Hi Tara. Topic 14 is the biggest thing Cambridge asks you to hold in your head, so here is the map before you set off. 14.1 is the nervous system: three kinds of neurone, the reflex arc, and what actually happens in the gap between one neurone and the next. 14.2 is receptors in general and then the eye in detail — the pupil reflex and accommodation, which are the two things examiners ask about almost every year. 14.3 is hormones, four glands and their four hormones, adrenaline, and the comparison with nervous control. 14.4 is homeostasis: blood glucose, and body temperature, both explained as negative feedback around a set point. 14.5 is plants — phototropism, gravitropism and auxin. 14.6 is the exam technique and the exact vocabulary that earns the marks.
Three warnings that between them are worth a grade. First: a spinal reflex is not controlled by the brain. The whole point of it is that the message does not have to travel to the brain and back before you move; the brain finds out afterwards. If your answer contains the word “brain” in a spinal reflex question, read it again. Second: impulses are electrical, neurotransmitters are chemical, and hormones travel in the blood. Nothing travels “along a nerve” except an electrical impulse, and no hormone ever does. Third: sweating cools you by evaporation. Sweat sitting on your skin does nothing at all. The mark is the word evaporates, and the reason is that evaporation takes heat energy (latent heat) from the skin.
One more thing worth saying now, because it will save you re-learning it four times. Every control system in this topic has the same shape: a stimulus (the change), a receptor (which detects it), a coordinator (the brain, the spinal cord, or a gland), an effector (a muscle or a gland) and a response. Write those five words at the top of your rough paper in the exam. A surprising number of six-mark questions in this topic are simply that list, filled in.
What the Nervous System Is, and What It Is For
Cambridge gives you one sentence for the purpose of the nervous system and it is worth memorising exactly: the nervous system is responsible for the coordination and regulation of body functions. Coordination means making separate parts of the body act together at the right moment; regulation means keeping conditions steady. Almost every mark in this section is one of those two ideas dressed up.
The system is divided in two, and the division is by position, not by job:
- The central nervous system (CNS) is the brain and the spinal cord — and nothing else.
- The peripheral nervous system (PNS) is all the nerves outside the brain and spinal cord.
Information travels around this system as electrical impulses, and it travels along cells called neurones. Notice the two words. An impulse is the electrical signal. A neurone is the cell that carries it. A nerve is a bundle of many neurone fibres wrapped together, the way a cable is a bundle of wires — so a nerve is not a cell, and you should not write “the nerve carries the impulse to the brain” when the question has given you a diagram of a single cell.
Neurone = one nerve cell. Nerve = a bundle of neurone fibres. Impulse = the electrical signal that travels along a neurone.
Test yourself with this sentence: “An impulse travels along the sensory neurone, which is one of thousands of fibres inside the nerve.” If that sentence sounds obvious, you have it.
Three Kinds of Neurone
Cambridge asks you to identify sensory, relay and motor neurones in diagrams. It does not ask you to write an essay on their internal structure, and it never asks about the fatty covering some of them have. What it does expect is that you can look at an unlabelled drawing and say which is which, using two clues: where the cell body sits and which way the impulse is travelling.
If the diagram shows the neurone running into the spinal cord from a sense organ, it is sensory. If it runs out of the spinal cord towards a muscle or a gland, it is motor. Sensory carries in, motor carries out — and “motor” is the same word as in motion, which is a reminder that its job ends in movement.
One direction question catches people out: the impulse in a sensory neurone travels past the cell body without stopping in it. It goes along the fibre; the cell body just sits to one side. So do not write that the impulse “goes to the cell body and then to the CNS”.
The Reflex Arc
Now the piece that Cambridge examines most often in this sub-topic. A reflex action is defined as a means of automatically and rapidly integrating and coordinating stimuli with the responses of effectors (muscles and glands). That is the syllabus wording and it is worth learning, because a question asking you to “define a reflex action” is looking for automatic and rapid, and most candidates give only one.
The reflex arc is the pathway the impulse takes. Five stages, in this order, and you should be able to write them out in five seconds:
“The impulse travels to the brain, which decides to move the hand.” This is refused, every time. In a spinal reflex the message is passed from sensory to motor neurone inside the spinal cord, and the response happens before the brain has been informed.
If you are asked why the reflex is arranged this way, the answer is about speed and about protection: the pathway is short, so the response is fast, so the tissue is damaged less. “It is faster” on its own is one mark; “the pathway is shorter so fewer synapses have to be crossed, so the response is faster and the skin is damaged less” is three.
Receptor (in the skin of the finger) → sensory neurone → relay neurone (in the spinal cord) → motor neurone → effector (a muscle in the arm). Three marks usually means the three neurones, but write the receptor and effector too: a list that starts at the stimulus and ends at the response cannot lose a mark for being incomplete.
The reflex arc is confined to the spinal cord. The impulse enters on the sensory neurone, crosses to the relay neurone and straight out on the motor neurone — a short pathway with only two synapses, so the response takes a fraction of a second.
A separate impulse travels up the spinal cord to the brain, and it is only when it arrives at the brain that the sensation of pain is produced. That journey is longer and involves more synapses, so it takes longer — which is why the hand has already moved.
Because the damage a hot or sharp object does depends on how long you are in contact with it. Waiting for a conscious decision would cost tenths of a second, which is the difference between a red mark and a blister. This is the “protective” mark that questions often ask for as a separate point.
