This is the biggest topic on the syllabus and the one where a single wrong word most often costs a whole mark. A spinal reflex does not involve the brain. Impulses are electrical and neurotransmitters are chemical. The lens changes shape; it never moves. Hormones travel in the blood, never along nerves. The liver stores the glycogen, not the pancreas. Sweat cools you only when it evaporates. Blood vessels change diameter, not position. Auxin gathers on the shaded side. Fourteen traps, six walkthroughs, seven lookalike pairs, a concept map, six wrong answers to dissect and ten full practice questions below — every one of them aimed at a place where a perfectly sensible sentence earns nothing at all.
Fourteen traps that cost marks on Topic 14 challenge papers. Every one is an answer that sounds right and that mark schemes refuse.
Six challenge-level questions worked through in the order you should actually think about them. Try each part before revealing the next step.
P is a spinal reflex: receptor → sensory → relay → motor → effector, all inside the spinal cord, with only two synapses. Q is a voluntary action: the impulse must travel to the brain, be processed there, and travel back out — a far longer pathway with many more synapses. Each synapse costs time, because a chemical must be released and diffuse across a gap.
The pupil reflex is coordinated in the brain, not the spinal cord, so its pathway is not the very short one P uses. And its effector is a small ring of muscle in the iris, which takes time to contract and to actually change the diameter of the pupil measurably. So R can be automatic and involuntary — genuinely a reflex — while still being slower than a practised voluntary button press.
This is the whole point of the question: it is testing whether you think “reflex” means “spinal”. It does not.
A reflex action integrates and coordinates a stimulus with the response of an effector automatically (involuntarily — you cannot choose to prevent it) and rapidly. Both words. “It is fast” alone is one mark of two.
Where is the pathway coordinated — spinal cord or brain? How many synapses are crossed? What kind of effector has to respond, and how quickly can it? Answer those three and you can explain any reaction-time comparison in this topic, including the ones you have never seen.
Y starts higher (9 against 5). Y rises further (by 7 units against 3). Y takes longer to peak (60 min against 30) and, crucially, does not recover — still 15 at 120 min, while X is back to its starting value. Four comparative statements, each with figures. Three marks is easily covered.
In X: the rise is detected by the pancreas, which secretes insulin into the blood; insulin causes the liver to convert glucose into glycogen for storage and muscles to take up glucose; the concentration falls back to the set point.
In Y the pancreas produces insufficient insulin, so the liver is never stimulated to store the glucose and it stays in the blood. Note what is not broken: absorption from the gut is normal, and Y’s liver is perfectly capable — it is simply not told.
Z detects and responds normally at first — the peak and the timing match X exactly. But the fall does not stop at the set point; it carries on down to 2 units. So the correction has been too large, or has gone on too long: too much insulin has been secreted, or it has continued to act after the concentration returned to normal.
The faintness is worth explaining: brain cells respire almost entirely using glucose, so a low blood glucose concentration limits the energy available to them.
Any negative feedback system corrects after a change has happened, so a small overshoot is unavoidable and you can see it on every homeostasis graph. What makes Z abnormal is the size of the overshoot and the fact that the level goes so far below the set point that a second correction — glucagon — is needed to rescue it.
Temperature receptors in the skin and the brain detect the cold. Impulses cause vasoconstriction: the arterioles supplying the surface capillaries narrow, so less blood flows through the capillaries near the skin surface. Less warm blood at the surface means less heat lost by radiation — and it also means the skin itself is now colder, which is exactly what the data show.
Say “arterioles”. Say “less blood flows”. Do not say the vessels moved.
Oxygen is used in aerobic respiration, so a rise of 160 cm³ per minute means respiration has increased by about 64 %. The person is sitting still, so the extra respiration is not for movement — it is shivering: rapid involuntary muscle contraction, which requires energy from respiration, and respiration releases heat energy as well.
That is the whole chain, and it is a direct link to Topic 12. The heat comes from respiration, never from friction.
Homeostasis is the maintenance of a constant internal environment. The temperature that matters is therefore the core, and the core has fallen by only 0.2 °C in 40 minutes in a room 31 °C colder than the body. That is regulation succeeding, not failing.
The falling skin temperature is not evidence against it — it is a consequence of the mechanism that is protecting the core. Sacrificing the surface to save the middle is the whole strategy.
