ESAT Biology: Enzymes and animal physiology, set 2

• Answer all questions. No calculator.
• Each question has exactly one correct answer.

1. 11 mark

   Catalase, an enzyme found in liver cells, catalyses the breakdown of hydrogen peroxide into water and oxygen. In an experiment, a small, fixed volume of hydrogen peroxide solution is left in a test tube with a small piece of fresh liver, and the total volume of oxygen gas produced is measured. Which statement about catalase in this reaction is correct?

   1. A Catalase makes the breakdown of hydrogen peroxide happen at all; without catalase present, hydrogen peroxide would never break down into water and oxygen under any conditions.
   2. B Catalase is entirely used up as it breaks down the hydrogen peroxide, so a completely fresh piece of liver would be needed to break down a second batch of hydrogen peroxide.
   3. C At the end of the reaction, catalase is chemically unchanged, so the same liver sample could go on to break down further hydrogen peroxide if more were added.
   4. D Catalase works by increasing the activation energy needed for hydrogen peroxide to break down, which is why the reaction proceeds faster in its presence.
2. 21 mark

   A molecule with a shape very similar to an enzyme's usual substrate is added to a reaction mixture, and the rate of the enzyme-catalysed reaction falls, even though temperature and pH are unchanged. Which explanation best accounts for this observation, based on the shape of the enzyme's active site?

   1. A The similar molecule binds into the active site because its shape resembles the true substrate's shape, so it occupies the site and blocks the substrate from binding there as often.
   2. B The similar molecule denatures the enzyme by breaking bonds that hold its tertiary structure together, permanently changing the shape of the active site.
   3. C The similar molecule changes the enzyme's optimum pH, so the active site is no longer able to catalyse the reaction at the pH being used.
   4. D The similar molecule creates an alternative, lower-energy active site elsewhere on the enzyme molecule, and substrate molecules bind there instead of at the original active site.
3. 31 mark

   Pepsin is a protease enzyme found in the stomach, where the environment is strongly acidic (around pH 2), close to pepsin's optimum pH. Trypsin is a different protease found in the small intestine, where the environment is alkaline (around pH 8). A student mixes a sample of pepsin with a protein substrate at pH 8 instead of pH 2. Which outcome, with its explanation, is most likely?

   1. A The reaction rate is unaffected by the change in pH, because an enzyme's active site shape is fixed by its amino acid sequence and does not change with the surrounding pH.
   2. B The reaction rate is faster at pH 8 than at pH 2, because all protease enzymes work fastest in alkaline conditions regardless of where they are normally found in the body.
   3. C The reaction stops completely and permanently within seconds, because moving an enzyme even slightly away from its optimum pH always denatures it instantly and irreversibly.
   4. D The reaction rate is much lower than at pepsin's optimum pH, because pH 8 changes the shape of its active site, so fewer substrate molecules bind successfully.
4. 41 mark

   A student sets up four test tubes, each containing a different food sample mixed with one digestive enzyme, and tests each mixture with Benedict's test for reducing sugar before and after the enzyme has acted. In which test tube would you expect the Benedict's test result to change from negative (stays blue) to positive (brick-red precipitate) after the enzyme has acted?

   1. A Egg white (protein) mixed with protease.
   2. B Starch solution mixed with amylase.
   3. C Vegetable oil mixed with lipase.
   4. D Starch solution mixed with protease.
5. 51 mark

   A student claims that anaerobic respiration in muscle cells releases the same amount of energy per glucose molecule as aerobic respiration, only faster. Which statement is the correct scientific response to this claim?

   1. A The claim is correct, because both processes fully break down a glucose molecule all the way to carbon dioxide and water, releasing identical amounts of energy each time.
   2. B The claim is incorrect, but for a different reason: anaerobic respiration actually releases more energy per glucose molecule than aerobic respiration, simply because it happens faster.
   3. C The claim is incorrect: anaerobic respiration only partially breaks glucose down to lactic acid, so it releases far less energy per glucose molecule than aerobic respiration.
   4. D The claim is correct, because oxygen plays no part in releasing energy from glucose in either process, only in removing waste products afterwards.
6. 61 mark

   A reflex action, such as withdrawing a hand from a hot object, happens without any conscious decision-making by the brain, and is faster than a response that does involve conscious thought. Which statement correctly explains why reflex responses are faster?

