Biology Paper 2 June 2004

1. Aa centriole, a barrel of nine triplets of microtubules that organises the spindle during cell division
2. Ba lysosome, a single membrane sac of hydrolytic enzymes that digests worn out organelles and bacteria
3. Ca mitochondrion, the site of the Krebs cycle and of oxidative phosphorylation in the cell
4. Da ribosome, a particle of two subunits of RNA and protein on which polypeptide chains are assembled

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1. Ais a single membrane that packages hydrolytic enzymes and keeps them away from the rest of the cytoplasm
2. Bis folded into cristae that carry the electron carriers used during oxidative phosphorylation in the cell
3. Cencloses the chromosomes and is pierced by pores through which messenger RNA passes to the cytoplasm
4. Dencloses the chromosomes and seals them off completely from the cytoplasm, which is where DNA is copied

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1. Athe surface tension of the water is high enough to support the ice, as it supports an insect walking on a pond
2. Bhydrogen bonds are broken as water freezes, so the molecules move further apart and the ice loses mass to the air above it
3. Chydrogen bonds hold the molecules in an open lattice, so a given mass of ice takes up more space than the same mass of water
4. Dthe molecules in the ice are colder and therefore lighter than the warmer water molecules in the liquid below them

1. Athe ice dissolves oxygen from the air into the lake, so aerobic respiration continues throughout the winter
2. Bthe ice insulates the water below it, so fish and other aquatic organisms live on in liquid water beneath
3. Cthe ice raises the temperature of the whole lake, so the metabolic rate of the fish living in it increases
4. Dthe ice reflects sunlight down into the water below, so the algae in the lake photosynthesise faster in winter

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1. Athe phosphate head attracts water while the tails repel it, so the molecules form the bilayer of a cell membrane
2. Bthe two hydrocarbon tails are oxidised during respiration, making this the cell's principal long term store of energy
3. Cthe phosphate group carries a charge that is passed along the membrane, which is how a nerve cell conducts an impulse
4. Dthe phosphate head repels water while the tails attract it, so the molecules line up as a single layer over the cytoplasm

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1. Aanaphase I and metaphase II, because the pairs are already separating in A and the two cells in C are dividing
2. Bmetaphase I and anaphase II, because the pairs line up in A and the chromatids are being pulled apart in C
3. Cmetaphase I and metaphase II, because homologous pairs line up in A and single chromosomes line up in C
4. Dprophase I and metaphase II, because the pairs are still condensing in A and the chromosomes line up in C

1. Achromatids of a homologous pair exchange equal lengths of DNA, putting new allele combinations on one chromatid
2. Bchromatids of a homologous pair are pulled to opposite poles, so each daughter cell keeps a full set of parent alleles
3. Cthe chromosome number is halved, so a gamete carries one allele of each gene instead of the two its parent had
4. Dthe DNA is copied a second time before the division, so extra alleles become available to the daughter cells

1. Aeach gamete receives chromosomes altered by a change in the base sequence of one of the genes they carry
2. Beach gamete receives an extra copy of one of the chromosomes, so the chromosome number of the offspring changes
3. Ceach gamete receives a different mixture of maternal and paternal chromosomes, one of many possible combinations
4. Deach gamete receives either the whole maternal set or the whole paternal set, giving two very different gametes

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1. Ain each organism the amount of adenine is close to that of guanine and thymine close to that of cytosine
2. Bin each organism the four bases occur in roughly equal amounts, each of them close to twenty five percent
3. Cin each organism the purine bases together are about twice as plentiful as the pyrimidine bases present
4. Din each organism the amount of adenine is close to that of thymine and guanine close to that of cytosine

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1. Aa purine is joined to a pyrimidine at every rung, so the two strands stay an even distance apart along the helix
2. Ba purine is joined to another purine at every rung, so the helix is wide enough for the bases to stack inside it
3. Cthe bases are joined to one another by strong covalent bonds, so the two strands hold together while it is copied
4. Dthe sugar phosphate backbone is turned inwards, where the bases shield it from the water surrounding the molecule

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1. Aconverting nitrate ions to nitrite ions and then nitrite ions to ammonium ions, which roots absorb
2. Bconverting nitrogen gas from the air into ammonia, using an enzyme that works in the absence of oxygen
3. Cconverting the protein of dead plants and animals into ammonia, which then escapes from the soil as a gas
4. Dconverting ammonium ions to nitrite ions and then nitrite ions to nitrate ions, which roots absorb

1. Aconvert ammonium ions into nitrite ions, which poison the root hair cells that take up water and minerals
2. Bconvert nitrate ions in the soil into nitrogen gas, which is then lost from the soil into the atmosphere
3. Cfix nitrogen gas from the air into ammonia, which raises the pH of the soil beyond the range most crops need
4. Dbreak the protein of dead organisms down to ammonia, which is then washed out of the soil by heavy rain

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1. Athe trees no longer take up mineral ions, so the ions build up and cement the soil grains into hard blocks
2. Bthe leaf litter left behind after felling seals the soil surface, so rainwater sinks straight down through it
3. Cthe roots that bound the soil particles together are gone and rain now strikes the bare surface directly
4. Dthe soil is left without shade, so it grows hot enough for the mineral ions in it to break down into gases

1. Athe bare ground reflects more of the sun's radiation back into the atmosphere, where the clouds hold on to it
2. Bthe soil bacteria in the open ground fix more nitrogen, and nitrogen is the main greenhouse gas in the air
3. Cthe felled trees no longer release oxygen, and it is the oxygen in the atmosphere that traps heat radiated from the sun
4. Dburning and decay of the felled trees release carbon dioxide, while the standing trees that took it up are gone