Synapses Supplement
A synapse is a junction between two neurones. That is the Core definition and it is worth exactly the words it uses — not “a gap between two nerves”, not “where two neurones join together” (they do not join; that is the whole point).
The Supplement asks you to know the structure of a synapse and the sequence of events at one. Structure first, three named parts:
- Vesicles containing neurotransmitter molecules, in the ending of the first neurone;
- the synaptic gap between the two neurones;
- receptor proteins on the membrane of the second neurone.
Why a gap at all? It looks like a design fault: why break a perfectly good electrical signal, convert it to a chemical, push the chemical across a gap by diffusion, and convert it back? The answer Cambridge wants is direction. Because the vesicles are only on one side and the receptor proteins only on the other, the message can only cross one way. An unbroken electrical wire would carry a signal both ways, and a nervous system in which impulses could run backwards would be useless.
Notice also that the gap is crossed by diffusion — the same process you met in Topic 3. It works here only because the gap is tiny, a fraction of a micrometre. Diffusion is fast over a very short distance and hopeless over a long one, which is exactly the argument you used for the alveolus in Topic 11.
Run the four events in order. (a) The impulse still arrives and neurotransmitter is still released — the first neurone is undamaged. (b) The molecules still diffuse across the gap. (c) They cannot bind with the receptor proteins, because those are blocked. (d) So no impulse is stimulated in the second neurone.
This is where the extra mark lives. The first neurone is completely normal and the impulse reaches the synapse perfectly well. The failure is entirely at the binding step. An answer that just says “the impulse cannot get through” has not shown that you know which of the four events failed.
If the synapse is the one between the relay and the motor neurone, then no impulse reaches the motor neurone, so no impulse reaches the effector, so the muscle does not contract and the reflex does not happen. The stimulus is still detected — the receptor and the sensory neurone are fine — but there is no response.
Electrical impulse along a neurone. Chemical neurotransmitter across a synapse. Never mix them: “the impulse jumps the gap” is worth nothing, because nothing electrical crosses the gap at all.
Diffuse, not “travel”. Bind (with receptor proteins), not “stick” or “attach to the neurone”. Vesicles, not “sacs” or “bubbles”. Synaptic gap, not “space”.
And effector means a muscle or a gland. A great many candidates lose a mark by defining an effector as “a muscle”, because plenty of reflexes end in a gland secreting something — salivating when you smell food, for instance.
What a Sense Organ Is
Cambridge defines sense organs as groups of receptor cells responding to specific stimuli. Take that sentence apart, because both halves are marks. “Groups of receptor cells” — a sense organ is not one cell, it is an organ built round thousands of them. “Specific stimuli” — each type of receptor answers to one kind of change and ignores everything else. Light receptors in your retina are not affected by sound; the pressure receptors in your skin do not respond to light.
The five stimuli named in the syllabus are light, sound, touch, temperature and chemicals. Learn them as a list of five, because “name three stimuli that human receptors respond to” is a free mark if you have and an awkward pause if you have not. Chemicals covers both taste and smell, which is why they are so closely linked.
The cells are receptor cells, or light receptors in the eye. Not “sensors”, not “detectors”, not “nerve endings”. When a question says “name the type of cell found in the retina”, the answer is light receptor cell — and at Supplement level you can name the two kinds, rods and cones.
The Structure of the Eye
You need seven structures by name at Core, plus three more at Supplement. Here they all are on one drawing. Spend a minute on this diagram now; every eye question in the topic refers back to it.
“The pupil lets light in.” It does, but so does a hole in a wall. The pupil is a hole, so it has no muscle and cannot do anything. It is the iris that controls how much light enters through it. A question that says “explain how the amount of light entering the eye is controlled” is asking about the iris.
“The lens focuses the light on to the eye.” On to the retina — naming the destination is the mark. And notice that the cornea also bends the light; in fact it does most of the refracting. The lens does the adjustable part.
“The optic nerve sends messages to the brain.” Use impulses, not “messages” or “signals” or “information”. The word costs nothing and is sometimes the whole mark.
The Pupil Reflex
At Core you must explain the pupil reflex in terms of light intensity and pupil diameter: in bright light the pupil gets smaller; in dim light it gets larger. At Supplement you must explain it in terms of the antagonistic action of the circular and radial muscles of the iris.
Antagonistic means the two sets pull in opposite directions, so one contracts while the other relaxes. Think of the circular muscles as a drawstring running round the pupil: contract them and the hole closes. The radial muscles run outwards like the spokes of a wheel: contract them and they pull the edge of the hole outwards, so the pupil opens.
Not because the eye “does not need as much light”. The reason Cambridge wants is protection: very bright light can damage the light receptor cells in the retina, so reducing the pupil diameter reduces the light reaching them. In dim light the pupil widens so that enough light enters for the receptors to be stimulated at all.
Also worth knowing: it is a reflex, so it is automatic and rapid and you cannot do it on purpose. That makes it a favourite example when a question asks you to name a reflex other than the withdrawal reflex.
Accommodation Supplement
Accommodation is the eye changing its focus between near and distant objects. Four things change together, and Cambridge asks for all four: the ciliary muscles, the tension in the suspensory ligaments, the shape of the lens, and the amount the light is refracted.