   1. A Reflex actions do not involve the central nervous system at all; only nerves outside the brain and spinal cord are involved, cutting out several processing steps.
   2. B Reflex arcs use a shorter sensory neurone than a conscious response would use, so the nerve impulse itself travels faster along the shortened distance to the spinal cord.
   3. C Reflex arcs involve only a single neurone that both detects the stimulus and triggers the muscle response directly, so there is no synapse for the impulse to cross.
   4. D Reflex arcs route the impulse through a relay neurone in the spinal cord, so a response is triggered without the impulse travelling to the brain for conscious processing.
7. 71 mark

   During inhalation (breathing in), which sequence of changes correctly describes what happens to the diaphragm, the ribcage, and the pressure inside the thorax, leading to air moving into the lungs?

   1. A The diaphragm contracts and flattens, the intercostal muscles contract so the ribcage moves up and outwards, thoracic volume increases, pressure inside the thorax falls below atmospheric pressure, and air moves in.
   2. B The diaphragm relaxes and moves upwards, the intercostal muscles relax so the ribcage moves down and inwards, thoracic volume decreases, and air still moves into the lungs because pressure inside the thorax falls below atmospheric pressure.
   3. C The diaphragm contracts and flattens, but thoracic volume decreases as a result, so pressure inside the thorax rises above atmospheric pressure, forcing air into the lungs.
   4. D The diaphragm and intercostal muscles both relax, thoracic volume increases as a result, pressure inside falls, and air is pushed into the lungs by the higher pressure outside acting directly against the relaxed muscles.
8. 81 mark

   An ECG (electrocardiogram) trace from a patient shows a series of regular spikes at rest. During a period of moderate exercise, the spikes become more frequent, while the overall shape of each spike pattern stays normal. Which statement correctly interprets this change and its likely cause?

   1. A The increase in spike frequency shows that each individual heartbeat is now pumping less blood, since a faster heart rate is a direct measure of a fall in the volume of blood pumped per beat.
   2. B The change is unrelated to the body's demand for oxygen and glucose, since a healthy heart rate is fixed and cannot be altered by the level of physical activity, only by disease.
   3. C The increase in spike frequency shows the heart rate has increased, driven by the greater demand for oxygen and glucose delivery to respiring muscles during exercise.
   4. D The change shows the heart has developed an abnormal rhythm (an arrhythmia), because a healthy heart rate should always stay exactly the same, regardless of the body's level of activity.
9. 91 mark

   The small intestine is lined with millions of tiny finger-like projections called villi, each containing a network of blood capillaries close to its surface. Which statement correctly explains how villi increase the efficiency of absorption of digested food molecules into the blood?

   1. A Villi work by absorbing large, undigested food molecules directly into the capillaries, allowing digestion and absorption to happen in a single combined step.
   2. B Villi greatly increase the surface area of the intestinal lining, increasing the rate at which digested food molecules diffuse, or are actively transported, into the capillaries running through them.
   3. C Villi increase absorption by producing extra digestive enzymes inside each villus, breaking food down further immediately before it is absorbed.
   4. D Villi slow down the movement of food through the small intestine using strong muscular contractions of their own, giving digestive enzymes elsewhere in the gut more time to act before absorption occurs.
10. 101 mark

    A urine sample from a patient is tested and found to contain glucose, which does not normally appear in the urine of a healthy person. Which explanation correctly accounts for this finding, in terms of how the nephron normally handles glucose?

    1. A Glucose is too large a molecule to be filtered out of the blood in the nephron under normal circumstances, so its presence in urine means an abnormal filtering pathway has developed.
    2. B Glucose only appears in urine when it leaks in from the bladder wall itself, rather than passing through the kidney, so this finding says nothing about how the nephron is working.
    3. C All substances that are filtered out of the blood by the nephron, including glucose, are normally excreted in urine without being reabsorbed, so this result is exactly what would be expected in every healthy person.
    4. D Glucose is normally filtered out of the blood, but a healthy nephron reabsorbs all of it back into the blood; glucose in urine means reabsorption is not returning all of it, which can happen when blood glucose is very high.
11. 111 mark

    A person drinks very little water on a hot day and becomes mildly dehydrated, so their blood plasma becomes more concentrated than normal. Which statement correctly describes how the hormone ADH (antidiuretic hormone) responds, and what effect this has on urine?