C still has its tip, so auxin is still made — and the growth figure proves it, because C grew 26 mm, almost as much as normal. The only difference is that C’s tip cannot detect the direction of the light. So the comparison shows that it is the tip that detects the light direction, and that detection there is what causes the bending.
C has two things done to it: it has been covered, and it has been darkened. Without D you could not tell which mattered — perhaps the physical presence of a cap, its weight, or the fact that it blocks air, stops the bending. D has the cap but not the darkness, and D bends normally. So the cap is harmless and it is specifically the exclusion of light from the tip that prevents the response.
“D is a control” on its own is worth one mark at most. Saying what it controls for is the second.
Auxin is made in the shoot tip. Remove the tip and no auxin is produced, so none diffuses down into the shoot. Auxin stimulates cell elongation, so with no auxin the cells hardly elongate and the shoot grows only 2 mm. It cannot bend either, because bending requires unequal elongation and here there is essentially none on either side.
“Light destroys auxin on the lit side” would indeed produce more auxin on the shaded side and therefore this bending, so it is consistent with the results. But so is “light causes auxin to move across to the shaded side”, which is the explanation the syllabus uses, and nothing in this experiment distinguishes the two. No auxin was measured anywhere.
To separate them you would have to compare the total amount of auxin below a lit tip with the total below an evenly lit one — destruction would reduce the total, redistribution would not.
Increase = 6.8 − 2.4 = 4.4 mm. Percentage increase = 4.4 ÷ 2.4 × 100 = 183 % (to 3 significant figures).
Dividing by 6.8 gives 65 %, which is the answer to a different question and is the wrong answer mark schemes watch for. Write the subtraction on its own line: the first mark is usually for the method, so a slip in the arithmetic still scores if the working is visible.
In dim light the radial muscles contract and the circular muscles relax, so the pupil widens and more light enters, allowing the light receptors in the retina to be stimulated. They are an antagonistic pair, because a muscle can only pull and never push, so a second set is needed to reverse the movement.
An answer that names only one set has thrown away a mark that costs nothing to secure.
The room is still bright, so the light-driven reflex should be holding the pupil small — and something is overriding it. The student is frightened, so the adrenal glands have secreted adrenaline into the blood, and one of its three Core effects is increased pupil diameter.
Note the compromise in the number: 4.1 mm is between the bright-room and dark-room values, which fits two influences pulling in opposite directions. Reading that off the data is worth saying.
Whenever a Topic 14 question gives you a change that appears in two different sub-topics — pupil diameter, heart rate, breathing rate — look at the stem for the cause. If it mentions light, it is the reflex. If it mentions fear, effort or excitement, it is adrenaline. Papers deliberately use this to see whether you are matching words to mechanisms or just to topics.
The impulse still arrives at the ending of the first neurone. But no neurotransmitter is released from the vesicles into the synaptic gap, so nothing diffuses across, nothing binds with the receptor proteins, and no impulse is stimulated in the second neurone. Transmission stops at the very first step.
Here release happens normally and the neurotransmitter diffuses across the gap as usual. But the receptor proteins are already occupied by the drug, so the neurotransmitter cannot bind to them — and since the drug itself does not stimulate an impulse, none is produced in the second neurone.
Same outcome, different point of failure. The reason the outcome is identical is that the four events form a chain: break any link and nothing after it can happen, so the result is the same wherever the break is.
The stimulus is detected by the receptor and an impulse travels along the sensory neurone exactly as normal — nothing in the question damages either. The block is further along the arc, so the impulse never reaches the motor neurone and therefore never reaches the effector. The animal detects and cannot respond.
If the block is at the synapse between the sensory and relay neurones, even the impulse to the brain may be interrupted; if it is between relay and motor, sensation is unaffected. Saying which synapse you mean is worth doing.
Never start by describing the fault. Write out the sequence as it should be, identify precisely which step is prevented, then state what still happens before it and what cannot happen after it. That structure earns the marks for the steps that still work as well as the ones that do not, and challenge papers award both.
Seven pairs that look almost identical and have different answers. The distinction is where the marks live.
Click each node. The whole topic is one idea repeated five times: something changes, something detects it, something responds, and the response cancels the change.
Six real student answers. Find the fault before you reveal it.
Ten Cambridge-style challenge questions. Write your answer first, then reveal the model answer and the examiner’s notes.