Why did I lose the mark?

The answer that scoresA light microscope resolves to about 0.2 µm, so it shows the nucleus, the cell wall, and at best the chloroplasts and mitochondria as shapes, but it cannot resolve ribosomes or the two membranes of the nuclear envelope. Because resolution depends on the wavelength of the radiation used, an electron microscope is needed: electrons have a far shorter wavelength than light, and a transmission electron microscope resolves to about 0.5 nm, which is enough to see ribosomes and the detail of membranes.

The answer that scoresactual size = image size / magnification = 40 mm / 5000 = 0.008 mm. Converting to micrometres: 0.008 mm x 1000 = 8 µm. A chloroplast about 8 µm across is a realistic size, and writing the unit with the value is part of the answer.

The answer that scoresPlant cells contain mitochondria, just as animal cells do, and a plant cell that photosynthesises also contains chloroplasts. Chloroplasts make carbohydrate and mitochondria release energy from it as ATP, and the cell needs ATP for active transport, protein synthesis, the movement of vesicles and cell division. A cell that works hard, such as a root hair cell taking up mineral ions by active transport, is packed with mitochondria even though it has no chloroplasts at all.

The answer that scoresA virus is a non-cellular structure made of a nucleic acid core surrounded by a protein capsid, and some viruses also have an outer envelope made of phospholipid. It has no cytoplasm, no ribosomes and no way of making ATP, so it cannot reproduce on its own and must use the ribosomes, ATP and enzymes of a host cell. Bacteria, by contrast, are prokaryotic cells with a peptidoglycan wall, circular DNA and 70S ribosomes, which is why antibiotics that target the wall or the ribosome work against bacteria but not against viruses.

The answer that scoresSucrose is a disaccharide of glucose and fructose joined by a glycosidic bond between carbon 1 of the glucose and carbon 2 of the fructose, so neither ring keeps a free reducing group. It is therefore a non-reducing sugar and gives a negative Benedict's result. Heating it with dilute hydrochloric acid hydrolyses the glycosidic bond, and after the acid has been neutralised with sodium hydrogencarbonate a Benedict's test on the products gives a brick-red precipitate, which shows that reducing sugars have been released.

The answer that scoresAdd dilute hydrochloric acid and heat the solution in a water bath to hydrolyse the glycosidic bonds and release the monosaccharides. Then add sodium hydrogencarbonate a little at a time until the solution stops fizzing and pH paper shows it is no longer acidic. Only then add Benedict's solution and heat in a water bath: a brick-red precipitate in this second test shows that reducing sugar has been released, so the original solution contained a non-reducing sugar.

The answer that scoresStarch is made of alpha-glucose joined by alpha-1,4 bonds, with alpha-1,6 branches in amylopectin, so the coils are compact and the branched ends allow rapid hydrolysis by amylase - good for storage. Cellulose is made of beta-glucose joined by beta-1,4 bonds, with alternate molecules inverted, so the chains are straight and are held in bundles by hydrogen bonds between their -OH groups. Those microfibrils have a high tensile strength and are resistant to hydrolysis, which is exactly what a plant cell wall needs. Humans digest neither beta-1,4 bonds nor the cellulose they form, because they have no cellulase.

The answer that scoresThe primary structure is held by peptide bonds, which are strong covalent bonds and survive boiling, so the sequence of amino acids is unchanged. What heat destroys is the tertiary structure: the hydrogen bonds, ionic bonds and hydrophobic interactions between R groups break, so the chain unfolds and the precise shape of the molecule is lost. Any protein that depends on that shape, such as an enzyme with an active site or an antibody with a binding site, stops working. Disulfide bonds are also covalent and usually survive, which is why the primary structure can be recovered while the function cannot.

The answer that scoresBelow the optimum temperature the enzyme is inactivated but not denatured: the active site keeps its shape and full activity returns when the mixture is warmed. Only a temperature above the optimum gives enough kinetic energy to break the hydrogen bonds and ionic bonds holding the tertiary structure. That changes the shape of the active site so that it is no longer complementary to the substrate, and the enzyme is permanently denatured.

