IGCSE Biology Paper 4 (Theory / Extended)

Topic 14: Coordination and Response -- Challenge Exam 1

Duration1 hour 15 minutes

Total Marks80

Questions7

Time Remaining75:00

Syllabus0610

Instructions

Answer all questions in the spaces provided.
Show all working for calculations.
Use appropriate scientific terminology.
Your answers will be automatically graded when you submit.

Question Navigation

This paper covers the whole of Topic 14. Like a real Cambridge paper it ranges across every sub-topic — 14.1 neurones, the reflex arc and synapses, 14.2 sense organs and the eye, 14.3 hormones, 14.4 homeostasis, and 14.5 tropic responses and auxin — and it mixes them inside single questions. All three Topic 14 papers do; they differ in the angle they come at it from, not in what they cover.

Question 1 — The Message That Has to Change Form Twice

Total: 12 marks

Fig. 1.1 shows a reflex arc. Nothing has been named for you. Work from position and direction.

(a) [4]

Name the structures labelled A, B, C and D on Fig. 1.1.

Model Answer — 1(a)

A — a receptor (accept temperature receptor / receptor cell in the skin) [1]

B — the sensory neurone [1]

C — the motor neurone [1]

D — the spinal cord [1]

(not needed for the marks: E is the effector, a muscle; F is the cell body of the sensory neurone; G is the relay neurone)

⚠ If you missed marks here: The direction of travel names the neurone, not its position on the page. B runs into D from a receptor, so it must be sensory; C runs out of D towards a muscle, so it must be motor. If you wrote “nerve” for B or C you have lost the mark on a technicality worth understanding: a nerve is a bundle of fibres, and a single drawn cell is a neurone. And D is the spinal cord, not the brain — the brain does not appear in this diagram at all, which is the whole point of it.

1 mark — A: receptor 1 mark — B: sensory neurone 1 mark — C: motor neurone 1 mark — D: spinal cord

(b) [3]

The person pulls their hand away before they feel any pain. Explain why.

Model Answer — 1(b)

the reflex pathway lies within the spinal cord and does not involve the brain [1]

it is a short pathway crossing only two synapses, so the response is very rapid [1]

a separate impulse travels up to the brain, and the sensation of pain is produced there; that journey is longer, so the pain arrives after the hand has moved [1]

⚠ If you missed marks here: There are two journeys in this answer and most candidates describe only one. If your answer never mentions the brain at all you cannot explain the pain; if the brain appears in the middle of the reflex arc you have described a voluntary action and lost the first two marks together. Also, “it is faster” on its own is worth one mark at most — say why a shorter pathway is faster, and the phrase “fewer synapses to cross” is what supplies it.

1 mark — the reflex pathway is within the spinal cord and does not involve the brain 1 mark — short pathway with few synapses, so the response is rapid 1 mark — a separate impulse travels to the brain, where pain is felt, and takes longer

Describe what happens at one of the structures ringed in orange on Fig. 1.1 when an impulse arrives.

Model Answer — 1(c)

the impulse stimulates the release of neurotransmitter molecules from vesicles into the synaptic gap [1]

the molecules diffuse across the gap and bind with receptor proteins on the next neurone [1]

this stimulates an impulse in the next neurone [1]

⚠ If you missed marks here: “The impulse jumps across the gap” replaces three marking points with nothing, and it is wrong in principle: nothing electrical crosses a synapse. The two verbs that earn the middle mark are diffuse and bind — “travels across” and “lands on the neurone” are both refused. Notice too that the question said “when an impulse arrives”, so it starts at the first neurone and ends at the second; describing the gap without describing the sequence scores nothing.

1 mark — neurotransmitter released from vesicles into the synaptic gap 1 mark — molecules diffuse across the gap and bind with receptor proteins 1 mark — an impulse is stimulated in the next neurone

(d) [2]

Explain why an impulse can pass across these structures in one direction only, and suggest why this is important in a reflex arc.