The hard part is the middle step, so get it straight once. The ciliary muscle is a ring. When a ring of muscle contracts, the hole in the middle of it gets smaller — think of a drawstring bag. The suspensory ligaments run from that ring inwards to the edge of the lens. So when the ring gets smaller, the ligaments go slack, and a lens that is not being pulled outwards springs back into its natural fat, rounded shape. A fat lens refracts light more, which is what you need for a near object.
Look at what makes the lens fat. Nothing pushes it — the ligaments go slack and the lens springs back into its rounded shape using its own elasticity. So the fat, near-vision shape is the one that depends on the lens being elastic. The thin shape is produced by the ligaments pulling, which still works on a hard lens.
A hard lens can still be pulled thin, so distant vision is largely unaffected. But it can no longer round itself up, so it cannot refract light enough for a near object, and the light from a near object is brought to a focus behind the retina. Near vision goes first — which is why people begin holding a book at arm’s length in their forties.
You have never been taught this condition. You were given one new fact (the lens hardens) and asked to run a mechanism you do know. That is exactly what challenge-level application means, and the marks are for the chain: elastic → rounds up → refracts more → near objects. Miss the chain out and “near vision” alone is worth one mark of four.
Rods and Cones Supplement
The retina contains two kinds of light receptor cell and the syllabus limits what you need to a very short list — do not learn more than this, and do not learn less.
| Rods | Cones | |
|---|---|---|
| Sensitivity | Greater sensitivity to light, so they work in dim light — they are responsible for night vision | Need brighter light to be stimulated |
| Colour | Do not give colour vision | Three kinds, each absorbing a different colour of light, giving colour vision |
| Where | Spread across most of the retina, but almost absent from the fovea | Concentrated at the fovea, becoming scarce further out |
The distribution graph
Examiners usually give you a graph. Position on the retina goes along the x-axis, with the fovea at 0 in the middle; the number of receptors per mm² goes up the y-axis. Read it like this:
- Cones: a tall, narrow peak at the fovea, then a very low level everywhere else.
- Rods: none at the fovea, rising to a maximum a short way out from it, then falling slowly towards the edges of the retina.
- The blind spot: a gap where both lines drop to zero, a short distance to one side of the fovea, where the optic nerve leaves and there are no receptors.
The fovea is the small part of the retina directly opposite the pupil, on which light from whatever you are looking straight at is focused. It is packed with cones and has almost no rods, which gives you two useful predictions:
- Detail and colour are sharpest at the centre of what you are looking at, and vaguer and less colourful at the edges of your vision. You can test that right now with something coloured held to one side.
- In very dim light the fovea is nearly blind, because the cones there are not sensitive enough and there are almost no rods to take over. Astronomers exploit this: to see a faint star you look slightly to one side of it, so that its light falls on a rod-rich part of the retina.
They are two different places and questions deliberately offer both. The fovea is where vision is best. The blind spot is where the optic nerve leaves the eye, so there are no receptor cells at all and nothing can be detected there. If a question describes a point on the retina where an image produces no impulses, that is the blind spot.
In dim light the cones are not stimulated — they are not sensitive enough. Vision in a dark room is therefore almost entirely due to rods, which have the greater sensitivity.
Rods do not give colour vision, so whatever the student is seeing is not being seen as a colour. But that is not the same as saying rods respond equally to all wavelengths — and the data say they do not, because the red light is harder to see.
The rods must absorb blue-green light more readily than red light, so dim blue-green light stimulates them and produces impulses while dim red light does not. Only the cones respond well to red, and in a dark room the cones are effectively switched off. Notice the honest wording: this is a suggestion that fits the observation, which is exactly what the question asked for.
The Definition, Word by Word
Cambridge’s definition of a hormone has four parts, and a four-mark question is usually one mark for each. Learn it as four clauses rather than one sentence:
Every one of those four clauses is a place candidates lose marks. “A chemical” — not a cell, not a nerve signal. “Produced by a gland” — specifically by an endocrine gland, one that secretes straight into the blood rather than down a tube. “Carried by the blood” — this is the clause worth guarding most fiercely, because hormones never travel along nerves, and writing that they do will cost you marks in several different questions. “Alters the activity of specific target organs” — a hormone reaches every part of the body, since blood goes everywhere, but only the target organs respond to it. The rest ignore it.
Insulin is carried in the blood to every part of the body, but only its target organs respond to it. The targets you need on this syllabus are the liver and the muscles. So a hormone is not aimed like a letter — it is broadcast to everybody and only the organs that can respond to it do. When a question asks “how does a hormone reach its target organ?”, the honest answer is in the blood, like everything else in the blood, and the selectivity is at the receiving end.
The Four Glands You Must Know
The syllabus limits you to four glands and their hormones. That is a small enough list that there is no excuse for getting it wrong, and large enough that it appears somewhere in nearly every paper.
| Gland | Hormone | What it does (as far as the syllabus goes) |
|---|---|---|
| Adrenal glands | Adrenaline | Secreted in “fight or flight” situations; increases breathing rate, heart rate and pupil diameter |
| Pancreas | Insulin (and, at Supplement, glucagon) | Insulin decreases blood glucose concentration |
| Testes | Testosterone | Development and regulation of male secondary sexual characteristics |
| Ovaries | Oestrogen | Development and regulation of female secondary sexual characteristics |
Where the glands are
Exam diagrams usually show a body outline with label lines and ask you to name the gland and its hormone, so learn the position as well as the name. Picture the body from the front:
- the two adrenal glands sit like small caps on top of the two kidneys, at the back of the abdomen, roughly level with the bottom of the ribs;
- the pancreas is a long, flat gland lying just below the stomach, in the loop of the first part of the small intestine (the duodenum);
- the testes hang outside the abdomen in the scrotum;
- the ovaries are a pair of small oval glands in the lower abdomen, one on each side of the uterus, close to the ends of the oviducts.