The answer that scoresTake absorbance readings at fixed time intervals from the moment the enzyme is added, plot absorbance against time, and take the rate of reaction as the gradient of the initial straight part of the curve, where the reaction is fastest. For an iodine end point the rate is 1 / time taken for the starch to disappear. Repeat each experiment and take a mean so that the gradient is reliable.

The answer that scoresA competitive inhibitor binds to the active site and raises the apparent Km, but at a high enough substrate concentration the substrate out-competes the inhibitor and the reaction still reaches the original Vmax. A non-competitive inhibitor is the one that lowers Vmax, because it binds away from the active site and changes the shape of the enzyme so that the active site is no longer complementary; adding more substrate does not overcome it, and Km is unchanged.

The answer that scoresThe same mass of immobilised enzyme normally breaks down less substrate in a given time than the free enzyme, for the reasons above. Immobilisation is still worthwhile because the enzyme can be recovered and reused many times, it does not contaminate the product so no purification step is needed to remove it, it is more stable to heat and to changes in pH so the reaction can be run at a higher temperature, and the process can be run continuously in a packed column.

The answer that scoresOsmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane. A dilute sugar solution has the higher (less negative) water potential and a concentrated sugar solution has the lower (more negative) water potential, so water moves from the dilute solution into the concentrated one, down the water potential gradient.

The answer that scoresFacilitated diffusion moves a substance down its concentration gradient through a channel protein or on a carrier protein and needs no ATP, so a cell short of ATP can still carry it out. Active transport uses a carrier protein that hydrolyses ATP to move a substance against its concentration gradient, so it stops when ATP is unavailable, for example when a root is treated with a respiratory inhibitor or the soil becomes waterlogged.

The answer that scoresThe 1 cm cube has the larger surface area to volume ratio. Its surface area is 6 x 1 x 1 = 6 cm2 and its volume is 1 cm3, giving 6:1. The 3 cm cube has a surface area of 6 x 3 x 3 = 54 cm2 and a volume of 27 cm3, giving 2:1. In the agar experiment the 1 cm cube changes colour first, because the acid reaches its centre over a shorter distance and every part of it is close to the surface.

The answer that scoresWater moved out of the potato cells by osmosis, from the higher water potential of the cell contents to the lower water potential of the 0.6 mol/dm3 sucrose solution, so the cylinders lost mass. The sucrose molecules are too large to cross the partially permeable cell surface membrane, so they stay outside the cells and lower the water potential there. If the cylinders are not blotted dry before they are reweighed, surface water adds mass and makes the loss look smaller than it really is.

The answer that scoresInterphase is the part of the cell cycle between divisions, and it is when the cell carries out its normal functions and prepares to divide. In G1 the cell grows and synthesises proteins and organelles, in S phase the DNA is replicated so that every chromosome consists of two sister chromatids, and in G2 the cell grows further and makes the proteins needed for the spindle. Interphase is much longer than mitosis, so most cells in a dividing tissue are in interphase at any moment.

The answer that scoresCells with their chromosomes arranged along the equator of the cell are in metaphase, and cells with two sets of chromosomes moving apart towards the poles are in anaphase. In a root tip squash most of the cells are in interphase, because interphase is much the longest part of the cell cycle, and only a small proportion are in prophase, metaphase, anaphase or telophase. Score the proportions by counting several fields of view and dividing the number of cells in mitosis by the total number of cells counted.

The answer that scoresIn an animal cell, cytokinesis happens when a ring of microfilaments inside the cell surface membrane contracts and forms a cleavage furrow, drawing the membrane inwards until the cell is pinched in two. In a plant cell, vesicles from the Golgi apparatus carry wall material to the equator and fuse to form a cell plate, which grows outwards to meet the existing cell wall. The plate becomes the middle lamella and a new cell surface membrane and cellulose cell wall are laid down on each side of it, so each daughter cell has its own wall.

The answer that scoresBoth benign and malignant tumours are masses of cells formed by uncontrolled cell division, but they behave differently. A benign tumour stays as a compact, encapsulated mass and does not invade the surrounding tissue or spread elsewhere, although it can cause problems by pressing on other structures. A malignant tumour invades and destroys the tissue around it, and its cells break away and travel in the blood or lymph to form secondary tumours, which is metastasis. The loss of control in both cases begins with a mutation in a gene that regulates the cell cycle, such as a proto-oncogene becoming an oncogene or the loss of a tumour suppressor gene such as TP53.

