

2) Genetic Recombination

    • Chromatids twist around each other
    • Creates tention and parts break off
    • These parts rejoin with the homologous chromosome
      • This is called Recombination (CROSSING OVER).
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Treatment on Cancer cells

Involves blocking parts of the cell cycle.

  • Chemotherapy drugs disrupt the cell cycle by:
    • Preventing DNA replication (S)
    • Inhibits metaphase by stopping spindle formation

Also effects normal cells;

  • Cancer cells divide more rapidly so are more damaged, however other rapidly dividing cells such as hair-producting cells are all vulnerable to damage.
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DNA Nucleotide structure

  • Chromosomes - Contains DNA and packing protein.
  • DNA & RNA are made up of nucleotides
  • Nucleotides are made of:
    • A phosphate, a base and a Deoxyribose (DNA) or a Ribose (RNA)


  • Nucleotides - 2 types of bases
    • Pyrimidines (short bases)                   Purines (long bases)
      • Cytosine - C  & Thymine - T               Adenine - A  &  Guanine - G
  • Adenine only bonds with Thymine (A & T) with double hydrogen bond
  • Guanine only bonds with Cytosine (G & C) with triple hydrogen bond
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DNA structure

  • DNA adaptations

    • Needs to be remarkably stable.
    • Extremely large molecule, carry imense amounts of genetic information.
    • Bases are protected by the sugar phosphate backbone, because of their coiled structure.
    • Bases bonded by weak hydrogen bonds which allows them to seperate easily during DNA replication and proteinsynthesis.


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DNA replication: semi-conservative

  • There are 4 requirements for semi-conservative replication to take place:
    • Both strands of the DNA molecule must act as a template
    • The 4 nucleotides (with different bases) must be present
    • DNA polymerase must be present
    • A source of chemical energy.
  • The stages in DNA replication:
    • DNA helix uncoils
    • Helicase (breaks H-bonds) seperates DNA strands
    • Free nucleotides are attracted to complementry bases (C to G, A to T, and vise versa)
    • Free nucleotides are joined to the DNA strand (acting like a template) by polymerase
    • 2 identical strands are formed.
  • Why do you think this is called semi-conservative replication?
    • semi = half
    • conservative = favouring tradition (original)
      • As it only contains half the original DNA strand so it is semi conservative.
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DNA semi- conservation replication model

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DNA, hereditary material? (model)

Virulent = Harmful

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DNA, hereditary material?

  • Mice were seperately injected with the non-virulant form (a) and the heat-killed virulant form (c) and both remained healthy.
  • Of course mice injected with living virulant form, became unhealthy and then died (b).
  • However mice which were injecting with both the non-virulant form (a) AND the heat-killed virulant form (c) became unhealthy and died.
  • 3 different possible explanations for this:
    • Experimental error (unlikely as repeats had same conclusion)
    • Living non-virulent form mutated into virulent form (possible, but unlikely)
    • Virulent form cause death via a toxin.
      • Heat-killed virulent form contains information on how to make the toxin, but cannot due to being dead.
      • Living non-virulent form has the means to do so, but not the genetic information to make the toxin.
      • Therefore the genetic information must have transfured from the heat-killed virulent form to the living non-virulent form.
      • Creating a living virulent strand.
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Genetic code

  • A sequence of nucleotide bases forms a code
  • Each codon codes for a specific amino acid.
    • e.g. GGG = proline, CGG = glycine, ATG = tryosine, ACT = stop (no amino acid)
      • 3 nucleotide (triplet code) = an amino acid
  • Triplet code features:
    • Only a few amino acids have a single triplet code
    • Most have between 2 and 6 codes each
    • This is known as the degenerative code
    • This triplet code also contains stop codons, which stop the polypeptides production
    • No overlap & mostly universal (across species)
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Gas exchange

  • All living organisms take gases and return them to their environment
    • Organisms which respire aerobically take Oxygen and give out Carbon Dioxide
    • Photosynthesizing plants take Carbon Dioxide and give out Oxygen
  • Many organisms have specialised surfaces through which gas exchange occurs
  • The amount of Oxygen needed by an organism is deturmined by:
    • Number of living cells (size of the organism)
    • Rate they need to respire (how active they are)
  • Gas exchange surfaces are designed to be effective by:
    • Having a large surface area
    • Being as thin as possible (short diffusion pathway)
    • Creates a high gas gradient for greater diffusion
    • Having a transport system (blood) which takes the gases to and from exchange site.
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Gas exchange in small and large organisms

  • Small organisms
    • High surface area to volume ratio
    • Small organisms get all the oxygen they need via diffusion through the body surface.
  • Large organisms
    • Small surface area to volume ratio
    • External surface area of the body is not large enough for diffusion of oxygen, and wouldn't take place quick enough to supply all the cells rapidly.
    • Have specialised exchange surfaces, eg. lungs.
  • Problems for land organisms:
    • Cells need to be exposed to air in order for Oxygen to diffuse into it
    • Land organisms bodies are made up of a high percentage of water
    • When living cells are exposed to the air, water molecules evaporate and the cell dehydrates.


