HookThe fourteen-year-old who should have died in 1922
On 11 January 1922, a fourteen-year-old boy named Leonard Thompson lay dying in Toronto General Hospital. He weighed under thirty kilograms and had the untreatable diabetes of the age — his body could no longer control the sugar in his blood, and every case like his ended the same way. That afternoon he was given an injection of a crude extract prepared by Frederick Banting and Charles Best from the pancreas of a dog. Within days his blood glucose fell towards normal and he began to recover. The extract was insulin, and its discovery turned a death sentence into a managed condition for millions. What Banting and Best had really done was replace one broken piece of a control system that healthy bodies run automatically, thousands of times a day, without anyone noticing.
That control system is homeostasis: the regulation of the internal conditions of the body — temperature, water, blood glucose — within narrow limits, so that cells and especially their enzymes keep working. This section is large because the body coordinates in two ways, and you need both. The nervous system is fast and electrical, carrying impulses along neurones for split-second responses like a reflex. The endocrine system is slower and chemical, releasing hormones into the blood for longer, whole-body changes like the menstrual cycle or the control of blood sugar. Around those two systems sit the eye, the brain, the kidney, thermoregulation, reproduction and even the hormones that steer a growing plant — all of them variations on a single idea: detect a change, respond to it, and correct it.
ModelThe control loop — receptors, coordination centres, effectors
Every homeostatic system in the body is built from the same three parts, and naming them is the backbone of a good six-mark answer. Receptors are cells that detect a change in the environment — a stimulus. A coordination centre, such as the brain, spinal cord or pancreas, receives and processes the information and decides on a response. Effectors, which are muscles or glands, carry the response out — a muscle by contracting, a gland by secreting a hormone.
The reason the body must do this at all is that its chemistry only works in a narrow band. Enzymes have an optimum temperature and pH; too much or too little water swells or shrinks cells by osmosis; the brain starves in minutes if blood glucose drops too far. Holding those conditions steady keeps every other process possible.
The control is achieved by negative feedback: whenever a condition moves away from its set point, the system triggers a response that pushes it back — and when the level returns to normal, the response switches off. If a level rises too high, the correction brings it down; if it falls too low, a different correction brings it up. A thermostat heating a room is the everyday analogy. In the body, thyroxine (from the thyroid) is controlled this way, held steady by negative feedback, while adrenaline (from the adrenal glands) is released in fright to ready the body for 'fight or flight' — a boost, not a set point.
MechanismThe nervous system and the reflex arc
The nervous system lets you react to your surroundings and coordinate behaviour. Its centre is the central nervous system (CNS) — the brain and spinal cord. Information travels as electrical impulses along cells called neurones. The full pathway is worth learning in order: a stimulus is detected by a receptor, which sends an impulse along a sensory neurone to the CNS; from there an impulse passes along a motor neurone to an effector, which produces the response. Where two neurones meet there is a tiny gap, the synapse, which the impulse crosses by releasing a chemical (a neurotransmitter) that diffuses across and triggers a new impulse in the next neurone.
A reflex is an automatic, rapid response that does not involve the conscious part of the brain — pulling your hand off a hot pan before you feel the pain. The reflex arc runs receptor → sensory neurone → relay neurone in the CNS → motor neurone → effector, skipping the conscious brain precisely so it can be fast and protective. That is the point examiners test: reflexes are quick because they bypass conscious thought.
Required practical 7 investigates reaction time — often by dropping a ruler between a partner's open fingers and reading the distance it falls before they catch it. That distance can be converted to a time. A ruler is caught after falling 12 cm (0.12 m). Because it accelerates under gravity, \(d = \tfrac{1}{2} g t^2\), which rearranges to \(t = \sqrt{\dfrac{2d}{g}}\). Substituting, \(t = \sqrt{\dfrac{2 \times 0.12}{9.8}} = \sqrt{0.0245} \approx 0.16\) s. Now test a factor such as caffeine: after a caffeinated drink the same person catches the ruler at 8 cm, giving \(t = \sqrt{\dfrac{2 \times 0.08}{9.8}} \approx 0.13\) s — a faster reaction. The control variables that make this fair are the same ruler, the same catching hand, no practice runs beforehand and the fingers starting at the same gap each time. That controlled comparison, not the raw number, is what earns the marks.
ModelThe brain and the eye — two studied organs
The brain (higher tier, biology only) controls complex behaviour and is made of billions of neurones. Three regions are named: the cerebral cortex, the outer folded layer responsible for consciousness, intelligence, memory and language; the cerebellum, which coordinates muscular activity and balance; and the medulla, which controls unconscious activities such as heartbeat and breathing. Neuroscientists map the brain by studying patients with damage to particular regions, by electrically stimulating parts of it, and by using MRI scanning. Treating brain damage and disease is hard because the brain is delicate, encased in the skull, and its structures are intricate and easily harmed.
The eye (biology only) is a receptor organ for light. Light enters through the transparent cornea, which does most of the focusing; the iris controls how much light passes through the pupil; the lens fine-tunes the focus onto the retina, where receptor cells fire impulses down the optic nerve. Focusing on near and far objects is called accommodation: for a near object the ciliary muscles contract, the suspensory ligaments slacken and the lens becomes fat and strongly curved; for a distant object the ciliary muscles relax, the ligaments pull tight and the lens is pulled thin. In short sight (myopia) the image forms in front of the retina, corrected with a concave lens; in long sight (hyperopia) it forms behind, corrected with a convex lens.
MechanismControlling body temperature
Human body temperature is held near 37°C because that is the optimum for the body's enzymes. The thermoregulatory centre in the brain monitors the temperature of the blood, and receptors in the skin feed in the temperature of the surroundings.