The answer that scoresThe leading strand is built continuously in the 5' to 3' direction, in the same direction as the fork is opening. DNA polymerase can only add nucleotides to a free 3' end, so on the lagging strand it must add nucleotides in short lengths in the 5' to 3' direction away from the fork, restarting each time more template is exposed. The short fragments are then joined into one continuous strand by DNA ligase, which forms the phosphodiester bonds between them.

The answer that scoresAfter one generation in nitrogen-14 there is a single band of intermediate density, because every DNA molecule contains one strand of the original heavy DNA and one newly synthesised light strand, which is semi-conservative replication. After two generations there are two bands, one intermediate and one light, and the presence of the light band shows that the strands made in the second generation were copied from light templates as well as heavy ones.

The answer that scoresThe code is degenerate because there are 64 possible triplets and only 20 amino acids, so some substitutions do not change the amino acid that is coded for. In the HBB gene the substitution changes the coding triplet from GAG to GTG, and although both triplets start with G, the third base G codes for glutamic acid while GUG codes for valine. The altered codon gives the beta chain a different amino acid, so the haemoglobin molecules stick together and the shape of the red cell changes.

The answer that scoresThe three bases after the deletion are read as a new triplet, so the reading frame is shifted from the mutation onwards. Nearly every amino acid after that point is different from the one the gene specified, and a stop codon is usually reached prematurely, so the polypeptide is both wrong in sequence and much shorter than normal. A deletion of three bases removes one whole amino acid without shifting the frame: the common mutation causing cystic fibrosis deletes three bases of the CFTR gene, so the protein lacks phenylalanine at position 508 and cannot fold correctly.

The answer that scoresTranspiration has two stages. First, water evaporates from the internal surfaces of the leaf: the film of water on the mesophyll cells and the cellulose walls exposed to the air spaces. Second, water vapour diffuses from the air spaces, which are saturated with water vapour, through the open stomata into the drier atmosphere outside, down a water vapour concentration gradient. The waxy cuticle is almost impermeable, which is why nearly all the water leaves through the stomata.

The answer that scoresWater is pulled up the xylem by the transpiration pull. Evaporation from the mesophyll cells lowers the water potential in the leaf, so water is drawn out of the xylem and a tension is set up in the column of water inside the vessel. Hydrogen bonding between water molecules (cohesion) transmits that tension down the column without it breaking, and adhesion of the water to the cellulose and lignin of the vessel wall holds the column against the walls and against gravity. Water then enters the root hair cells by osmosis from the soil, keeping the column continuous. Root pressure adds only a small amount, mainly when transpiration is slow, such as at night.

The answer that scoresThe Casparian strip of suberin is impermeable to water, so it blocks the apoplast pathway through the endodermal cell walls. Water must therefore enter the cytoplasm of an endodermal cell, crossing its partially permeable cell membrane, and continue through the symplast pathway into the xylem. This makes the endodermis a checkpoint: because a membrane has to be crossed, the root controls which ions and how much water enter the xylem, and the mineral ions that have been actively pumped into the xylem cannot leak back out through the cortex.

The answer that scoresThe companion cell loads the sucrose. It uses ATP from its many mitochondria to drive proton pumps that move hydrogen ions (H+) out of the cell into the apoplast. The H+ then flows back in down its concentration gradient, through a cotransporter protein, and the energy released carries sucrose into the companion cell at the same time, against the sucrose concentration gradient (cotransport). The sucrose then passes through the plasmodesmata into the sieve tube element, lowering its water potential so that water follows by osmosis from the xylem.

The answer that scoresThe left ventricle pumps blood into the systemic circulation and around the whole body, whereas the right ventricle pumps blood only to the lungs, which are close by. The systemic circulation is much longer and offers much greater resistance to flow, so a much higher pressure is needed and therefore a much thicker layer of cardiac muscle to produce the greater force of contraction. The pressure in the pulmonary circulation is kept low because the walls of the alveoli are extremely thin and a high pressure would force tissue fluid out of the lung capillaries into the alveoli. The atria have the thinnest walls of all because they only push blood a short distance into the ventricle below them.