Adrenaline and “Fight or Flight”
Adrenaline is secreted by the adrenal glands in situations where the body may have to fight or run away — fear, anger, sudden danger, the start of a race, the moment before you walk into an exam hall. Its job is to get the body ready for hard physical work in the next few seconds.
The Core effects are limited to three, and it is worth resisting the temptation to add more:
- increased breathing rate — so more oxygen is taken into the blood;
- increased heart rate — so oxygen and glucose are delivered to the muscles faster;
- increased pupil diameter — so more light enters the eye and you can see more.
Supplement. You must also know adrenaline’s role in metabolic activity, limited to two things: it increases the blood glucose concentration, and it increases the heart rate.
Both make the same sense once you connect them to Topic 12. A muscle that is about to work hard must respire faster; respiration needs glucose and oxygen; glucose comes from the blood and oxygen comes from the blood; and the rate at which both arrive depends on how fast the blood is pumped. So adrenaline raises the supply of the fuel, raises the supply of the oxygen, and speeds up the delivery. Three separate actions with one purpose.
In 14.2 the pupil widens because the light is dim, and that is a reflex coordinated by the nervous system, happening in a fraction of a second.
In 14.3 the pupil widens because adrenaline has been secreted, and that is hormonal, arriving in the blood, slower to start and slower to wear off. Somebody who is frightened has wide pupils in a fully lit room, which is how you can tell the two apart.
A question that gives you a person with dilated pupils and asks you to explain it has therefore two possible answers, and the stem will tell you which. Read whether it mentions light or fear.
Nervous Control Compared with Hormonal Control
The syllabus limits this comparison to two things: speed of action and duration of effect. You may know more, but those two are what is examined, so build your answer round them.
| Nervous control | Hormonal control | |
|---|---|---|
| What travels | Electrical impulses along neurones | Chemicals in the blood |
| Speed of action | Very fast — a fraction of a second | Slower — seconds to minutes, or longer |
| Duration of effect | Short — the effect stops almost as soon as the impulses stop | Long — may last minutes, days or years |
| Where it acts | A precise place — wherever the neurone ends | Widespread — any target organ the blood reaches |
Ask: how fast did it have to be, and how long does it need to last?
Pulling your hand off a hot pan has to happen in milliseconds and needs to last no time at all — nervous. Growing from a child into an adult happens over years and must not stop — hormonal. Being ready to run away from a dog is somewhere in between, which is why adrenaline takes a couple of seconds to work and why you still feel shaky ten minutes later.
“Within one second” and “still raised three minutes later” are two different observations, and the whole point of the question is that no single system explains both. Nervous control is fast but short-lived; hormonal control is slow but long-lasting. If both features are present, both systems are involved.
The sound is detected by receptors, impulses travel along neurones at very high speed, and the heart is stimulated almost immediately. Electrical transmission along a neurone is the fastest thing in the body, which is why a one-second response has to be nervous.
The adrenal glands secrete adrenaline into the blood. It takes several seconds for the blood to carry it round the body to the heart, so it cannot account for the first second — but once it is circulating it goes on acting until it is broken down, which is why the heart rate is still up three minutes later.
State it explicitly: nervous control is rapid but brief; hormonal control is slower to act but its effects last much longer. That sentence is worth a mark on its own in nearly every question of this shape.
What Homeostasis Is
Homeostasis is the maintenance of a constant internal environment. Six words, and Cambridge will take them almost verbatim. “Internal environment” means the conditions inside your body — the temperature, the water content, the glucose concentration of the blood, the carbon dioxide concentration — as opposed to the weather outside, which does whatever it likes.
The obvious question is why bother, and the answer connects straight back to Topic 5. Every reaction in your body is catalysed by an enzyme, and enzymes work at a particular temperature and pH. Let your body temperature drift and the enzymes work too slowly; let it rise too far and they are denatured and stop working altogether. Keeping the inside steady is what allows the chemistry of being alive to carry on regardless of what the outside is doing.
Negative feedback and the set point Supplement
Homeostatic control works by negative feedback around a set point. Here is the idea in one sentence: a change away from the set point triggers a response that reverses the change. Not stops it, not prevents it — reverses it. That is what the word negative means here, and it is why a graph of any homeostatic variable is a wavy line hovering around a level rather than a flat one.
The general shape, which fits blood glucose, temperature and everything else:
- the level rises above the set point;
- the change is detected;
- a response is triggered that makes the level fall;
- the level returns towards the set point — and often overshoots slightly, which triggers the opposite correction.
Two things follow that examiners like to test. First, the level is never perfectly constant; it fluctuates around the set point, because a correction cannot begin until a change has already happened. Second, two opposite mechanisms are needed, one for each direction — which is exactly why the pancreas makes two hormones and why the skin has both sweating and shivering.
Blood Glucose
Core content here is one line: insulin decreases blood glucose concentration. If you learn nothing else in this section, learn that, and learn that insulin comes from the pancreas.