    1. A The blood becomes more concentrated, so ADH secretion increases; the kidney tubules reabsorb more water, producing a smaller volume of more concentrated urine.
    2. B The blood becomes more concentrated, so ADH secretion decreases; this causes the kidney tubules to reabsorb less water, producing a larger volume of more dilute urine to dilute the blood.
    3. C ADH is released from the kidney itself and acts by adding water directly to the blood from an internal store, without changing the volume or concentration of urine produced.
    4. D Dehydration has no direct effect on ADH secretion; ADH is released only in response to a fall in blood glucose concentration, not to changes in how concentrated the blood is.
12. 121 mark

    During a sudden fright, a person's heart rate and breathing rate both increase rapidly, and more glucose becomes available in the blood. Which statement correctly explains the hormonal basis of this response?

    1. A Adrenaline is released from the pancreas and acts to lower blood glucose concentration, providing the burst of energy needed for a rapid 'fight or flight' response.
    2. B Adrenaline is released from the adrenal glands and prepares the body for rapid action by increasing heart rate, breathing rate, and blood glucose availability.
    3. C Adrenaline is a hormone released only during physical exercise, and it plays no role in the body's response to sudden fright or perceived danger.
    4. D Adrenaline acts by directly causing muscle fibres to contract, rather than by changing heart rate, breathing rate or blood glucose concentration.
13. 131 mark

    HIV (human immunodeficiency virus) is a pathogen that specifically infects and destroys a particular type of white blood cell involved in the immune response. Left untreated, this progression can eventually lead to AIDS (acquired immune deficiency syndrome). Which statement correctly explains why a person with advanced, untreated HIV infection becomes vulnerable to many other infections?

    1. A HIV directly causes the symptoms of every other infection itself, so a weakened immune system is not actually involved in a patient's vulnerability to other diseases.
    2. B HIV destroys red blood cells, reducing the oxygen supply to body tissues, and it is this reduced oxygen supply that makes the body more vulnerable to infection.
    3. C HIV only affects the skin's barrier function, physically making it easier for pathogens to enter the body, without affecting the body's internal immune responses at all.
    4. D HIV progressively destroys the white blood cells needed for an effective immune response, so as their numbers fall the body becomes unable to fight off pathogens it would normally overcome.
14. 141 mark

    A doctor explains to a patient that their risk of developing cardiovascular disease is not caused by any single factor, but arises from several factors acting together, such as diet, smoking history, exercise levels and family history. Which statement about non-communicable diseases best reflects this idea?

    1. A Non-communicable diseases such as cardiovascular disease are always caused by a single, identifiable pathogen, in the same way that a specific bacterium causes a bacterial infection.
    2. B Because non-communicable diseases cannot be transmitted from person to person, no lifestyle or environmental factor can influence whether or not a person develops one.
    3. C Non-communicable diseases such as cardiovascular disease typically result from several interacting risk factors, for example diet, exercise, smoking history and genetics, rather than from a single cause.
    4. D Non-communicable diseases are entirely determined by a person's genetics at birth, so lifestyle changes such as diet and exercise cannot affect a person's risk of developing one.
15. 151 mark

    On a hot day, a person's skin appears flushed (redder than usual) and they begin to sweat. Which explanation correctly links these two responses to the body's control of core temperature by negative feedback?

    1. A Blood vessels supplying the skin dilate (vasodilation), increasing heat loss by radiation, while sweat evaporating from the skin surface removes further heat; together these responses cool the body back towards its normal temperature.
    2. B Blood vessels supplying the skin constrict (vasoconstriction), moving blood away from the skin surface, which is why the skin appears redder than usual; sweating then adds further heat to the body, helping it match the temperature of a hot environment.
    3. C Sweating warms the skin directly through a chemical reaction with substances in the sweat, and vasodilation is a separate response triggered by a rise in blood glucose concentration rather than by body temperature.
    4. D The flushed skin and sweating are both signs that the body's temperature control mechanism has failed, since a healthy body should show no visible response at all to being in a hot environment.