The answer that scoresValves are passive: each one opens when the pressure on the side it leads to falls below the pressure behind it, and closes when that pressure reverses. At the start of ventricular systole the ventricles contract and the pressure inside them rises above the pressure in the atria, so the AV valves are forced shut, which makes the first heart sound; their chordae tendineae and papillary muscles stop them turning inside out. During diastole the ventricles relax, the pressure inside them falls below the atrial pressure, and the AV valves are pushed open again so that the ventricles refill. The semilunar valves do the same job at the base of the aorta and the pulmonary artery.

The answer that scoresAt the arterial end of the capillary the hydrostatic pressure of the blood is higher than the osmotic pull of the plasma proteins, so water, glucose, amino acids, ions and oxygen are forced out through the capillary wall and its pores by pressure filtration, forming tissue fluid. Blood cells and large plasma proteins cannot fit through, so they stay in the capillary. At the venous end the hydrostatic pressure has fallen and the plasma proteins give the blood a lower water potential than the tissue fluid, so water re-enters by osmosis. The fluid that is not returned is collected by lymph capillaries and returned to the blood through the lymphatic system and the subclavian vein.

The answer that scoresA small fraction of the carbon dioxide dissolves in the plasma, a small fraction combines with the amino groups of haemoglobin to form carbaminohaemoglobin, and the great majority is carried in the plasma as hydrogencarbonate ions. Inside the red blood cell, carbonic anhydrase catalyses CO2 + H2O -> H2CO3, which dissociates into H+ and HCO3-. The H+ ions are buffered by haemoglobin, forming haemoglobinic acid, and the hydrogencarbonate diffuses out into the plasma. The chloride shift is the inward movement of chloride ions that accompanies this, and its importance is that it keeps the charge inside the cell balanced so that a large amount of carbon dioxide can be carried as hydrogencarbonate without an electrical gradient stopping the process.

The answer that scoresDuring inspiration the diaphragm contracts and flattens and the external intercostal muscles raise the ribs, so the volume of the thorax increases. Because the volume has increased, the pressure inside the thorax and the airways falls below atmospheric pressure, and air flows down that pressure gradient through the trachea and bronchi into the alveoli. In the bell-jar model the same thing happens when the rubber sheet is pulled down: the balloons inflate because the pressure inside the jar has fallen, not because the sheet pushes air into them.

The answer that scoresCartilage forms C-shaped rings in the trachea and irregular plates in the bronchi, and it holds those airways open against the fall in pressure that occurs during inhalation. The bronchioles have no cartilage; their diameter is controlled by smooth muscle. The alveolar wall is only one squamous cell thick, so that the diffusion distance is kept to a minimum, and the alveoli are prevented from collapsing by surfactant and by the pressure in the pleural cavity, not by cartilage.

The answer that scoresThe diagram should be drawn with a sharp pencil in single, continuous, unbroken lines showing the boundaries between the layers, in proportion and filling the space available, with no shading, no colour and no individual cells. Each label line should touch the structure it names and carry only that name, such as 'cartilage', 'ciliated epithelium' or 'smooth muscle', and the drawing should carry a title such as 'transverse section of the wall of the trachea'.

The answer that scoresOxygen diffuses from the alveolar air into the blood because the concentration of oxygen in the alveolar air is higher than in the blood arriving in the pulmonary capillaries, which has just returned from the respiring tissues. Ventilation keeps the oxygen concentration in the alveolar air high and blood flow carries the oxygen away, so the difference in concentration is maintained. Because the alveolar wall and the capillary wall are each one cell thick, the distance is short and the diffusion is very rapid.

The answer that scoresAntibiotics work on bacteria, so they are effective against Mycobacterium tuberculosis, which has a peptidoglycan cell wall that penicillin-type drugs can disrupt. They have no effect on HIV, which is a retrovirus: with no cell wall, no ribosomes and no enzymes of its own, it offers no target. HIV is treated with antiretroviral drugs such as reverse transcriptase inhibitors, which block an enzyme the virus brings with it and the human cell does not have.

The answer that scoresMalaria is caused by protoctists of the genus Plasmodium, such as Plasmodium falciparum, and is transmitted by the bite of an infected female Anopheles mosquito. The mosquito picks up gametocytes when it feeds on an infected person; sexual reproduction takes place in the mosquito gut and the parasite reaches the salivary glands as sporozoites. When the mosquito feeds again, sporozoites are injected into the blood of a new host, where the parasite multiplies in the liver and then inside red blood cells.