The Supplement asks for the whole system: the control of blood glucose by the liver, and the roles of insulin and glucagon. The critical thing to get right is which organ does what, because this is where nearly all the marks are lost.
Now the two halves of the cycle:
- Blood glucose too high (after a meal). The pancreas secretes insulin. Insulin travels in the blood to the liver, which converts glucose into glycogen for storage. Muscle cells also take up more glucose. The blood glucose concentration falls.
- Blood glucose too low (a few hours later, or during exercise). The pancreas secretes glucagon. Glucagon travels in the blood to the liver, which converts glycogen back into glucose and releases it into the blood. The blood glucose concentration rises.
1. Saying insulin converts glucose into glycogen. Insulin is a hormone, not an enzyme; it does not carry out the conversion. It causes the liver to do it. Write “insulin stimulates the liver to convert glucose to glycogen” and you are safe.
2. Saying the conversion happens in the blood. It happens inside the liver. Glycogen is not found in blood plasma.
3. Writing glucagon when you mean glycogen, or the reverse. They differ by two letters and they are completely different things: glucagon is a hormone made by the pancreas; glycogen is a storage carbohydrate in the liver and muscles. Say them out loud — glu-CA-gon, GLY-co-gen — until the sounds are separate in your head.
4. Saying the pancreas stores the glycogen. The pancreas secretes; the liver stores.
Type 1 diabetes Supplement
In Type 1 diabetes the pancreas does not produce enough insulin, so blood glucose concentration rises after a meal and stays high. The syllabus asks you to outline the treatment, and the outline is short:
- Insulin is injected, usually before meals, in an amount matched to the food being eaten. It has to be injected rather than swallowed because insulin is a protein — it would be digested in the stomach and small intestine into amino acids and never reach the blood.
- The diet is managed, particularly the amount and timing of carbohydrate, so that the rise in blood glucose is predictable.
- Blood glucose is monitored regularly, so that the dose can be matched to the actual concentration.
- Exercise is taken into account, because working muscles use glucose and can bring the concentration down sharply.
That is as far as 0610 goes. You are not asked about the causes of diabetes, about Type 2, or about the long-term complications.
Starting level: B starts much higher (9 against 5 units). Size of the rise: B rises by 7 units, A by only 3. Return: A returns to its starting value within two hours; B has fallen by only 1 unit and is still far above where it began.
A comparison that says “B is higher” earns one mark of three. Attach a number to every statement — that is what turns a description into a comparison.
In A, the rise in blood glucose is detected by the pancreas, which secretes insulin into the blood. Insulin causes the liver to convert the excess glucose into glycogen for storage, and muscle cells to take up glucose. The concentration therefore falls back to the set point.
In B the pancreas does not produce enough insulin. So although the glucose is absorbed just as fast, there is no signal to the liver, the excess glucose is not converted to glycogen, and it stays in the blood. Note what is not wrong: B’s liver is perfectly capable of storing glycogen. It is simply never told to.
Temperature Control
Your core body temperature is held at about 37 °C, and the whole apparatus for doing it is in the skin, coordinated by the brain. Cambridge asks you to identify seven structures in a diagram of the skin, so learn the drawing rather than a list.
“Sweating cools you down.” Incomplete. Sweat lying on the skin does nothing. The mark is for evaporation: as the water in sweat evaporates it takes heat energy from the skin, which cools the body. That is also why sweating works badly on a humid day — the sweat cannot evaporate.
“The blood vessels move closer to the surface.” They do not move at all. Blood vessels are fixed in position. What changes is their diameter: the arterioles supplying the surface capillaries widen (vasodilation) or narrow (vasoconstriction), so more or less blood flows through the capillaries near the surface. Notice also that it is the arterioles that dilate, not the capillaries — capillary walls are one cell thick and contain no muscle, so they cannot change their own diameter.
“Shivering makes heat by friction.” No. Muscles contracting need energy, which comes from respiration, and respiration releases heat energy. Increase the rate of respiration in your muscles and you warm up. That is the chain, and it links straight back to Topic 12.
And one more, worth remembering because it is so widely believed: hairs standing up trap a layer of air, and it is the air, being a poor conductor of heat, that insulates. The hairs themselves insulate very little, which is why the mechanism does almost nothing on a human and a great deal on a cat.
Vasodilation. The arterioles supplying the surface capillaries have widened, so more blood flows through the capillaries near the skin surface and more heat is lost by radiation to the surroundings. That is why the skin looks flushed — the redness is the extra blood, and it is direct visual evidence of the mechanism.
Sweating. The sweat glands are producing sweat, which reaches the surface, and as it evaporates it takes heat energy from the skin, cooling the body.
The cooling comes from evaporation, not from the sweat itself. On a humid day the air already contains a great deal of water vapour, so sweat evaporates much more slowly — it runs off the skin instead. Less evaporation means less heat lost, so the core temperature rises further. This is why the wet skin in the stem is a clue rather than a reassurance.
Enzymes are proteins, and above their optimum temperature they are denatured: the shape of the active site changes permanently, so the substrate no longer fits and the reaction stops. Since every reaction in the body is enzyme-catalysed, a sustained rise to 41 °C would slow or stop metabolism, which is why a high fever is treated as an emergency.
Use denatured, never “killed”. An enzyme is not alive, so it cannot be killed.