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Gas exchange: Flatworm & Earthworm

  • Flatworm
    • Simple animals, lack specialised exchange surfaces.
    • flattened, tubular, thin shaped bodies, which are most efficient for gas exchange
    • Outer surface is used for gas exchange, via diffusion
    • Only possible because cells are localized relatively near to the exterior since gases diffuse cell by cell.
  • Earthworm
    • Have a series of thin capillaries
    • Gas exchange occurs at capillaries located throughout the body as well as by the repiratory surface.
    • Adaptations:
      • Cells on the surface for gas exchange
      • Live in moist conditions, prevents water loss
      • Slimy covering reducting evaporation
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Gas exchange: Insects

  • Respiratory system is located inside of the body
  • Trachea (tubes) are throughout the body, carrying air to every tissue and cell
  • Spiracles are openings at the body surface that lead to the tracheae which then branch into the tracheoles (smaller tubes).
  • Gases enter and leave through the spiracles ( 
  • Spiracles are usually grouped in 10's and surrounded by hairs to prevent water loss
  • Spiracles open into a system of Traceae (which are supported by chitin)
  • In times of high activity, the increase in metabolic rate rasies the solute concentration of cells which means water moves out the traceoles, and more air is drawn into the tracheal system.
  • Higher  Activity= high metabolic rate = high solute concentration = H2O lower in tracheoles = more air
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Gas exchange: Fish (gills)

  • Gills have between 2-4 filliments to increase exchange surface area, increasing the diffusion of oxygen from the water.
  • Deoxygenated blood flows in the gills.
  • Water holds less oxygen than air, so gas exchange system needs to be effcient.
  • Water and blood flow in different directions (counter current flow)

  • As a result the water is always moving on to blood with less oxygen in it.
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Gas exchange: Counter current flow

  • Blood and water flow in opposite directions, therefore diffusion from water to blood is ALWAYS taking place.
  • There is a fairly constant rate of diffusion across the gills, around 80% of oxygen is diffused from the water passing through the gills and into the fish.
    • If blood and water flowed in the same direction only 50% of oxygen would diffuse.


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Gas exchange: Plants

  • Gases needed for Photosynthesis and respiration
  • Leaves help the exchange by:
    • Having a thin flat shape which provides a large surface area
    • Stomata allow many rotes into and out of the leaf
    • Spongy mesophyll layer offers interconnected air spaces (allows gases to diffuse around easily)

  • Stomata:
    • Tiny pores, surrounded by guard cells
    • Allows control of gas exchange and control of water loss (via evaporation)
    • Guard cells control the opening and closing of the Stomata (by manipulating water potential)
      • Guard cells turgid (swollen) means stomata open
      • Guard cells shrunken means stomata closed
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  • Haemoglobin - A group of proteins molecules that have a quaternary structure (made of four polypeptides)
  • The role of Haemoglobin is to transport oxygen. To be efficent it must:
    • Readily associate with oxygen at the gas exchange surface
    • Readily diassociate with oxygen at the tissues requiring it
    • It achieves this by changing its affinity for oxygen under different conditions
  • There are many different types of Haemoglobin which exhibit different properties related to how they take up and release oxygen. At each end of the scale;
    • High affinity for oxygen - takes up oxygen more easily, releases it less readily
    • Low affinity for oxygen - takes up oxygen less easily, releases it more readily
  • Affinity of Haemoglobin at different regions of the body:
    • Gas exchange surface, high oxygen concentration, low carbon dioxide concentration, means high affinity for oxygen, so oxygen is attached readily.
    • Respiring tissues, low oxygen concentration, high carbon dioxide concentration, means low affinity for oxygen, so oxygen is released readily.
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Oxygen dissociation curve

  • Haemoglobin is a respiratory pigement that easily picks up and releases oxygen.
  • High affinity of oxygen when concentration is high and low affinity when concentrations are low. (oxygen concentration is measured in partial pressure, PP, or oxygen tension)
  • When haemoglobin is exposed to a gradual increase in partial pressure it absorbs oxygen easily at first and then less readily as the partial pressure increases.
    • This relationship is the oxygen dissociation curve.