The answer that scoresThe control measures work by breaking the faecal-oral route: treat drinking water by filtration, chlorination or boiling; build and maintain sewage disposal and sanitation so that faeces cannot reach a water supply; and educate people about hand washing and about not using rivers for both waste and drinking water. Treatment of individual patients is oral rehydration therapy to replace water and ions, with antibiotics only as an additional measure in severe cases.

The answer that scoresMutations that give resistance occur at random, before the antibiotic is used, so a bacterial population contains variation. When the antibiotic is present the sensitive bacteria are killed while the resistant mutants survive, and because bacteria reproduce rapidly by binary fission the resistant genotype quickly becomes the most common in the population. Resistance genes can also pass between bacteria on plasmids, which is why resistance spreads so fast. Using antibiotics only when necessary, completing the course and reducing their use in farming lower the selection pressure.

The answer that scoresPhagocytes such as neutrophils and macrophages carry out the same non-specific response to any pathogen. The phagocyte is attracted to the site by chemicals, binds to non-self antigens on the surface of the pathogen, engulfs it by endocytosis to form a phagosome, and digests it with hydrolytic enzymes released from lysosomes. The specific response is provided by lymphocytes, and the link between the two is that a macrophage displays the antigens of the digested pathogen on its own surface membrane, which activates the T-helper cell whose receptor fits that antigen.

The answer that scoresAn antibody binds to an antigen on the surface of a pathogen or to a toxin. Binding can agglutinate pathogens into clumps, neutralise a toxin by covering its active site or a virus by covering its attachment proteins, or act as a marker so that the constant region binds to receptors on a phagocyte and stimulates phagocytosis. Antibodies bound to a bacterial surface can also activate the complement system, which makes pores in the cell surface membrane so that the bacterium bursts. The antibody does not digest the pathogen; the phagocyte's lysosomes do that.

The answer that scoresA vaccine contains antigens - killed or weakened pathogens, toxoids or purified antigen subunits, often with an adjuvant. Those antigens stimulate the person's own macrophages, T-helper cells and B-lymphocytes, so plasma cells secrete antibody and memory cells are produced without the person suffering the disease. Because the person's own lymphocytes did the work, this is active immunity and it is long-lasting. Passive immunity is different: it results from antibodies made elsewhere being injected, such as tetanus antitoxin after a wound or anti-D immunoglobulin for a rhesus-negative mother, and it protects at once but only for a few weeks.

The answer that scoresA T-helper cell carries receptors complementary to an antigen presented on the surface of a macrophage. When it binds, it is activated, divides by mitosis and releases cytokines, which stimulate phagocytes, the T-killer cells and the B-lymphocyte whose antibody receptor fits the same antigen. A T-killer cell, once activated, binds to a body cell that is displaying that antigen - an infected cell or a cancer cell - and makes its cell surface membrane permeable, so hydrolytic enzymes inside the target cell are activated and the cell dies. Destroying the infected cell prevents the pathogen from multiplying inside it.

The answer that scoresGlycolysis happens in the cytoplasm; the link reaction and the Krebs cycle happen in the mitochondrial matrix, where their enzymes are dissolved; oxidative phosphorylation happens on the inner mitochondrial membrane, where the carriers of the electron transport chain and the ATP synthase molecules are embedded. The inner membrane is folded into cristae to provide a large surface area for those proteins.

The answer that scoresThe alkali absorbs the carbon dioxide that the seeds produce, so the only gas that changes the volume in the tube is oxygen. As the seeds use oxygen the pressure inside falls and the fluid droplet moves towards the seeds, so the distance moved divided by the time gives the rate of oxygen uptake. To find the RQ you also need a second respirometer with no alkali, where the movement shows the difference between the carbon dioxide produced and the oxygen used, and then RQ = carbon dioxide produced / oxygen used.

The answer that scoresIn a muscle that is short of oxygen, glycolysis still converts glucose to pyruvate with a net gain of two ATP per glucose and two molecules of reduced NAD. The pyruvate is then reduced to lactate, using hydrogen from the reduced NAD, which regenerates NAD so that glycolysis can continue. No carbon dioxide is produced. The lactate passes into the blood to the liver, is converted back to pyruvate, and can be respired aerobically later.