Two Definitions, and a Warning About Them
A tropism is a growth response of a plant to a stimulus coming from a particular direction. The two you need are:
- Gravitropism — a response in which parts of a plant grow towards or away from gravity.
- Phototropism — a response in which parts of a plant grow towards or away from the direction of the light source.
The word grow is doing serious work in both definitions, and it is the thing most answers leave out. A plant does not move towards the light the way an animal would. It grows more on one side than the other, and the difference in growth bends it. That is why a tropism takes hours or days rather than seconds, and why it is permanent — a shoot that has bent towards a window does not straighten again when you turn the pot round; it bends the other way instead, and the old bend stays.
Directions, which are worth knowing cold because a question can ask about either organ:
| Light | Gravity | |
|---|---|---|
| Shoots | Grow towards the light — positively phototropic | Grow away from gravity, i.e. upwards — negatively gravitropic |
| Roots | Grow away from the light — negatively phototropic | Grow towards gravity, i.e. downwards — positively gravitropic |
Ask what each organ is for. A shoot needs light for photosynthesis, so growing towards light and away from gravity puts the leaves where the light is. A root needs water and mineral ions, and needs to anchor the plant, so growing down into the soil and away from the light puts it where the water is.
That is the survival value, and questions ask for it directly. But be careful how you phrase it, because there is a trap waiting in the next box.
“The root grows down because it needs water” is not an explanation of anything. A root has no needs and no plan. The correct structure of an answer is to keep the two ideas in separate sentences:
Mechanism: auxin is unequally distributed, cells on one side elongate more, the organ bends.
Advantage: the resulting direction of growth means the root reaches water and anchors the plant.
Notice that a question saying “explain how” wants the mechanism, and one saying “suggest the advantage” wants the second. Giving the wrong one earns nothing, however true it is.
Investigating Tropisms
You are expected to be able to investigate and describe gravitropism and phototropism in shoots and roots. The experiments are simple and the marks are almost all in the controls.
Testing gravity is awkward, because you cannot switch gravity off. The usual answer is a klinostat — a slowly rotating disc that a germinating seed is pinned to. Because the seedling turns steadily, every side of it spends equal time facing downwards, so the gravity stimulus is applied equally in all directions rather than removed. A seedling on a stationary disc is the comparison, and that one shows the response.
If an exam question gives you a klinostat, you have been given the control. Say what it does in that language: it does not remove gravity, it gives the stimulus equally from all directions.
Investigating gravitropism and phototropism in roots and shoots
Gravitropism. Soak some bean seeds and let them germinate until the root is about 1 cm long. Pin three seedlings to the cork disc of a klinostat, each pointing a different way: one root pointing up, one sideways, one down. Pin three more seedlings the same way to a second klinostat. Keep both in the dark, so light cannot be the cause of any bending, and line them with damp cotton wool so the seedlings do not dry out. Switch the first klinostat on, so it turns slowly; leave the second one still. Leave both for two days.
Results. On the still disc every root has curved to grow downwards and every shoot has curved to grow upwards, whichever way it started. On the turning disc the roots and shoots grow straight on, in the direction they were pinned. Conclusion: roots are positively gravitropic and shoots are negatively gravitropic. The turning disc is the control: it shows that the bending needs gravity to act from one direction.
Phototropism in roots. Grow seedlings with their roots in clear water or on damp paper inside a box with one slit, so light comes from one side only. The shoots bend towards the slit; the roots bend away from it (negatively phototropic). A control box lit evenly from above shows straight growth.
Measuring the response. Measure the angle between the original direction of growth and the new tip with a protractor, or measure how far the tip has moved sideways. Use several seedlings of the same type and age for each condition and calculate a mean. Keep the temperature, the water supply and the light conditions the same for every seedling.
Auxin Supplement
Now the mechanism. The Supplement asks you to explain phototropism and gravitropism in a shoot as examples of the chemical control of plant growth, and it limits the role of auxin to four points. Here they are, and it is worth learning them in this order because they form a chain:
- auxin is made in the shoot tip;
- it diffuses through the plant from the shoot tip;
- it is unequally distributed in response to light and gravity;
- it stimulates cell elongation.
Everything else follows from those four. In a shoot lit from one side, auxin accumulates on the shaded side. More auxin means more cell elongation, so the cells on the shaded side get longer than the cells on the lit side, and a stalk whose cells are longer on one side than the other must bend — towards the light.
Gravitropism in a shoot. The same four rules explain a shoot laid on its side in the dark. Auxin becomes unequally distributed in response to gravity, accumulating on the lower side. More auxin on the lower side means more cell elongation there, so the lower side grows longer and the shoot bends upwards, away from gravity.
What about the root? The syllabus asks you to explain phototropism and gravitropism of a shoot in terms of auxin, so a root explanation is not required — and that is just as well, because roots respond to auxin differently and the mechanism goes beyond 0610. For roots, describe what they do (positively gravitropic, negatively phototropic) and leave the auxin explanation to the shoot. If a question asks you to explain a root response, read it carefully: it is almost certainly asking for the observation and its advantage, not for a chemical mechanism.
Agar is a jelly made mostly of water, and small molecules can diffuse through it. Plastic is impermeable, so nothing can pass. That is the only difference the experiment has introduced, so whatever is being blocked must be a substance that moves by diffusion.
Auxin is made in the tip, becomes unequally distributed there in response to the light, and then diffuses down through the agar into the shoot below. More auxin passes down the shaded side, those cells elongate more, and the shoot bends towards the light. The agar does not stop it, so the response is normal.