Model Answer — 1(d)

vesicles of neurotransmitter are only in the first neurone and receptor proteins only on the second, so the signal can only be sent one way [1]

this ensures the impulse travels from receptor to effector and cannot run backwards, so the response always follows the stimulus and the arc has a fixed direction [1]

⚠ If you missed marks here: “Because the gap is too small” is not a reason — a small gap is crossed just as easily in either direction. The mark is for saying where the two structures are, and if you listed them properly in an earlier part you already had this answer. The second mark asks for a consequence, and it is worth noticing that a nervous system in which impulses could run backwards would be worse than useless: a muscle could stimulate its own receptor and the loop would never stop.

1 mark — vesicles only in the first neurone, receptor proteins only on the second 1 mark — so the impulse always travels from receptor towards effector and cannot run backwards

Question 2 — Two Rings of Muscle Doing Two Different Jobs

Total: 12 marks

Fig. 2.1 shows the human eye in horizontal section. Light enters from the left.

(a) [4]

Name the structures labelled S, T, W and X, and state the function of X.

Model Answer — 2(a)

S — the ciliary muscle [1]

T — the suspensory ligaments [1]

W — the blind spot [1]

X — the optic nerve, which carries impulses to the brain [1]

(not needed: P is the cornea, Q the iris, R the lens, U the retina, V the fovea)

⚠ If you missed marks here: The pair that goes wrong most often is V and W: the fovea is where vision is best, the blind spot is where the optic nerve leaves and there are no receptor cells at all. They sit close together on every diagram and questions offer both deliberately. For X, “sends messages to the brain” is usually refused — use the word impulses, which costs nothing and is sometimes the whole mark.

1 mark — S: ciliary muscle 1 mark — T: suspensory ligaments 1 mark — W: blind spot 1 mark — X: optic nerve, carries impulses to the brain

(b) [3]

A bright torch is shone into the eye. Describe what happens to structure Q, and explain the advantage of the response.

Model Answer — 2(b)

the circular muscles of the iris contract and the radial muscles relax [1]

so the pupil diameter decreases and less light enters the eye [1]

this protects the light receptor cells in the retina from damage by very bright light [1]

⚠ If you missed marks here: Name both sets of muscles. Because they are an antagonistic pair, a mark scheme usually wants one contracting and the other relaxing, and “the iris muscles contract” cannot score it. For the advantage, “so it does not hurt” or “because the eye does not need so much light” earns nothing — the answer names the receptor cells. And do not write that “the pupil contracts”: the pupil is a hole and has no muscle.

1 mark — circular muscles contract, radial muscles relax 1 mark — pupil diameter decreases, so less light enters 1 mark — protects the light receptor cells in the retina from damage

The person then looks at a book held close to their face. Describe the changes in S, T and R that allow the light to be focused on the retina.

Model Answer — 2(c)

S, the ciliary muscle, contracts, so the ring of muscle becomes narrower [1]

T, the suspensory ligaments, slacken / become loose [1]

R, the lens, becomes fatter and more curved, so it refracts the light more [1]

⚠ If you missed marks here: Almost every lost mark in this part comes from one step: a ring of muscle contracting makes its own hole smaller, like a drawstring, so the ligaments attached to it go slack. Write “contracts, so the ligaments tighten” and every step after it is reversed as well. Two other refusals to watch for: the ligaments cannot contract, because they are not muscle, and the lens does not move — it changes shape.

1 mark — ciliary muscle contracts, ring becomes narrower 1 mark — suspensory ligaments slacken 1 mark — lens becomes fatter and more curved, refracting light more

(d) [2]

In very dim light the person can see the outline of the book but cannot read the coloured writing on the cover. Explain this, referring to structures V and U.

