Coordination and response

• One sentence that prevents four of the seven. "Circular closes the pupil in bright light; ciliary contracts for near objects; insulin lowers high glucose; arterioles, not capillaries, change width." Write it at the top of your rough working before you start the paper — and if you are an Extended candidate, add "shaded side, longer side".
• The direction test. If you can only remember one thing, remember what the names mean. Sensory comes from "sense" — it brings the sensation in. Motor comes from "motion" — it sends the instruction to move out. Relay means to pass something on — it sits in the middle and hands the signal over. Get the direction right and the diagram cue usually falls into place behind it.
• Which is which, under exam pressure. Picture the word circular as a drawstring on a bag: pull it and the opening closes. Picture radial as the spokes of a wheel: pull outwards on the rim of a hole and the hole opens. Bright light closes the drawstring; dim light pulls the spokes.
• Insulin puts glucose in. In to the cells, in to store, and therefore out of the blood — so the concentration goes down. One letter, one direction, and it is the single most reversed fact in the topic.
• Two words, one letter apart, opposite jobs. Insulin puts glucose into cells and into store, so it lowers blood glucose. Glucagon makes stored glucose gone from the liver and into the blood, so it raises blood glucose. If you remember nothing else from this section, remember which direction each one pushes.
• Say it out loud, twice. "High glucose, insulin, glucose into glycogen, concentration falls." "Low glucose, glucagon, glycogen into glucose, concentration rises." Two sentences, four seconds. They are worth more marks per second of revision than anything else in this chapter.
• Hot and cold are mirror images. Every response in this section is the reverse of one in the previous section: more sweat / less sweat, dilate / constrict, hairs flat / hairs raised. Learn one column properly and you get the other by reversing it — with one exception. Shivering has no opposite. There is no "anti-shivering" when you are hot; that response exists on the cold side only.
• Shoots grow towards the light and away from gravity. Roots grow towards gravity and away from the light. Four responses, two organs, two stimuli — and an advantage attached to each one.

• State that electrical impulses travel along neurones.
• Describe the mammalian nervous system as the central nervous system, consisting of the brain and the spinal cord, and the peripheral nervous system, consisting of the nerves outside them.
• Describe the role of the nervous system as the coordination and regulation of body functions.
• Identify sensory, relay and motor neurones in diagrams and images.
• Describe a simple reflex arc in terms of receptor, sensory neurone, relay neurone, motor neurone and effector.
• Describe a reflex action as a means of automatically and rapidly integrating and coordinating stimuli with the responses of effectors, which are muscles and glands.
• Describe a synapse as a junction between two neurones.
• Describe sense organs as groups of receptor cells responding to specific stimuli: light, sound, touch, temperature and chemicals.
• Identify the seven structures of the eye the syllabus lists: cornea, iris, pupil, lens, retina, optic nerve and blind spot.
• Give the five functions the syllabus lists: cornea refracts light, iris controls how much light enters the pupil, lens focuses light on to the retina, retina contains light receptors some of which are sensitive to light of different colours, optic nerve carries impulses to the brain.
• Explain the pupil reflex in terms of changes in light intensity and pupil diameter.
• Describe a hormone as a chemical substance, produced by a gland and carried by the blood, which alters the activity of one or more specific target organs.
• Identify the four endocrine glands the syllabus lists and state the hormone each secretes: adrenal glands and adrenaline, pancreas and insulin, testes and testosterone, ovaries and oestrogen.
• Describe adrenaline as the hormone secreted in ‘fight or flight’ situations, and give all three of its effects: increased breathing rate, increased heart rate, increased pupil diameter.
• Compare nervous and hormonal control on speed of action and on duration of effect.
• Describe homeostasis as the maintenance of a constant internal environment.
• State that insulin decreases blood glucose concentration.
• Describe gravitropism as a response in which parts of a plant grow towards or away from gravity.
• Describe phototropism as a response in which parts of a plant grow towards or away from the direction of the light source.
• Investigate and describe gravitropism and phototropism in shoots and roots, naming the variables and choosing a valid control.
• Describe the structure of a synapse: vesicles containing neurotransmitter molecules, the synaptic gap, and receptor proteins.
• Describe the four events at a synapse in order, from the impulse stimulating release to an impulse being stimulated in the next neurone.
• State that synapses ensure that impulses travel in one direction only.
• Explain the pupil reflex in terms of the antagonistic action of the circular and radial muscles in the iris.
• Explain accommodation for near and distant objects using the ciliary muscles, the tension in the suspensory ligaments, the shape of the lens and the refraction of light.
• Describe the distribution of rods and cones in the retina of a human.
• Outline the function of rods and cones: the greater sensitivity of rods for night vision, and three different kinds of cones absorbing light of different colours for colour vision.
• Identify the position of the fovea and state its function.
• State that glucagon is secreted by the pancreas.
• Describe the role of adrenaline in the control of metabolic activity: increasing the blood glucose concentration, and increasing heart rate.
• Explain homeostatic control by negative feedback, with reference to a set point.
• Describe the control of blood glucose concentration by the liver, and the roles of insulin and glucagon.
• Outline the treatment of Type 1 diabetes.
• Identify hairs, hair erector muscles, sweat glands, receptors, sensory neurones, blood vessels and fatty tissue in diagrams and images of the skin.
• Describe the maintenance of a constant internal body temperature in terms of insulation, sweating, shivering and the role of the brain.
• Describe it also in terms of vasodilation and vasoconstriction of the arterioles supplying the skin surface capillaries.
• Explain phototropism and gravitropism of a shoot as examples of the chemical control of plant growth.
• Explain the role of auxin in controlling shoot growth: it 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, and it stimulates cell elongation.