1. Athe rainfall over the whole catchment increases sharply once the trees are gone, so much more water enters the channel
2. Bthe river channel is deepened by the loss of the trees along its banks, so it carries the water away more slowly
3. Cthe soil left behind holds far more water than the forest floor did, and releases it into the river during a storm
4. Dthe canopy no longer intercepts the rain and the roots no longer draw water up, so more of it runs off the surface

1. Achemoreceptors detect the rise in carbon dioxide and the fall in blood pH, and the medulla sends more impulses to the intercostal muscles and the diaphragm
2. Bstretch receptors in the alveoli detect the extra oxygen dissolved in the blood and act on the rib muscles directly without involving the brain
3. Cthe heart muscle contracts more strongly and pushes the diaphragm downwards, drawing a larger volume of air into the lungs at each breath
4. Dchemoreceptors detect the fall in carbon dioxide and the rise in blood pH, and the medulla then sends fewer impulses to the intercostal muscles and the diaphragm

1. Apyruvate is oxidised to carbon dioxide and water in the mitochondria, which supplies the oxygen the muscle needs
2. Bpyruvate is reduced to lactate, which regenerates the NAD that glycolysis needs to keep producing ATP
3. Cpyruvate is converted to glycogen and stored in the liver until the oxygen supply to the muscle improves
4. Dpyruvate is broken down into ethanol and carbon dioxide, which releases enough ATP to keep the muscle contracting

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1. Athe lungs now hold fewer and larger air spaces, so there is less surface area for oxygen to diffuse across
2. Bthe lungs now hold more and smaller air spaces, so the air is trapped and cannot reach the blood capillaries
3. Cthe airways fill with mucus, so air cannot reach the alveoli at the end of the bronchioles at all
4. Dthe alveoli fill with tissue fluid, so oxygen has to dissolve in it before it can reach the red blood cells

1. Athe airways are widened, so air rushes past the alveoli too quickly for the oxygen in it to diffuse across
2. Bthe airways are narrowed, so airflow to the alveoli falls and the trapped mucus harbours bacteria that infect the lungs
3. Cthe ribs are held down, so the chest cavity cannot expand and the tidal volume falls below a hundred cubic centimetres
4. Dthe alveolar walls are destroyed, so the lungs lose their elastic recoil and stale air is trapped in the chest

1. Athe pleural membranes stick to one another, so the two lungs can no longer move independently of each other
2. Bthe loss of elastic tissue in the lungs makes it hard to force air out, so stale air is left behind in the chest
3. Cthe loss of elastic tissue in the lungs makes it hard to draw air in, so the ribs are pulled inwards instead
4. Dthe extra mucus produced in the lungs dissolves the oxygen before it can reach the blood in the capillaries

1. Acollect the carbon dioxide given off from hydrogen peroxide in a gas syringe, recording the volume collected each minute
2. Bmeasure the fall in temperature of the hydrogen peroxide with a thermometer as the reaction proceeds
3. Cweigh the enzyme left at the end of the reaction and compare it with the mass that was added at the start
4. Dcollect the oxygen given off from hydrogen peroxide in a gas syringe, recording the volume every thirty seconds

1. Ausing a different volume of hydrogen peroxide in each tube so that the results cover a wider range
2. Bkeeping the temperature, the pH and the hydrogen peroxide concentration the same in every tube tested
3. Ckeeping the temperature the same in every tube while allowing the pH to vary with the tissue used
4. Dchanging both the temperature and the pH together, so that the enzyme works at its own optimum in each tube

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1. Adoubles, because twice as many active sites are free to form enzyme substrate complexes
2. Bhalves, because the enzyme molecules now collide with one another instead of with the substrate
3. Cis unchanged, because the rate is fixed by the number of substrate molecules present at the start
4. Drises briefly and then drops to zero, because the extra enzyme uses up all of the substrate at once

1. Aevery active site is filled again as soon as it is free, so the enzyme concentration now limits the rate
2. Bthe substrate molecules begin to bind to one another rather than entering the active sites of the enzyme
3. Cthe enzyme is denatured by the high concentration of substrate, so its active sites change shape
4. Dthe product that has formed builds up and reacts with the substrate, turning it back into the enzyme itself

1. Avoltage gated sodium channels open and sodium ions flood into the axon, making the inside positive
2. Bchloride ions diffuse into the axon through open channels, making the inside of the membrane more negative
3. Cvoltage gated potassium channels open and potassium ions move inwards, making the inside strongly positive
4. Dvoltage gated sodium channels close and sodium ions are pumped out, making the inside far more negative

1. Athe sodium potassium pump works faster in a myelinated axon, so the resting potential is restored sooner
2. Bthe impulse jumps from one node of Ranvier to the next, since ions cross the membrane only at the nodes
3. Cthe myelin sheath conducts the impulse as a current, in the way that a copper wire carries an electric current
4. Dthe myelin sheath widens the axon, and a wider axon carries an action potential faster than a narrow one

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1. Acalcium ions leave the knob, vesicles form from the membrane and acetylcholine is taken back into the neurone
2. Bpotassium ions enter the knob, the mitochondria release acetylcholine and it diffuses back along the axon
3. Csodium ions cross the cleft through protein channels and depolarise the postsynaptic membrane without a transmitter
4. Dcalcium ions enter the knob, vesicles fuse with the membrane and acetylcholine crosses the cleft to receptors

1. Ait pumps sodium ions back across the postsynaptic membrane, so a second impulse can begin straight away
2. Bit builds acetylcholine from choline and ethanoic acid, so the knob holds a store ready for the next impulse
3. Cit breaks acetylcholine into choline and ethanoic acid, so the postsynaptic membrane is not held depolarised
4. Dit binds to the receptors of the postsynaptic membrane, so the sodium channels there are held open longer

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