Worked solutions

Every question below carries the reasoning, not just the answer. The official material for this test publishes a correct option letter and nothing else.

1. Question 1Answer: C

   1. Hydrogen peroxide can break down into water and oxygen without any catalyst present, but the uncatalysed reaction is extremely slow; catalase provides an alternative reaction pathway with a lower activation energy, so the same reaction happens far more quickly.
   2. A defining property of any catalyst, biological or otherwise, is that it is chemically unchanged at the end of the reaction it catalyses, so it is not consumed and can go on to catalyse further reactions.
   3. This rules out both B (which treats catalase as consumed) and D (which reverses the effect on activation energy), and A overstates catalase's role by claiming the reaction could not happen at all without it.
   4. Only C correctly states that catalase remains chemically unchanged and so the same sample can continue to break down further hydrogen peroxide, so C is correct.
   • Why not A: Overstates a catalyst's role: hydrogen peroxide does break down without catalase present, just far more slowly; a catalyst speeds up a reaction that can already occur, it does not switch an impossible reaction into a possible one.
   • Why not B: Wrongly treats the enzyme as a reactant that is consumed; a defining property of any catalyst is that it is not used up by the reaction it catalyses, so the same liver sample keeps working.
   • Why not D: Reverses the mechanism: a catalyst lowers the activation energy needed for a reaction, providing an alternative pathway, rather than increasing it.

2. Question 2Answer: A

   1. In the lock-and-key model, an enzyme's active site has a shape complementary to its substrate, which is why the enzyme is specific to that substrate.
   2. A molecule whose shape closely resembles the true substrate can also fit into the same active site, occupying it without necessarily being converted into a product.
   3. While that molecule occupies the active site, the true substrate cannot bind there, so fewer successful enzyme-substrate reactions occur per unit time and the overall rate falls, exactly as observed.
   4. Denaturation (B) and a shift in optimum pH (C) both require a change in temperature or pH, which the question rules out, and an enzyme has only one active site, not an alternative one elsewhere (D), so A is the only explanation consistent with the observation.
   • Why not B: Denaturation is a permanent, structural change usually caused by extremes of temperature or pH, not by another molecule of similar shape simply being present; the question states temperature and pH are unchanged.
   • Why not C: An enzyme's optimum pH is a fixed property of its structure and does not shift just because another molecule is present in the mixture; this option also does not explain why shape similarity to the substrate would matter.
   • Why not D: An enzyme has one active site with one particular shape; a molecule cannot create a second active site elsewhere, and this does not fit the lock-and-key model of a single, shape-specific binding site.

3. Question 3Answer: D

   1. Each enzyme has an optimum pH at which its active site has the ideal shape for binding its substrate; pepsin's optimum is strongly acidic, matching the stomach's pH of about 2.
   2. Moving pepsin to pH 8, far from its acidic optimum, disturbs the bonds that hold its active site in its normal shape, so the active site becomes a poorer fit for the substrate.
   3. This means fewer substrate molecules bind successfully to the active site per unit time, so the reaction rate falls well below the rate seen at pepsin's own optimum pH, without necessarily destroying the enzyme outright.
   4. This rules out A (pH does affect enzyme shape), B (pepsin's optimum is acidic, not alkaline) and C (the effect is a reduced rate, not necessarily an instant, total, permanent loss of activity), leaving D as the correct interpretation.
   • Why not A: The amino acid sequence sets the enzyme's primary structure, but its three-dimensional active site shape is held together by ionic and hydrogen bonds that are sensitive to pH as well as temperature, so the shape does change away from the optimum pH.
   • Why not B: Wrongly generalises trypsin's alkaline optimum to every protease; pepsin's optimum is acidic (around pH 2), matching the stomach environment it works in, so moving it to pH 8 would not make it faster.
   • Why not C: Overstates how quickly and completely a pH change acts; moving away from the optimum pH reduces the rate of reaction, and only a sufficiently extreme and sustained pH difference denatures an enzyme, rather than every shift instantly and permanently destroying it.