Model Answer — 2(d)

in dim light only the rods are sensitive enough to be stimulated, and rods do not give colour vision [1]

the cones, which give colour vision, are concentrated at V, the fovea, and need brighter light, so they are not stimulated [1]

⚠ If you missed marks here: Rods and cones get swapped constantly. Tie them down with a letter: c for cones and c for colour. The distribution matters as much as the function — the fovea is packed with cones and has almost no rods, which is why you can see a faint star only by looking slightly to one side of it. If your answer said “the light is not bright enough to see colour” without naming a cell type, it described the observation rather than explaining it.

1 mark — only rods are stimulated in dim light, and rods do not give colour vision 1 mark — cones, concentrated at the fovea, need brighter light

Question 3 — One Second and Four Minutes

Total: 12 marks

A student sits quietly while their heart rate and blood adrenaline concentration are recorded. At time 0 a very loud noise is made behind them. Table 3.1 shows the results.

time / s	heart rate / beats per minute	blood adrenaline / arbitrary units
−10	68	2
1	103	2
10	112	26
60	108	31
240	94	19
600	71	3

(a) [3]

Describe the changes shown in Table 3.1, using figures from the table.

Model Answer — 3(a)

heart rate rises sharply from 68 to 103 beats per minute in the first second, and peaks at 112 at 10 s [1]

blood adrenaline concentration does not change at 1 s (still 2 units) but has risen to 26 units by 10 s and peaks at 31 units at 60 s [1]

both then fall, and by 600 s both are close to their original values (71 beats per minute and 3 units) [1]

⚠ If you missed marks here: “Describe” means say what the numbers do, with the numbers. “They both go up and then come down” is worth one mark of three. The single most valuable observation in the table is the one nearly everybody skips: at 1 s the heart rate has already jumped 35 beats per minute while the adrenaline has not moved at all. That mismatch is the whole point of the question, and part (c) depends on you having noticed it.

1 mark — heart rate rises from 68 to 103 within 1 s, peaking at 112 1 mark — adrenaline unchanged at 1 s, rising later to a peak of 31 units 1 mark — both return towards their original values by 600 s

(b) [3]

Define the term hormone.

Model Answer — 3(b)

a chemical substance produced by a gland [1]

carried by the blood [1]

which alters the activity of one or more specific target organs [1]

⚠ If you missed marks here: The clause candidates leave out is specific target organs, and the one they get wrong is carried by the blood — if the word “nerve” appears anywhere in your definition, that mark is gone and so is any later mark that depends on it. Learn the definition as separate clauses rather than as one long sentence, and write them on separate lines so the examiner can see all three.

1 mark — a chemical substance produced by a gland 1 mark — carried by the blood 1 mark — alters the activity of specific target organs

Use the data to explain why two different control systems must be involved in the changes in heart rate.

Model Answer — 3(c)

at 1 s the heart rate has risen by 35 beats per minute while the adrenaline concentration is unchanged, so the first rise cannot be caused by adrenaline [1]

it must therefore be nervous: impulses travel electrically along neurones to the heart, which is extremely fast [1]

the adrenaline concentration rises only after several seconds, because the hormone has to be secreted by the adrenal glands and carried in the blood to the heart [1]

the heart rate is still raised at 240 s, long after a nervous response would have ended, so the sustained effect is hormonal and lasts until the adrenaline is broken down [1]

⚠ If you missed marks here: The command was “use the data”, so an answer that recites the difference between nervous and hormonal control without quoting a single figure loses half the marks even if every sentence is true. The argument you are being asked to make is a genuine piece of reasoning: the adrenaline had not changed yet, therefore adrenaline cannot be the cause, therefore something faster must be. Write it in that order and the marks follow.

1 mark — at 1 s adrenaline is unchanged, so it cannot have caused the first rise 1 mark — the immediate rise must be nervous, using fast electrical impulses 1 mark — adrenaline is secreted by the adrenal glands and carried in the blood, which takes time 1 mark — the sustained rise is hormonal and lasts until the adrenaline is broken down

(d) [2]

State two other effects that adrenaline would have on this student, and for each explain how it would be useful if they had to run away.