Why "chemical control" is the right phrase, and why 14.5.4 uses it. An animal coordinates itself with neurones, which are fast, targeted and reversible. A plant coordinates itself with a substance that diffuses through its tissues — slow, spread out, and expressed as a permanent change of shape. 14.5.4 asks you to treat a shoot's tropisms as examples of the chemical control of plant growth, and that phrase captures exactly this difference. Use the words when the statement is being examined.

• One mark. "The plant grows towards the light because it needs light to photosynthesise." True, and it answers a different question. Nothing here is a mechanism: no auxin, no distribution, no elongation, and "because it needs" is a purpose rather than a cause.
• Nerve or neurone? These are not synonyms and the exam distinguishes them. A neurone is a single nerve cell. A nerve is a bundle containing the fibres of many neurones, wrapped together in connective tissue. So the peripheral nervous system is made of nerves, and each nerve contains many neurones. Writing "a nerve carries an impulse to the CNS" is loose; "a sensory neurone carries an impulse to the CNS" is precise, and precise is what gets the mark.
• A wiring point students invent. Not every pathway contains all three neurones, and the syllabus does not claim it does. What you must be able to describe is the reflex arc, which does contain all three. Do not write that a sensory neurone connects directly to a muscle: sensory neurones end in the CNS, and the instruction to a muscle always leaves along a motor neurone.
• Does the brain know? Yes — just not first. While the reflex is being carried out in the spinal cord, impulses also travel up to the brain, which is why you become aware of the pain a moment later. The examinable point is the ordering: the brain is informed, it does not decide. Writing "the brain is not involved at all" is as wrong as writing "the brain decides to move the hand"; the accurate statement is that the response begins without waiting for the brain.
• Write it carefully. "The eye sees the object" will not earn a mark on a question about receptors. "Light receptors in the retina detect the light and impulses are sent along the optic nerve to the brain, where they are interpreted" earns several.
• Two things the pupil is not. It is not a muscle, and it is not a black disc. It is a hole. The blackness you see is simply the inside of the eye, which is dark. Because it is a hole, it cannot contract or dilate by itself — the iris changes its size. This is why 14.2.4 is phrased in terms of pupil diameter: write "the pupil becomes smaller" or "the pupil diameter decreases", never "the pupil contracts".
• Say pupil diameter, not the pupil contracts. The pupil is a hole. A hole has a diameter; it has no muscle and cannot contract. The syllabus uses the phrase pupil diameter in this statement and again in 14.3.3(c), and it is the safest wording in both places.
• Both halves of every pair, every time. 14.2.5 says antagonistic action, so a full answer names what both sets of muscles are doing: in bright light the circular muscles contract and the radial muscles relax. Giving only the contracting set leaves the word antagonistic unanswered.
• "Greater sensitivity" is a comparison, so finish it. Rods are more sensitive than cones. Writing "rods are sensitive" says nothing, because cones are sensitive too — just not sensitive enough to work in dim light. The whole of the rod statement is: rods have a greater sensitivity than cones, so they are used for night vision.
• The fovea is not the blind spot, and neither is "the middle of the eye". They lie close together on the retina and questions put them side by side. The fovea is on the light axis and has the most receptors; the blind spot is a little to one side of it and has none. If a diagram asks you to mark the fovea, mark the point the light rays converge on.
• Three things a hormone is not. It is not an enzyme — it is not a catalyst and it is not used to speed up a specific reaction. It is not an electrical impulse — it is a chemical, and it travels in blood, not along a neurone. And it is not a neurotransmitter — that acts across a single synapse, not around the whole body.
• Two pupils, two different causes — do not merge them. In the pupil reflex (section H) the pupil changes because the light intensity changed. Here it changes because a hormone is acting on the iris in a demanding situation, and the light may not have changed at all. Same structure, same measurement — pupil diameter — two entirely separate statements: 14.2.4 and 14.3.3(c).
• Do not write "always". An answer claiming that every hormonal response is slower than every nervous one is making a stronger claim than the biology supports. Keep the hedge: "generally", "usually", "often". The syllabus statement itself is only compare nervous and hormonal control, limited to speed of action and duration of effect — it names the two features and leaves the comparison to you, so the hedged version is the one that is safe to write.