4. Question 4Answer: B

   1. Starch itself does not give a positive Benedict's test, since it is not a reducing sugar, so the starch-plus-amylase tube starts negative.
   2. Amylase hydrolyses starch into shorter sugar units, including maltose and glucose, which are reducing sugars.
   3. Once amylase has acted, the same tube now contains reducing sugars where it previously did not, so its Benedict's test result changes from negative to positive.
   4. The other three combinations either use the wrong enzyme for starch (D), or use a food and enzyme pair that never produces a reducing sugar in the first place (A, C), so only B shows the negative-to-positive change described.
   • Why not A: Protease breaks down protein into amino acids, not into reducing sugars, so Benedict's test on this mixture would stay negative even though the protein has been digested; a biuret test, not Benedict's test, would be needed to detect the protein here.
   • Why not C: Lipase breaks down fat into fatty acids and glycerol, neither of which is a reducing sugar, so the Benedict's test result would stay negative throughout; an emulsion test would be needed to detect the fat being broken down.
   • Why not D: Protease has no effect on starch, since amylase (not protease) is the enzyme that breaks starch down into sugars; without a suitable enzyme acting on the starch, the Benedict's test would stay negative throughout, showing no change.

5. Question 5Answer: C

   1. Aerobic respiration fully oxidises each glucose molecule, ultimately to carbon dioxide and water, releasing a relatively large amount of energy per glucose molecule.
   2. Anaerobic respiration in animal cells does not use oxygen and only partially breaks glucose down, to lactic acid, so it releases considerably less energy from each glucose molecule than aerobic respiration does.
   3. Anaerobic respiration can supply energy quickly during intense exercise precisely because it skips the slower, oxygen-dependent steps, but that speed comes at the cost of a much lower energy yield per glucose molecule, not a higher or equal one.
   4. So the student's claim is incorrect, and the correct reason is the lower energy yield of the incomplete anaerobic pathway, exactly what option C states.
   • Why not A: Wrongly assumes anaerobic respiration fully oxidises glucose; in fact it only partially breaks glucose down to lactic acid, so it cannot release the same amount of energy per glucose molecule as full aerobic oxidation.
   • Why not B: Confuses the rate at which a process proceeds with the total amount of energy released per glucose molecule; anaerobic respiration can occur quickly precisely because it is an incomplete breakdown, which is why it releases less, not more, energy per glucose molecule.
   • Why not D: Misunderstands the role of oxygen: oxygen is required as part of fully oxidising glucose in aerobic respiration, which is why aerobic respiration releases far more energy per glucose molecule than the oxygen-independent anaerobic pathway.

6. Question 6Answer: D

   1. In a reflex arc, the sensory neurone carries the impulse from a receptor into the spinal cord, part of the central nervous system, where it meets a relay neurone.
   2. The relay neurone passes the impulse directly to a motor neurone within the spinal cord, which then carries it out to an effector such as a muscle.
   3. Because the impulse is relayed within the spinal cord rather than travelling all the way up to the brain, being consciously processed there, and then travelling back down again, the whole reflex pathway takes noticeably less time than a response that involves conscious thought.
   4. This rules out A (the CNS is involved via the spinal cord), B (impulse speed along a neurone is not simply a matter of it being 'shortened') and C (multiple neurones and synapses are involved), leaving D as the correct explanation.
   • Why not A: The reflex arc does involve the central nervous system: the sensory neurone carries the impulse into the spinal cord (part of the CNS), and a relay neurone within the spinal cord connects it to a motor neurone; what is bypassed is the brain's conscious processing, not the CNS as a whole.
   • Why not B: The speed of a nerve impulse along a given neurone is a property of that neurone (for example, whether it is myelinated), not simply a matter of the sensory neurone being shorter; reflexes are fast because of the short route through the spinal cord and relay neurone, not because sensory neurones are unusually short.
   • Why not C: A reflex arc involves at least three neurones (sensory, relay and motor), connected by synapses, not a single neurone acting alone; removing the synapses would remove the coordinated pathway a reflex actually relies on.