Model Answer — 3(d)

increased breathing rate, so more oxygen enters the blood for respiration in the muscles [1]

increased pupil diameter, so more light enters the eye and more can be seen [1]

(also acceptable: increased blood glucose concentration, so more glucose is available to muscle cells for respiration)

⚠ If you missed marks here: Each mark needs an effect and a use — a bare list of effects earns nothing here, because the question asked you to explain. Do not invent extra effects: the Core list is exactly three, with raised blood glucose added at Supplement, and anything else you have read elsewhere cannot be credited. Note also that increased heart rate is already in the stem, so it cannot be one of your two.

1 mark — increased breathing rate, so more oxygen for respiration 1 mark — increased pupil diameter (or raised blood glucose), with its use explained

Question 4 — Three People and One Glucose Drink

Total: 12 marks

Three people each drink the same glucose solution at time 0. Their blood glucose concentrations are measured for two hours. Fig. 4.1 shows the results.

(a) [3]

Compare the results for person J and person K.

Model Answer — 4(a)

K begins at a higher concentration than J (about 9 units against about 5) [1]

K rises further, to about 16 units, whereas J peaks at about 8 [1]

J returns to its starting value within the two hours, whereas K remains high throughout [1]

⚠ If you missed marks here: A comparison needs both people in every sentence and a comparative word — higher, further, longer. “K is high” is a description of one line, not a comparison, and it is worth one mark at most. Notice that there are three separate things to compare here: where each line starts, how far it rises, and whether it comes back. Candidates who spot only the peak lose two marks with a single correct sentence.

1 mark — K starts higher than J, with figures 1 mark — K rises further than J, with figures 1 mark — J returns to its starting value while K does not

(b) [4]

Explain fully how the blood glucose concentration of person J is brought back down.

Model Answer — 4(b)

the rise is detected by the pancreas [1]

which secretes insulin into the blood [1]

insulin is carried to the liver, which it stimulates to convert glucose into glycogen for storage; muscle cells also take up more glucose [1]

so the blood glucose concentration falls back towards the set point — the change has been reversed, which is negative feedback [1]

⚠ If you missed marks here: Three refusals to check your answer against. Insulin does not convert anything — it is a hormone, not an enzyme, so it stimulates the liver to do the converting. The conversion happens inside the liver, never in the blood. And the pancreas does not store glycogen; it detects and secretes, and the liver stores. Naming the wrong organ once usually costs two marks, because the second half of the chain is then attached to the wrong place.

1 mark — the rise is detected by the pancreas 1 mark — insulin is secreted into the blood 1 mark — the liver is stimulated to convert glucose into glycogen for storage 1 mark — the concentration falls back towards the set point

Person L behaves like J for the first 30 minutes, but by 90 minutes their concentration has fallen well below the set point and they feel faint. Suggest an explanation for the shape of L’s line.

Model Answer — 4(c)

detection and the initial response are normal, since L rises and begins to fall exactly like J [1]

but too much insulin has been secreted, or it has gone on acting after the set point was reached, so too much glucose has been converted to glycogen and the concentration overshoots downwards [1]

the faintness occurs because brain cells respire almost entirely using glucose, so a low blood glucose concentration limits the energy available to them [1]

(the concentration then rises again, which would be the effect of glucagon causing the liver to convert glycogen back into glucose)

⚠ If you missed marks here: The commonest wrong answer is “L has diabetes”, which reads the symptom rather than the graph — L’s problem is a correction that is too large, which is the opposite of too little insulin. Say what is normal as well as what has gone wrong; the first mark is for recognising that the detection worked. And note the last part of the line: it comes back up, which is glucagon rescuing the situation, and mentioning it shows you understand that the system has two halves.