Auxin is still made in the tip, but it cannot reach the cells below. Since auxin stimulates cell elongation, and there is now none in the growing region, the shoot hardly elongates at all and cannot bend — a bend requires one side to grow more than the other, and here neither side is growing.
That the signal from the tip is a chemical that diffuses, not an electrical signal and not something carried by living tissue — because the tip was cut off entirely and the response still worked. This is one of the classic pieces of evidence for the whole idea of a plant growth substance, and it is exactly the kind of reasoning challenge papers reward.
“The shoot moves towards the light.” It grows. Movement and growth are different processes and the definition uses the second.
“Auxin makes the plant bend.” Auxin stimulates cell elongation. The bend is a consequence of unequal elongation, not an action auxin performs.
“Auxin builds up on the light side.” The shaded side. Say it out loud a few times; it is the most reversed fact in the topic.
“The cells on the shaded side divide more.” They elongate — they get longer. Cell division is not what auxin is doing here as far as this syllabus is concerned.
“The root grows down to find water.” Roots do not look for anything. State the response, then state the advantage, in separate sentences.
“Auxin is a plant hormone that travels in the plant’s blood.” Plants have no blood. Auxin diffuses.
Why This Topic Is Marked So Strictly
Topic 14 is unusually unforgiving, and it is worth understanding why. In most topics you can express an idea several ways and still be credited. Here, the correct and incorrect versions of an answer often differ by a single word — evaporates, arterioles, elongate, shaded, denatured, diffuse — and the mark scheme is built round exactly those words, because they are the ones that show you understand a mechanism rather than a story.
So the technique for this topic is different from the technique for, say, Topic 11. There, you were mainly assembling ideas. Here, you are choosing words. What follows is the list of choices worth making.
The Vocabulary Table
| Do not write | Write | Because |
|---|---|---|
| the impulse jumps the gap | neurotransmitter diffuses across the gap and binds with receptor proteins | nothing electrical crosses a synapse |
| the brain decides to move the hand | the impulse passes from sensory to motor neurone via a relay neurone in the spinal cord | a spinal reflex does not involve the brain |
| the pupil lets more light in | the iris controls how much light enters the pupil | the pupil is a hole and cannot act |
| the lens moves to focus | the lens changes shape — fatter for near, thinner for distant | this is not a camera |
| the ligaments contract | the ciliary muscle contracts; the ligaments slacken | ligaments are not muscle |
| hormones travel along nerves | hormones are carried in the blood | this is the defining property of a hormone |
| insulin turns glucose into glycogen | insulin causes the liver to convert glucose into glycogen | insulin is a hormone, not an enzyme |
| the pancreas stores glycogen | the liver stores glycogen | pancreas detects and secretes; liver stores |
| sweating cools you down | sweat evaporates, taking heat energy from the skin | evaporation is the cooling process |
| blood vessels move to the surface | vasodilation of the arterioles supplying the surface capillaries | vessels change diameter, not position |
| shivering makes heat by friction | muscle contraction increases respiration, which releases heat energy | heat comes from respiration |
| the shoot moves towards the light | the shoot grows towards the light | a tropism is a growth response |
| auxin makes the shoot bend | auxin stimulates cell elongation on the shaded side | bending is the consequence, not the action |
| the enzymes are killed | the enzymes are denatured | an enzyme is not alive |
How to Read a Command Word in Topic 14
State / name — one word or one short phrase, no reason needed. Do not waste a sentence explaining; you get no extra credit and you lose time.
Describe — say what happens, in order, with figures if there are figures. No causes.
Explain — say why. Every point needs a “so that” or a “because” attached to it. In this topic an explain question almost always wants a chain, and the marks are for the links, not the ends.
Suggest — you have not been taught this exact case; take a mechanism you know and apply it. Reasoning that is sensible and biological is credited even if it is not the printed answer. The reason is the mark, never the bare statement.
Compare — both things in every sentence, with a comparative word (higher, faster, longer, more). “Nervous control is fast” is not a comparison. “Nervous control acts faster than hormonal control” is.
How to Attack a Topic 14 Data Question
1. Read the axes and the units before the line. Blood glucose in arbitrary units, temperature in °C and pupil diameter in mm all produce similar-looking graphs and mean entirely different things.
2. Find the set point. Most homeostasis graphs have a level the line keeps returning to. Mark it. Every description you write should be relative to it: above, below, returning to.
3. Mark the events on the axis. The meal, the injection, the moment the person entered the cold room, the moment the light was switched on. Everything before an event is the baseline and everything after it is the response.
4. Quote figures whenever you describe. “It rises” is worth one mark at most; “it rises from 5 to 9 units in the first 30 minutes and returns to 5 by 120 minutes” is the whole answer.
5. Look for the lag. The gap between a change and its correction is often the point of the question — it is why the level overshoots, and it is the evidence that a hormone was involved rather than a nerve.
6. Do not claim more than the data show. A graph showing that glucose stays high shows that the correction failed; it does not by itself tell you whether the pancreas, the insulin or the liver was at fault. Say what would need to be measured to find out.
Three Scenarios to Test Yourself On
In a spinal reflex the impulse passes from the sensory neurone to a relay neurone inside the spinal cord and straight out along the motor neurone. The brain receives a separate impulse afterwards, which is why the pain is felt after the hand has moved. Put the brain in charge and you have described a voluntary action, not a reflex.