7. Question 7Answer: A

   1. On inhalation, the diaphragm contracts and flattens, moving downwards, while the intercostal muscles contract, moving the ribcage upwards and outwards.
   2. Both of these actions increase the volume of the thorax (chest cavity).
   3. Increasing the volume of a fixed amount of gas lowers its pressure, so the pressure inside the thorax falls below the atmospheric pressure outside the body.
   4. Gas flows from an area of higher pressure to an area of lower pressure, so air moves in through the airways and into the lungs, down this pressure gradient, exactly as option A describes; B, C and D each pair the wrong muscle action with the wrong volume or pressure change.
   • Why not B: Describes the muscle actions and volume change of exhalation (diaphragm relaxing and moving up, ribcage moving down and in, decreasing volume) but wrongly pairs this with air moving in; a decrease in thoracic volume raises, not lowers, the internal pressure, so this combination would push air out, not draw it in.
   • Why not C: Gets the muscle action right (diaphragm contracting) but reverses its effect on volume and pressure: contracting and flattening the diaphragm increases thoracic volume, which lowers internal pressure below atmospheric, rather than decreasing volume and raising pressure as this option claims.
   • Why not D: Relaxation of the diaphragm and intercostal muscles is the set of actions that occurs on exhalation, which decreases (not increases) thoracic volume; this option also mischaracterises the physical explanation, since it is the pressure difference created by the muscles' own contraction that draws air in, not outside pressure acting directly on relaxed muscles.

8. Question 8Answer: C

   1. An ECG spike represents one heartbeat, so the number of spikes per minute is a direct measure of heart rate.
   2. During exercise, respiring muscles need a greater supply of oxygen and glucose, and produce more carbon dioxide, so the body responds by increasing heart rate to deliver blood, and the substances it carries, more quickly.
   3. Adrenaline, released as part of this response, is one of the signals that increases heart rate during exercise or excitement.
   4. Because the overall shape of each spike stays normal, and only the frequency of spikes increases, this is consistent with a normal, healthy increase in heart rate rather than an arrhythmia (D) or a change unrelated to activity (B), and the frequency change alone does not tell you anything about the volume pumped per beat (A), so C is the correct interpretation.
   • Why not A: Confuses heart rate (spikes per minute) with the volume of blood pumped per individual beat; a faster heart rate does not, by itself, tell you anything about how much blood each beat is pumping, and this option invents a link between the two that the ECG trace alone does not show.
   • Why not B: Wrongly claims heart rate is fixed; heart rate normally rises during exercise in direct response to the body's increased demand for oxygen and glucose delivery to active muscles, which is a normal, healthy physiological response, not evidence against a link to activity.
   • Why not D: Misreads a normal exercise response as disease; the trace's overall spike pattern (shape) staying normal, with only the frequency increasing, is consistent with a healthy heart beating faster, not with an arrhythmia, which would typically show an irregular or abnormally shaped pattern.

9. Question 9Answer: B

   1. Villi are tiny, finger-like projections that greatly increase the surface area of the small intestine's lining compared with a flat surface of the same length.
   2. A larger surface area allows more digested food molecules to diffuse, or be actively transported, across the gut wall into the blood at the same time, increasing the overall rate of absorption.
   3. Each villus also contains a network of capillaries very close to its surface, keeping the diffusion distance short and maintaining a concentration gradient by carrying absorbed molecules away in the blood.
   4. This surface-area-based explanation is what distinguishes villi's true role from the incorrect mechanisms in A (absorbing undigested food), C (producing enzymes) and D (slowing gut transit), so B is correct.
   • Why not A: Absorption specifically refers to small, already-digested, soluble molecules moving into the blood; large, undigested food molecules are still too big to cross the villus surface and must be broken down by digestion first, at an earlier stage.
   • Why not C: Digestive enzymes are secreted by glands and gland cells elsewhere in the digestive system, not manufactured specifically inside each villus purely to enable absorption; the villus's own specialised role is increasing surface area for absorption, not enzyme production.
   • Why not D: Muscular contraction that moves food along the gut is peristalsis, a separate process from absorption, and villi themselves are not the structures responsible for this muscular movement; villi's defining structural feature is their large surface area, not a role in slowing gut transit.