The answer that scoresFor oleic acid, C18H34O2 + 25.5O2 -> 18CO2 + 17H2O, the RQ is 18/25.5 = 0.71, well below 1. An RQ greater than 1 only happens when carbon dioxide is produced without oxygen being used, which occurs when some respiration is anaerobic, or when the substrate is an organic acid that already contains a great deal of oxygen, such as malic acid with an RQ of 4/3 = 1.33.

The answer that scoresIn non-cyclic photophosphorylation the oxygen-evolving complex catalyses the photolysis of water: 2H2O -> 4H+ + 4e- + O2. The electrons replace those lost by the reaction centre of photosystem II and the protons are released into the thylakoid space. Carbon dioxide is fixed much later, in the stroma, where rubisco combines it with RuBP to give two molecules of GP.

The answer that scoresCyclic photophosphorylation uses only PSI, and the only product is ATP. It is needed because the Calvin cycle uses more ATP than reduced NADP, so the extra ATP is made this way. Non-cyclic photophosphorylation is the route that uses both PSI and PSII, splits water and produces ATP, reduced NADP and oxygen.

The answer that scoresThe absorption spectrum of chlorophyll a has peaks at about 430 nm in the blue-violet and about 660 nm in the red, with a trough in the green. The action spectrum for photosynthesis matches these peaks, which shows that the pigments absorbing the light are the ones driving photosynthesis; the small amount of photosynthesis that happens in green light is the contribution of the accessory pigments carotene and xanthophyll.

The answer that scoresThe grower's result shows only that temperature was the limiting factor over the range tested. The rate of photosynthesis has an optimum temperature, because the Calvin cycle is enzyme-catalysed: up to the optimum the enzymes and substrates collide more often, and above it the tertiary structure of rubisco and the other enzymes is destroyed by denaturation, so carbon fixation slows and then stops. A warmer glasshouse also increases the rate of respiration, which uses up the products of photosynthesis, so the net gain in yield is smaller than the increase in gross photosynthesis.

The answer that scoresNeurosecretory cells in the hypothalamus synthesise ADH, which is stored in and released from the posterior pituitary gland into the blood. ADH binds to receptors on the cells of the collecting duct and increases the number of aquaporins in their cell surface membranes, so more water is reabsorbed by osmosis down the water potential gradient into the blood. A high concentration of ADH therefore produces a small volume of concentrated urine, and a low concentration, which is what follows drinking a large volume of water, produces a large volume of dilute urine.

The answer that scoresUltrafiltration is filtration under pressure. Water, glucose, amino acids, salts and urea are forced out of the fenestrated glomerular capillaries, through the basement membrane and between the pedicels of the podocytes, into the capsular space. Blood cells and large plasma proteins remain in the capillary, so a healthy glomerular filtrate contains no cells and no protein. Protein appearing in the urine is therefore a sign that the filtration barrier has been damaged.

The answer that scoresGlucagon binds to a specific receptor on the cell surface membrane of the liver cell, and the binding causes a conformational change that activates a G-protein, which in turn stimulates adenylyl cyclase. Adenylyl cyclase converts ATP into cyclic AMP, the second messenger, and cAMP activates protein kinase A, which begins an enzyme cascade. The cascade ends with the activation of glycogen phosphorylase, which catalyses the breakdown of glycogen to glucose, so the blood glucose concentration rises.

The answer that scoresAbscisic acid binds to receptors on the guard cell membrane and causes calcium ions to be released into the cytosol, where they act as a second messenger. The calcium ions open channels so that chloride and potassium ions leave the guard cells, which raises the water potential inside them, so water leaves by osmosis. The guard cells become flaccid, they straighten, and the stoma closes, which reduces transpiration during water stress.

The answer that scoresThe action potential is produced by diffusion through voltage-gated channels, not by the pump. A stimulus opens voltage-gated sodium channels, and sodium ions diffuse into the axon down their electrochemical gradient, so the inside becomes less negative and then positive, reaching about +40 mV. The sodium channels then inactivate and voltage-gated potassium channels open, so potassium ions diffuse out and the inside becomes negative again, which is the repolarisation. The sodium-potassium pump then restores the sodium and potassium concentration gradients, using ATP, so that the neurone can conduct another impulse.