The student has gone sensory → motor with nothing between them. The arc has three neurones and therefore two synapses. Leaving the relay neurone out is worth a mark on its own in most mark schemes.
Nothing electrical crosses a synapse. An impulse arriving at the ending causes vesicles to release neurotransmitter into the synaptic gap; the molecules diffuse across and bind with receptor proteins on the next neurone, which stimulates a new impulse in it. Four stages, and the word “jumps” replaces all of them with nothing.
“A receptor in the skin detects the heat. An impulse travels along the sensory neurone into the spinal cord, where it is passed to a relay neurone and then to a motor neurone. At each synapse the impulse causes neurotransmitter to be released from vesicles; it diffuses across the gap and binds with receptor proteins, stimulating an impulse in the next neurone. The motor neurone carries the impulse to a muscle, the effector, which contracts and pulls the hand away. A separate impulse travels to the brain, where the pain is felt.”
Homeostasis maintains a constant internal environment. The temperature that matters is the core temperature, because that is where the organs and the enzymes are. Skin temperature is not being held constant, and it is not supposed to be.
The core has fallen by 0.2 °C in thirty minutes in a room 32 °C colder than the body. That is a spectacularly successful piece of regulation, not a failure. The classmate has looked at the wrong number.
The skin is cold precisely because the control system is working. Vasoconstriction of the arterioles supplying the surface capillaries has reduced the blood flow near the surface, so less warm blood reaches the skin. That is what keeps the heat in the core — and it necessarily makes the skin colder. The cold skin is evidence of success.
Also happening: the hair erector muscles have contracted, raising the hairs and trapping a layer of air, which is a poor conductor of heat; sweat production has stopped; and shivering has begun, so muscles are contracting rapidly, respiring faster and releasing heat energy. Fatty tissue beneath the skin insulates throughout.
Judgement: the conclusion is wrong. The data show temperature control working well, and the fall in skin temperature is a consequence of the mechanism rather than a sign of its failure.
Say this first — an evaluation that only attacks is incomplete. The comparison does show that the shoot tip is necessary for the bending response, because removing it abolished the response while everything else was kept the same.
The word auxin appears nowhere in the evidence. Nothing here was measured chemically. Removing the tip could have stopped the bending for other reasons — it might simply have injured the seedling, or the tip might be needed for growth of any kind. In fact seedling Q has probably barely grown at all, and a shoot that is not growing cannot bend whatever the cause. The student has jumped from “the tip matters” to “a named chemical from the tip matters” without evidence for the second.
There is no control for the injury of cutting, and no seedling grown with light from all sides to show what “straight” looks like without a directional stimulus. There is also only one seedling of each kind, so no repeats and no way to know whether this is typical.
Remove the tip and replace it with a block of agar through which a substance can diffuse; if the response returns, the signal is chemical and diffusible. Better still, replace the tip with a block of agar that a tip has been standing on but with the tip itself removed — if that restores growth, the chemical alone is enough. Add a control in which the tip is replaced with an impermeable barrier, grow several seedlings of each type, and keep temperature, water and light intensity the same throughout.
The Night-Before Checklist
The two parts of the nervous system and exactly what is in each. The role of the nervous system in Cambridge’s own words. The difference between a neurone, a nerve and an impulse. How to tell sensory, relay and motor neurones apart in a diagram, by cell body position and by direction. The five stages of a reflex arc in order. The definition of a reflex action, with both automatically and rapidly. Why a spinal reflex does not involve the brain, and why that is an advantage. What a synapse is. The three named parts of a synapse. The four events at a synapse, in order, with the words diffuse and bind. Why transmission is one-way.
The definition of a sense organ. The five stimuli. The seven Core structures of the eye and the function of each. The pupil reflex in bright and in dim light, with circular and radial muscles and the word antagonistic. Why the pupil closes in bright light. Accommodation for near and for distant objects, all four steps, starting with what a contracting ring of muscle does to its own diameter. Rods for dim light, three kinds of cone for colour. Where the fovea is and what is special about it. Why the blind spot is blind.
The four-clause definition of a hormone. The four glands and their hormones. The three Core effects of adrenaline and the two Supplement ones. Nervous against hormonal control, in terms of speed of action and duration of effect. Why a hormone reaches every organ but only some respond.
The definition of homeostasis. Negative feedback and set point, and why the level always fluctuates. Insulin decreases blood glucose. The full loop for high and for low blood glucose, with the pancreas detecting and the liver storing. The difference between glucagon and glycogen. The outline treatment of Type 1 diabetes and why insulin is injected. The seven structures in a skin diagram. Sweating with the word evaporates. Vasodilation and vasoconstriction of arterioles. Shivering as respiration releasing heat. Hairs trapping air. The role of the brain. Why 37 °C matters, in terms of enzymes.
The definitions of gravitropism and phototropism, both containing the word grow. The four directions for shoots and roots. The four points about auxin: made in the shoot tip, diffuses, unequally distributed, stimulates cell elongation. Which side auxin accumulates on, and the check that proves it. The phototropism experiment with the tip removed and the tip covered, and what each shows. What a klinostat does — and what it does not do.
That list is the whole topic. It is long, because this is the longest topic on the syllabus. If you can say it out loud in about ten minutes, you are ready.