10. Question 10Answer: D

    1. As blood passes through a nephron, small molecules including water, salts, urea and glucose are all filtered out of the blood into the nephron tubule.
    2. In a healthy nephron, all of this filtered glucose is then reabsorbed back into the blood further along the tubule, so no glucose is normally left in the fluid that becomes urine.
    3. If glucose appears in the urine, this suggests that not all of the filtered glucose has been reabsorbed; this can occur when blood glucose concentration is unusually high, since more glucose is filtered than the nephron's reabsorption capacity can return to the blood.
    4. This links the urine test result to a specific step (reabsorption) in nephron function rather than to filtration alone (A), the bladder (B), or the mistaken idea that nothing is ever reabsorbed (C), so D is correct.
    • Why not A: Glucose is a small molecule and is filtered out of the blood in the nephron under normal, healthy conditions, along with water, salts and urea; the issue is not whether filtration happens, but what happens to the glucose afterwards.
    • Why not B: Misattributes the source of urinary glucose to the bladder rather than to the kidney's own filtering and reabsorption processes; the bladder simply stores urine that has already been produced by the kidneys, it does not add substances to it.
    • Why not C: Wrongly assumes filtration and excretion are the same step; in a healthy nephron, useful substances such as glucose (and most of the water) are filtered out and then selectively reabsorbed back into the blood, so normal urine does not contain glucose at all.

11. Question 11Answer: A

    1. Becoming dehydrated makes blood plasma more concentrated, which is detected and leads to an increase in ADH secretion from the pituitary gland.
    2. ADH travels in the blood to the kidneys, where it increases the permeability of the kidney tubules to water, allowing more water to be reabsorbed from the filtrate back into the blood.
    3. This reabsorbed water helps to dilute the blood back towards its normal concentration, while less water is left to be excreted, producing a smaller volume of more concentrated urine.
    4. This rules out B (which reverses the direction of the response), C (which misplaces ADH's origin and mechanism) and D (which confuses ADH's trigger with blood glucose), leaving A as the correct description.
    • Why not B: Reverses the direction of the response: a more concentrated blood plasma should trigger more ADH secretion (to conserve water), not less, and the resulting urine should become more concentrated and lower in volume, not more dilute and higher in volume.
    • Why not C: Misplaces ADH's origin and mechanism: ADH is released from the pituitary gland, not manufactured or released by the kidney itself, and it acts by increasing the permeability of the kidney tubules to water so that more water is reabsorbed from the filtrate, rather than adding water directly to the blood from a stored supply.
    • Why not D: Confuses the trigger for ADH release with the trigger for insulin and glucagon; ADH release responds to how concentrated the blood plasma is (its water content), not to blood glucose concentration.

12. Question 12Answer: B

    1. Adrenaline is a hormone released from the adrenal glands, which sit above the kidneys, in response to stress, excitement or perceived danger such as a sudden fright.
    2. Adrenaline increases heart rate and breathing rate, so that more oxygenated blood can reach respiring tissues more quickly.
    3. Adrenaline also increases the availability of glucose in the blood, providing extra fuel for respiration in the brain and muscles.
    4. Together, these effects prepare the body for rapid physical action, matching option B; the other options either misidentify the source and direction of adrenaline's effect on blood glucose (A), wrongly limit adrenaline's trigger to exercise alone (C), or misdescribe its mechanism as acting directly on muscle contraction (D).
    • Why not A: Misidentifies both the source and the effect: adrenaline is released from the adrenal glands, not the pancreas (which releases insulin and glucagon), and adrenaline raises, rather than lowers, blood glucose concentration, to provide more fuel for respiring cells.
    • Why not C: Wrongly restricts adrenaline's trigger to exercise alone; adrenaline is released in response to a range of stressful or exciting situations, including sudden fright, as part of a rapid 'fight or flight' response, not only during physical exertion.
    • Why not D: Misdescribes adrenaline's mechanism; adrenaline does not act by directly making muscle fibres contract, but by increasing heart rate, breathing rate and blood glucose availability, which together prepare the muscles to work harder if they are called on to do so.