1 mark — detection and initial response are normal 1 mark — too much insulin secreted or acting too long, so the level overshoots downwards 1 mark — brain cells need glucose for respiration, so a low concentration causes faintness

(d) [2]

Person K is treated with injections of insulin. Explain why the insulin cannot be given as a tablet to be swallowed.

Model Answer — 4(d)

insulin is a protein [1]

so it would be digested into amino acids by proteases in the stomach and small intestine, and would never reach the blood intact [1]

⚠ If you missed marks here: This is Topic 7 doing work inside Topic 14, and challenge papers are built on exactly this kind of link — if a question here seems to need something you learned in another topic, it probably does. “It would be destroyed in the stomach” scores one mark at best; name it as a protein and say what it would be digested into.

1 mark — insulin is a protein 1 mark — it would be digested into amino acids before reaching the blood

Question 5 — Cold Hands, Warm Middle

Total: 10 marks

Fig. 5.1 shows a section through human skin.

(a) [3]

Name the structures labelled A, C and F on Fig. 5.1.

Model Answer — 5(a)

A — the hair erector muscle [1]

C — a sweat gland [1]

F — fatty tissue [1]

(not needed: B is a hair, D a receptor, E a blood vessel)

⚠ If you missed marks here: The syllabus names seven structures in a skin diagram — hairs, hair erector muscles, sweat glands, receptors, sensory neurones, blood vessels and fatty tissue — and any of them can be labelled. Learn the drawing rather than the list, because a label points at a position. For A, “muscle” alone is not enough: say which muscle, since the skin contains no other kind in this diagram.

1 mark — A: hair erector muscle 1 mark — C: sweat gland 1 mark — F: fatty tissue

(b) [4]

A person moves into a room at 5 °C. Describe four changes that take place in their body, and for each state how it helps to maintain body temperature.

Model Answer — 5(b)

vasoconstriction: the arterioles supplying the surface capillaries narrow, so less blood flows near the skin surface and less heat is lost [1]

sweating stops, so no heat energy is lost by the evaporation of sweat [1]

the hair erector muscles contract, raising the hairs so that a thicker layer of air is trapped; still air is a poor conductor of heat [1]

shivering: muscles contract rapidly, increasing respiration, which releases heat energy [1]

⚠ If you missed marks here: Four marks means four changes each with a consequence — a bare list of four changes usually scores two. Three phrases that lose marks: the blood vessels do not move, they change diameter, and it is the arterioles that do so, not the capillaries; the hairs trap air, and it is the air that insulates; and shivering warms you through respiration, not friction.

1 mark — vasoconstriction of arterioles, so less blood near the surface and less heat lost 1 mark — sweating stops, so no heat lost by evaporation 1 mark — hair erector muscles contract, trapping an insulating layer of air 1 mark — shivering increases respiration, which releases heat energy

After 40 minutes in the cold room the person’s skin temperature has fallen from 33 °C to 24 °C but their core temperature has fallen only from 37.1 °C to 36.9 °C. A student says these results show that temperature homeostasis is failing. Evaluate this statement.

Model Answer — 5(c)

homeostasis maintains a constant internal environment, so it is the core temperature that is being controlled [1]

the core has changed by only 0.2 °C in a room 32 °C colder than the body, which shows the control is working very well [1]

the large fall in skin temperature is a consequence of vasoconstriction, which is part of the mechanism protecting the core, so it is evidence of success rather than failure [1]

⚠ If you missed marks here: When a question gives you two temperatures, always ask which one homeostasis is actually defending. The trap is that the bigger number looks more impressive: 9 °C is a large change and 0.2 °C is a small one, so the eye goes to the skin. But the body is deliberately sacrificing the surface to save the middle, which means the cold skin is the mechanism working. An evaluation should also say what the data do support, not only what they do not.