• Decreases, not destroys. Insulin does not use glucose up, break it down or get rid of it. It lowers the concentration in the blood, which is a different claim — the glucose is still in the body, it has simply been moved out of the plasma. Where it goes is Supplement 14.4.4, in section R.
• The overshoot mistake. Do not write that the response continues at full strength until the variable is exactly at the set point and then stops dead. The corrective response is reduced gradually as the deviation shrinks, which is why the variable settles into a narrow band rather than swinging wildly past the set point in the other direction.
• Glycogen, glucagon, glucose. Three words, and in an exam under pressure they blur. Glucose is the sugar dissolved in the blood. Glycogen is the storage carbohydrate made of many glucose molecules, held in the liver. Glucagon is the hormone. Write each one out slowly the first time you use it in an answer.
• Direction check. Insulin goes with high and pushes down. Glucose to glycogen. Out of the blood, into store. If your answer has insulin doing anything that raises blood glucose, you have written glucagon's job under insulin's name.
• Do not stop at "the liver converts glycogen to glucose". That is only half the response. The glucose is no use to anyone while it is still inside a liver cell — the liver must also release it into the blood. Answers that omit the release step routinely drop a mark.
• Do not make the skin do the brain's job. Skin receptors detect the temperature of the environment at the surface, not the temperature of the core. If a question asks how the body detects that its own temperature has risen, the answer must include the brain monitoring the temperature of the blood.
• Do not let the temperature fall for ever. These responses do not run until the body is cold. As the temperature returns towards the set point, the deviation being detected gets smaller and the corrective response is reduced — sweating slows, the arterioles return towards their normal diameter. That is negative feedback, and mentioning it turns a good answer into a complete one.
• Growing a set in total darkness is not a control for this investigation. It removes the light itself rather than its direction, and seedlings grown in the dark grow tall and pale for reasons that have nothing to do with the question. If you offer it as the control, you have changed two things at once.
• 14.5.4 says "of a shoot", and it means it. The auxin explanation is required for a shoot only. A root's response to gravity is Core content — describe what it does and give the advantage, as in section X — but the chemical mechanism behind it is not in the 0610 syllabus, and carrying the shoot account across to a root produces an answer that is confidently wrong: greater elongation on the lower side would lift a root, not lower it. An answer that reasons its way to the opposite of the observed result is worse than one that does not reason at all.
• "Count for 15 seconds and multiply by four" loses precision. A miscount of one beat becomes an error of four beats per minute. Counting for a full minute, or for 30 seconds at least, reduces that error — and stating the counting period is part of the method mark.

• Which papers you sit depends on your route, and Topic 14 behaves differently on each kind of paper. Knowing which behaviour you are facing changes how you write.
• Core and Extended do not share a theory paper. A Core candidate sits Papers 1 and 3; an Extended candidate sits Papers 2 and 4. Supplement content cannot appear on a Core paper, and Core content certainly can appear on an Extended one — Extended means Core plus Supplement, so every Core statement in this chapter is examinable on both routes.
• Distractors here are almost always reversals. Four options, two of which are the correct mechanism with one word swapped: circular for radial, insulin for glucagon, contract for relax, shaded for lit. Read the whole option before choosing, and check the direction of every arrow in a given diagram before you look at the answers.
• Expect one labelling question (the eye, a neurone, a reflex arc, or on Paper 4 the skin), one sequencing question worth four to six marks, and one data question on a blood-glucose curve, a temperature trace or a tropism result. The sequencing question is where the marks concentrate and where they are most often thrown away.
• Two items on the official experimental-contexts list belong to this topic: tropic responses and heart rate and breathing rate. Both are worked through in the practical-skills section near the end of this chapter, and both apply to whichever route you are on.
• State wants a fact, no reason. Describe wants what happens, in order. Explain wants what happens and why — every "because" is a mark. Compare wants both sides of each point in the same sentence, not two separate lists. Identify means a diagram: the mark is on the drawing, not in a paragraph.