13. Question 13Answer: D

    1. HIV specifically targets and infects a type of white blood cell that plays a central role in coordinating the body's immune response to pathogens.
    2. Over time, as HIV replicates, it progressively destroys increasing numbers of these white blood cells.
    3. As their numbers fall, the immune system becomes less and less able to recognise and respond effectively to pathogens that a healthy immune system would normally deal with easily, leaving the person vulnerable to infections that would not usually cause serious illness.
    4. This progressive loss of effective immune function, rather than a direct effect on red blood cells (B), the skin (C), or HIV directly causing every other infection's symptoms (A), is why advanced, untreated HIV infection leads to AIDS and increased vulnerability to other diseases, exactly what option D describes.
    • Why not A: Confuses HIV's own action with the action of the many different pathogens a person later becomes vulnerable to; HIV itself does not directly cause the symptoms of, for example, a later fungal or bacterial infection, but its destruction of key white blood cells removes the immune defences that would normally control those infections.
    • Why not B: Misidentifies HIV's target cell type: HIV specifically infects and destroys certain white blood cells involved in the immune response, not red blood cells, and reduced oxygen transport is not the mechanism behind the increased vulnerability to infection seen in AIDS.
    • Why not C: Understates HIV's mechanism to a purely physical, skin-barrier effect; HIV's central effect is on internal immune function, through destroying white blood cells, rather than any change to the skin's barrier role.

14. Question 14Answer: C

    1. Non-communicable diseases, unlike communicable diseases, are not caused by a single transmissible pathogen and cannot be passed directly from one person to another.
    2. Diseases such as cardiovascular disease typically develop because of the combined, interacting effects of several risk factors over time, which can include diet, physical activity levels, smoking history and inherited (genetic) factors.
    3. This means an individual's overall risk depends on the particular combination of factors that applies to them, rather than on any one factor alone, and it also means that changing a modifiable risk factor, such as diet or smoking, can alter that risk.
    4. This rules out treating non-communicable disease causation like a single-pathogen model (A), denying any lifestyle influence because the disease is non-transmissible (B), or treating the outcome as fixed by genetics alone (D), leaving C as the statement that correctly reflects the multi-factor nature of non-communicable disease causation.
    • Why not A: Wrongly applies the causal model of communicable disease (one identifiable pathogen causing one disease) to a non-communicable disease; cardiovascular disease is not caused by a pathogen at all, and arises instead from the interaction of multiple risk factors over time.
    • Why not B: Wrongly extends the meaning of 'non-communicable' (cannot be passed between people) to imply that no factor at all can influence risk; lifestyle and environmental factors, such as diet, smoking and exercise, can still strongly influence the risk of developing a non-communicable disease, even though the disease itself cannot be caught from another person.
    • Why not D: Overstates genetic determinism and denies any role for lifestyle; family history (genetics) is only one of several contributing risk factors, and changes to diet, exercise and smoking are well established to be able to reduce or increase an individual's risk.

15. Question 15Answer: A

    1. When core body temperature rises above its normal range, this is detected, and the body responds through negative feedback to bring temperature back down.
    2. Blood vessels supplying the skin dilate (vasodilation), increasing blood flow close to the skin's surface; this is what makes the skin look flushed, and it increases heat loss from the blood to the surroundings by radiation.
    3. At the same time, sweat glands produce more sweat; as this sweat evaporates from the skin surface, it takes heat energy with it, further cooling the body.
    4. Together, increased heat loss by radiation (from vasodilation) and by evaporation (from sweating) act to lower core body temperature back towards normal, exactly what option A describes; the other options either reverse the blood vessel response and sweating's effect (B), misdescribe the mechanism and its trigger (C), or misinterpret normal responses as failure (D).
    • Why not B: Reverses the blood vessel response: vasoconstriction moves blood away from the skin surface and would make skin look paler, not more flushed, and it is vasodilation, moving blood towards the skin, that produces the flushed appearance; sweating also cools the body through evaporation, rather than adding further heat to it.
    • Why not C: Misdescribes the mechanism of sweating, which cools the body through the evaporation of water from the skin surface removing heat energy, not through a warming chemical reaction, and wrongly links vasodilation to blood glucose regulation rather than to temperature control.
    • Why not D: Misreads normal, functioning homeostatic responses as a sign of failure; vasodilation and sweating are exactly the responses a healthy temperature control system is expected to produce when core temperature rises above normal, and their appearance shows the system working, not failing.