1 mark — homeostasis controls the internal, core temperature 1 mark — the core changed by only 0.2 °C, showing effective control 1 mark — the fall in skin temperature is a result of vasoconstriction, part of the mechanism

Question 6 — Four Seedlings and a Sheet of Plastic

Total: 12 marks

Four identical cress seedlings were grown for three days at 20 °C, each lit from the left only. The results are shown in Table 6.1.

seedling	treatment	increase in height / mm	angle of bending / degrees
W	none	28	34
X	shoot tip removed	3	0
Y	opaque cap over the shoot tip	26	1
Z	transparent cap over the shoot tip	27	31

(a) [2]

Define the term phototropism, and state the type of phototropic response shown by seedling W.

Model Answer — 6(a)

a response in which parts of a plant grow towards or away from the direction of a light source [1]

W shows a positive phototropic response — it grows towards the light [1]

⚠ If you missed marks here: The word grow is the mark. “The plant moves towards the light” is refused, because a tropism is a growth response — which is why it takes days and why it is permanent. The definition must also allow for away from, since roots are negatively phototropic; a definition that only covers shoots is incomplete.

1 mark — a growth response towards or away from the direction of a light source 1 mark — W is positively phototropic

(b) [4]

Explain, in terms of auxin, how seedling W came to bend towards the light.

Model Answer — 6(b)

auxin is made in the shoot tip and diffuses down through the shoot [1]

it becomes unequally distributed, accumulating on the shaded side [1]

auxin stimulates cell elongation, so the cells on the shaded side become longer than those on the lit side [1]

that side of the shoot therefore grows longer, so the shoot bends towards the light [1]

⚠ If you missed marks here: If you wrote that auxin collects on the lit side, every step after it is reversed and the answer describes a shoot bending the wrong way. Use the five-second check: the shoot bends towards the light, so the far side must have grown longer, so the auxin was on the far side. And auxin stimulates cell elongation — it does not “make the plant bend”, which is the conclusion rather than the mechanism.

1 mark — auxin is made in the shoot tip and diffuses down the shoot 1 mark — it becomes unequally distributed, accumulating on the shaded side 1 mark — auxin stimulates cell elongation, so those cells become longer 1 mark — the shaded side grows longer, so the shoot bends towards the light

Explain the results for seedlings X and Y, using the figures in Table 6.1.

Model Answer — 6(c)

X grew only 3 mm because its tip was removed, so no auxin was produced and there was almost no cell elongation [1]

a shoot that is not elongating cannot bend, because bending requires one side to grow more than the other [1]

Y grew 26 mm, almost as much as W, so auxin was still being produced; it did not bend because the tip could not detect the direction of the light, so the auxin remained evenly distributed [1]

⚠ If you missed marks here: X and Y both failed to bend, and it is easy to give them the same explanation — but the growth figures show they failed for opposite reasons. X made no auxin at all; Y made plenty and simply could not tell where the light was. Quoting 3 mm against 26 mm is what proves you have read the table rather than recited the theory, and that is where the marks are.

1 mark — X grew only 3 mm because no tip means no auxin and no elongation 1 mark — a shoot that is not elongating cannot bend 1 mark — Y grew 26 mm so auxin was made, but the tip could not detect the light direction

(d) [3]

Explain why seedling Z was included, and state two other variables that should have been controlled in this investigation.

Model Answer — 6(d)

Z controls for the presence of the cap itself: it has a cap but still bends 31°, so the failure of Y cannot be caused by the cap and must be caused by the exclusion of light from the tip [1]

control the light intensity (only the direction should differ) [1]

control the water supply, and use seedlings of the same species and age (temperature was already controlled at 20 °C) [1]

⚠ If you missed marks here: “Z is a control” on its own is worth nothing — the mark is for saying what it controls for. Y had two things done to it, being covered and being darkened, and without Z you could not tell which mattered. On the variables, do not offer temperature: the stem already tells you it was 20 °C throughout, and repeating a control that was given wastes a mark you could have earned elsewhere.