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Haemoglobin & carbon dioxide (Bohr effect)

  • Haemoglobin has a reduced affinity for oxygen when carbon dioxide is present.
    • Greater carbon dioxide concentration means haemoglobin releases its oxygen more readily. (This is know as the Bohr effect)
  • This explains affinity changes in different regions of the body:
    • Gas exchange surface - low carbon dioxide levels (as it diffuses out). Affinity for oxygen increases + high oxygen concentration = oxygen is readily loaded.
    • Respiring tissues - high carbon dioxide levels (aerobic respiration). Affinity for oxygen is reduced + low oxygen concentration = oxygen is readily unloaded.
    • Line shifted to the right = Bohr effect  (
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Oxygen dissociation curve in other animals

  • The curve can shift left or right for animals for many reasons.
  • (
  • For larger and more inactive animals (e.g. elephant, lugworm)the curve shifts to the left. The haemoglobin takes up oxygen more readily this happens for many reasons; size of the animal (oxygen needs to reach every tissue), or to prevent loss of oxygen in a oxygen deprived environment.
  • For smaller and more active animals (e.g. mouse) the curve shifts to the right. The haemoglobin releases oxygen more readily, this happens because; high metabolic rate (more active) large surface area to volume ratio (oxygen is more available)
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Circulatury systems, why?

  • An efficent supply of materials over large distances requires mass transport systems.
  • Specialized exchange surfaces are needed to absorb nutrient and respiratory gases, and to remove excretory products, these are located in specific regions of ALL organisms.
  • Whether or not there is a specilised transport medium (blood) and whether or not there is a circulatory pump (heart) depends on two factors:
    • The surface area to volume ratio
    • How active the organism is
  • The lower the surface area to volume radio and the more active an organism is, the greater need for a specialised transport system with a pump.
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Circulatory system features

  • Transport systems of many organisms have many common features:
    • Suitable medium to carry materials, e.g. blood (usually a water based liquid)
    • Form of mass transport in which the transport medium is moved in bulk over large distances
    • Closed tubular system that contains the transport medium (branching network to reach all the tissues of the organism)
    • Mechanism for moving the transport medium, (requires pressure difference between two parts of the system) achieved two main ways:
      • Animals - Muscular contractions either of the body or specialised pumping organ
      • Plants - No muscles, rely on passive (no enery required) natural physical processes, e.g. evaporation of water
    • Machanism to maintain the mass flow of movement in one direction, e.g. valves
    • Means of controlling the flow of the transport medium to suit the changing needs of the different parts of the organism.
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Circulatory system, mammals

  • Closed blood system, blood is confinded to vessels
  • Muscular pump (heart) circulates blood around the body
  • Double circulatory system (blood passes through the heart twice for each complete circuit of the body)
    • Blood passes through the lungs at a lower pressure
    • If blood passes immediately from the lungs to the body, the pressure would be too low and circulation would be very slow
    • Blood therefore returns to the heart to be put under higher pressure and pumped to the rest of the tissue, making circulation quick.
  • Substances are delivered to the body quickly (necessary as mammals have high body temperature, hence high rate of metabolism)
  • Three types of vessels: Arteries, Veins and Capilliares (reference unit 1)
  • Transport system, used to move substances long distance
  • Substances diffuse into cells
    • Rapid because it takes place over large surface area, across short distances and there is a steep diffusion gradient.
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Circulatory system, mammals diagram

  • Plan of mammalian circulatory system:(
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Arteries, Veins and Capillaries

  • Arteries
    • Narrow lumen - high pressure
    • Highly elastic - expand and recoil
    • Thick muscular wall - to withstand force; more elastic fibres (recoil)
    • No valves (except aortic and pulmonary semi-luner at the start)
    • Carries oxygenated blood from the heart (except pulmonary artery)
    • Pulsatile blood flow, caused by expansion and recoil (pulse can be felt)
  • Veins
    • Wide lumen - low pressure
    • Thin wall - less elastic fibres and less muscular tissue
    • Valves - prevent backflow of blood
    • Carries deoxygenated blood to the heart (except pulmonary vein)
    • Non pulsatile, smooth flow
  • Capillaries
    • Large number - creating large surface area for exchange
    • Wall, one cell thick - short diffusion distance
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Tissue fluid

  • The fluid which allows the exchange of substances between the blood and cells
  • (
  • Substances found in the tissue fluid:
    • Delivered to the cells - Glucose, amino acids, fatty acids, salts and oxygen
    • Removed from the cells - Carbon dioxide and waste substances
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Tissue fluid movement

  • Hydrostatic pressure
    • As capillaries are narrower than the arteries, a pressure builds up which forces fliud tissue out of the blood plasma - HYDROSTATIC PRESSURE
    • This pressure is resisted by:
      • pressure of tissue fluid on the capillaries (from outside)
      • The lower water potential (concentration) of the blood (caused by plasma protiens - too large to leave blood)
    • Overall, pressure pushes tissue fliud and small molecules out of the capillary leaving cells and large proteins behind - ULTRAFILTRATION
  • Most tissue fluid is returned to the blood plasma via capillaries
    • Hydrostatic pressure at the end of the capillary is higher outside the capillary and tissue fluid is forced back in.
    • Osmotic forces (resulting from protiens in the plasma) pull water back into the capillaries
  • Remaining tissue fluid enters the lympth vessles, goes back into veins close to the heart
  • Lympth - moved by:
    • Hydrostatic pressure & contractions of the body muscles (aided by valves)
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