The answer that scoresThe myelin sheath insulates the axon, so the only places where ions can cross the membrane and an action potential can be generated are the nodes of Ranvier. Depolarisation at one node sets up a local circuit of current that flows through the axoplasm of the myelinated stretch to the next node and depolarises the membrane there, so the action potential appears to jump from node to node: this is saltatory conduction. Because action potentials are generated only at the nodes, fewer sodium and potassium ions cross the membrane, so fewer sodium-potassium pump cycles are needed and the neurone uses less ATP, as well as conducting much faster.

The answer that scoresIn the knee-jerk reflex the tap stretches the quadriceps tendon and stimulates stretch receptors in the muscle. Impulses travel along a sensory neurone into the spinal cord, where the sensory neurone synapses directly with a motor neurone, and the motor neurone carries impulses out to the quadriceps, which contracts so that the lower leg kicks forward. Only two neurones are involved and the pathway is short, so the response happens before the brain can be consulted, and the same stimulus at the same time inhibits the motor neurones of the antagonistic flexor muscles so that they do not oppose the movement. Branches of the sensory neurone also pass impulses up to the brain, so the person is aware of the tap, but the brain plays no part in producing the movement and cannot prevent it.

The answer that scoresThe myosin heads attach to the exposed binding sites on the actin filaments to form cross-bridges and then change angle, pulling the actin filaments towards the centre of the sarcomere, so the actin filaments slide over the myosin filaments and the filaments themselves stay the same length. The Z lines are drawn closer together and the sarcomere therefore shortens. Because the actin filaments slide further in among the myosin filaments, the I band and the H zone become narrower and can almost disappear, while the A band remains exactly the same width.

The answer that scoresA gamete is haploid (n) and contains one chromosome from each homologous pair - 23 chromosomes in a human sperm or egg. The pairs exist in the diploid (2n) parent cell before meiosis and are separated in anaphase I, so one chromosome of each pair goes into each gamete. At fertilisation the two haploid sets come together and restore the diploid number, so the homologous pairs are re-formed in the zygote.

The answer that scoresConvert the ratio to proportions first. Round and yellow is 9/16, so out of 64 offspring you expect 9/16 x 64 = 36 round and yellow. Round and green is 3/16 x 64 = 12, wrinkled and yellow is 3/16 x 64 = 12, and wrinkled and green is 1/16 x 64 = 4, and the four expected numbers add up to 64.

The answer that scoresTwo purple-flowered plants could be PP or Pp. A test cross distinguishes them: cross each plant with a white-flowered plant, which must be pp. If all the offspring are purple the parent was probably PP, because every offspring received a P allele; if about half the offspring are white the parent was Pp, because its p gametes produced white offspring with the pp parent.

The answer that scoresWith 1 degree of freedom the critical value of chi-squared at the 0.05 probability level is 3.84. The calculated value of 2.13 is smaller than 3.84, so the difference between the observed and expected numbers is not significant: the results are consistent with a 3 : 1 ratio, and the deviation is due to chance. The number of degrees of freedom and the probability level must both be stated.

The answer that scoresA random mutation has already given a few bacteria an allele for resistance before the antibiotic is used. When the antibiotic is given, the bacteria without the allele are killed while the resistant ones survive and reproduce rapidly by binary fission, so the frequency of the resistance allele in the population rises and the antibiotic no longer works. Resistance can also be passed between bacteria on plasmids by conjugation, which spreads it even faster.

The answer that scoresp and q are allele frequencies that add up to 1, while p2, 2pq and q2 are genotype frequencies: p2 for homozygous dominant, 2pq for heterozygous and q2 for homozygous recessive. If 9 people in every 100 show the recessive phenotype then q2 = 0.09, so q = 0.3 and p = 0.7; 2pq = 0.42 means 42 people in every 100 are carriers who show no sign of the condition.

The answer that scoresAllopatric speciation needs three things: a barrier that stops gene flow between the two populations; different selection pressures and independent mutations, so the allele frequencies of the two gene pools diverge; and finally reproductive isolation, so that even if the populations meet again they can no longer interbreed to produce fertile offspring. Only then are they separate species. Sympatric speciation, by contrast, can occur with no barrier at all, through ecological or behavioural isolation between groups living in the same area.