1 mark — Z controls for the cap itself, showing the effect in Y is due to excluding light 1 mark — light intensity should be kept the same 1 mark — water supply, or species and age of seedlings, should be kept the same

Question 7 — The Animal That Cannot Sweat

Total: 10 marks

Read all of this information before you begin. Everything that is new is given here.

A dog has almost no sweat glands in the skin of its body. When it becomes too hot it pants: it breathes rapidly and shallowly through an open mouth, so that air passes over the moist surfaces of the mouth and tongue. Like a human, a dog keeps its core temperature close to 38 °C, has hairs with erector muscles, and controls the blood flow through the vessels of its skin. A dog also has a pancreas, a liver and adrenal glands that work in the same way as a human’s.

In an experiment a dog rested in a room at 35 °C. Its breathing rate rose from 24 to 190 breaths per minute over 20 minutes, and its core temperature rose by only 0.4 °C.

(a) [3]

Explain how panting cools the dog.

Model Answer — 7(a)

rapid movement of air passes over the moist surfaces of the mouth and tongue [1]

so water evaporates from them [1]

evaporation takes heat energy from those surfaces and from the blood flowing beneath them, cooling the dog [1]

⚠ If you missed marks here: This is the sweating answer applied to an unfamiliar organ, and the mark is still the word evaporates. “The cool air cools the dog down” misses the mechanism entirely — and in a room at 35 °C the air is warmer than the dog’s skin, so it could not cool anything by contact. It is the change of state from liquid water to vapour that removes the energy.

1 mark — air passes rapidly over the moist surfaces of the mouth and tongue 1 mark — water evaporates from these surfaces 1 mark — evaporation takes heat energy from the body, cooling it

(b) [4]

Suggest two other changes that would occur in the dog’s skin in this room, and explain how the brain brings them about.

Model Answer — 7(b)

the brain monitors the temperature of the blood flowing through it and receives impulses from temperature receptors in the skin [1]

it sends impulses along neurones to the effectors in the skin [1]

vasodilation: the arterioles supplying the surface capillaries widen, so more blood flows near the surface and more heat is lost by radiation [1]

the hair erector muscles relax, so the hairs lie flat and a thinner layer of insulating air is trapped, allowing more heat to escape [1]

⚠ If you missed marks here: Two marks are for the coordination and two for the changes, so an answer that lists only the skin responses throws away half of them — the question asked how the brain brings them about. Precision on the changes: it is the arterioles that dilate, not the capillaries, and blood vessels change diameter rather than position.

1 mark — the brain monitors blood temperature and receives impulses from skin receptors 1 mark — the brain sends impulses along neurones to the effectors 1 mark — vasodilation of arterioles, so more blood near the surface and more heat lost 1 mark — hair erector muscles relax, hairs lie flat, less air trapped

A student says that a rise of only 0.4 °C shows that the dog is not really controlling its temperature, because a good control system would allow no change at all. Evaluate this statement, and explain why a rise of 4 °C would be dangerous.

Model Answer — 7(c)

the statement is wrong: a correction cannot begin until a change has already occurred, so some fluctuation around the set point is unavoidable in any negative feedback system [1]

a rise of only 0.4 °C in a room 35 °C warm is in fact evidence that the control is working very effectively [1]

a rise of 4 °C would begin to denature enzymes, so the reactions of metabolism would slow down or stop [1]

⚠ If you missed marks here: The general principle is worth learning as a sentence, because it appears in every homeostasis question: negative feedback corrects a change after it has happened, so the level always fluctuates around the set point. A perfectly flat line is not a higher standard, it is an impossible one. For the last mark use denatured, never “killed” — an enzyme is not alive — and say what the consequence is, because “the enzymes are denatured” without a consequence is only half an answer.

1 mark — a correction can only follow a change, so fluctuation is unavoidable 1 mark — a rise of only 0.4 °C shows the control is working effectively 1 mark — a rise of 4 °C would denature enzymes, slowing or stopping metabolism

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