The answer that scoresNatural selection is a non-random change in allele frequencies caused by differences in survival and reproductive success. Genetic drift is a random change caused by chance events, and both the founder effect and the bottleneck effect increase its importance because they reduce the population to a small sample of the original alleles: the colonists of a new area, or the few survivors of a disaster, carry only a random sample, so allele frequencies differ from the source population even though no allele is better than another.

The answer that scoresThe biological species concept defines a species as a group of organisms with similar characteristics that can interbreed to produce fertile offspring and that do not normally interbreed with other groups. If the two populations meet and interbreed freely, they are one species; if they do not interbreed, or their offspring are infertile, they are separate species even when they look alike. The common pipistrelle and the soprano pipistrelle are an example of two species that were separated only when their echolocation calls and DNA were compared, not by their appearance. Where the biological concept cannot be applied, for fossils or asexual organisms, morphology or ecology is used and the limitation is stated.

The answer that scoresThe total number of individuals is N = 40 + 30 + 20 + 10 = 100. The squares of the counts add to 1600 + 900 + 400 + 100 = 3000, so the sum of (n / N)2 is 3000 / 10000 = 0.30 and D = 1 - 0.30 = 0.70. A value of 0.70 is close to 1, so diversity is fairly high, which agrees with the data: four species are present and no single species makes up more than 40 per cent of the individuals.

The answer that scoresA frame quadrat is a square frame of known area, placed at random coordinates to estimate abundance in an area, and its abundance can be recorded as density, percentage cover or frequency. A line transect is a tape stretched along the gradient, on which the species touching the tape or the points where each species begins are recorded, which shows distribution along the gradient but not abundance. A belt transect is a strip of known width sampled at regular intervals along the tape, so that both the distribution and the abundance of each species are recorded at each point along the gradient; light intensity is measured at the same points with a light meter so that the two sets of data can be correlated.

The answer that scoresIn situ conservation protects the habitat and the whole community at once, which is why conserved areas such as national parks and marine parks are the main method: they protect the species, the interactions between species and the potential for the population to keep evolving, and they protect species that could never be kept in captivity. Ex situ measures, including zoos, botanic gardens, frozen zoos and seed banks, are valuable back-ups that keep genetic material and provide animals for reintroduction, but they are used alongside habitat protection, not instead of it.

The answer that scoresThe donor DNA and the plasmid are both cut with the same restriction endonuclease, which recognises a specific base sequence and cuts the sugar-phosphate backbone, leaving complementary sticky ends. The gene and the opened plasmid then base pair through those sticky ends, and DNA ligase catalyses the formation of phosphodiester bonds between the sugar of one nucleotide and the phosphate of the next, so the gene becomes part of one continuous DNA molecule with the plasmid.

The answer that scoresTaq polymerase comes from the thermophilic bacterium Thermus aquaticus and is not denatured at about 95 °C, so it survives the step that separates the two strands and does not have to be replaced every cycle. Each cycle doubles the number of copies of the target sequence, because every strand made in one cycle acts as a template in the next, so after 20 cycles there are 2 to the power of 20, about one million, copies rather than 20.

The answer that scoresThe phosphate groups in the sugar-phosphate backbone give every DNA fragment a net negative charge, so the fragments move towards the positive electrode, the anode. The agarose gel acts as a sieve: small fragments pass through the pores easily and travel further in a given time, while large fragments are held back, so a band further from the wells contains shorter fragments. A sample of fragments of known length is run in one lane so that the distance moved can be converted into a length in base pairs.

The answer that scoresThe gene is made as complementary DNA instead. mRNA is extracted from human cells that are expressing the gene, such as pancreatic beta cells, and reverse transcriptase makes a DNA copy of it. Because mRNA has already been spliced, the cDNA contains only the coding sequence, so the bacterial ribosomes can translate it correctly into human insulin exactly as the human cell does. A strong bacterial promoter is transferred with the gene at the same time, because RNA polymerase from the bacterium will not bind to a human promoter, and a marker gene is included so that the bacteria that have taken up the plasmid can be identified.