When you are too hot, sweat is released from sweat glands and its evaporation transfers energy away from the skin; the blood vessels supplying the skin capillaries dilate (vasodilation), so more blood flows near the surface and more energy is lost to the air. When you are too cold, sweating stops, you begin to shiver — muscle contractions that release heat through extra respiration — and the vessels constrict (vasoconstriction), so less blood reaches the surface and less energy is lost. The tiny muscles at the base of body hairs can also raise the hairs to trap an insulating layer of air.
The single most common error here is to write that 'the capillaries move towards or away from the skin surface'. Capillaries do not move. It is the small arterioles supplying them that widen or narrow, changing how much blood flows through the surface capillaries. Say 'vasodilation' and 'vasoconstriction' and describe blood flow, not moving vessels.
ModelHormones, the endocrine system and blood glucose
The endocrine system is made of glands that secrete hormones — chemical messengers — directly into the bloodstream, which carries them to target organs. It is slower to act than the nervous system but its effects are more general and longer-lasting; that speed-versus-duration contrast is a guaranteed exam comparison. The pituitary gland in the brain is the 'master gland': it secretes several hormones that in turn control other glands. Other key glands are the thyroid, pancreas, adrenal glands, ovaries and testes.
Blood glucose control is the model case. The pancreas monitors and controls the concentration of glucose in the blood. When blood glucose rises too high — after a meal — the pancreas secretes insulin, which makes liver and muscle cells take up glucose and store it as glycogen, bringing the level down. Higher-tier students also need the other direction: when blood glucose falls too low, the pancreas secretes glucagon, which makes the liver convert glycogen back into glucose, raising the level. That is negative feedback with two opposing hormones.
Type 1 diabetes is a disorder in which the pancreas produces little or no insulin, so blood glucose can rise to dangerous levels; it is normally treated with insulin injections. Type 2 diabetes is where the body's cells stop responding properly to insulin; it is strongly linked to obesity and is usually controlled first by a carbohydrate-controlled diet and exercise. Confusing the two — or assuming every diabetic injects insulin — is a classic lost mark.
MechanismWater and nitrogen balance — the kidney
The body loses water constantly: from the lungs when you breathe out, from the skin in sweat (along with ions and urea), and in urine. It cannot control the first two, so the kidneys do the fine-tuning, filtering the blood and adjusting how much water is reabsorbed. If body cells lose or gain too much water by osmosis they do not function efficiently, so this balance matters.
The nitrogen story runs alongside it. Excess amino acids cannot be stored, so in the liver they are broken down — deamination removes the nitrogen part, which is converted to ammonia and then to the less toxic urea. Urea passes into the blood, is filtered out by the kidneys and leaves the body in urine.
Water reabsorption is controlled by negative feedback using ADH (anti-diuretic hormone), released from the pituitary. When the blood is too concentrated, more ADH is released, the kidney tubules become more permeable and reabsorb more water, so little, concentrated urine is made; when the blood is too dilute, less ADH is released and more dilute urine is produced. If the kidneys fail, waste builds up, and a patient is treated by dialysis — a machine that filters the blood — or by a kidney transplant, which is a longer-term fix but depends on a matching donor and drugs to prevent rejection.
CaseReproductive hormones, contraception and fertility
At puberty, reproductive hormones trigger the secondary sex characteristics. The main male hormone, testosterone from the testes, stimulates sperm production; the main female hormone, oestrogen from the ovaries, drives the menstrual cycle. Four hormones control that monthly cycle and you must know each source and role: FSH (from the pituitary) matures an egg in the ovary; oestrogen (from the ovary) builds up the uterus lining and switches off FSH; LH (from the pituitary) triggers the release of the egg (ovulation) around day 14; and progesterone (from the ovary) maintains the lining in the second half of the cycle.
Contraception works by interrupting this system. Hormonal methods — the pill, implant, injection and patch — use oestrogen and/or progesterone to inhibit FSH so no egg matures; they are very effective but can have side effects and do not protect against infection. Non-hormonal methods include condoms and diaphragms (barriers that stop sperm meeting the egg), intrauterine devices, spermicides, sterilisation, and abstaining from intercourse when an egg may be present. Each is a trade-off between reliability, side effects and protection.
Hormones also treat the opposite problem. If a woman does not produce enough FSH, a 'fertility drug' containing FSH and LH can stimulate egg maturation. In IVF (in vitro fertilisation), FSH and LH mature several eggs, which are collected and fertilised by sperm in the laboratory; the resulting embryos develop for a few days before one or two are inserted into the uterus. IVF gives many couples a child, but it is emotionally and physically demanding, has a relatively low success rate, and can lead to risky multiple births — the balanced evaluation examiners reward.
MechanismPlant hormones — tropisms and their uses
Plants coordinate too, using hormones to respond to light and gravity — responses called tropisms. Growth towards light is phototropism; growth in response to gravity is gravitropism (geotropism). The hormone responsible is auxin, made in the growing tips. Auxin controls growth by accumulating unevenly. In a shoot tip lit from one side, auxin moves to the shaded side, where it makes cells elongate more, so the shoot bends towards the light. The twist that catches students is that auxin has the opposite effect in roots: there, a higher auxin concentration inhibits growth, so a horizontal root's lower side grows less and the root bends downwards, into the soil and its water supply.
Required practical 8 investigates exactly this, measuring the effect of light or gravity on newly germinated seedlings — controlling variables such as temperature, water and the type of seed, and using seedlings kept in the dark or turned on their side as comparisons.
These hormones are commercially valuable. Auxins are used as selective weedkillers (they make broad-leaved weeds outgrow themselves and die while grasses survive), as rooting powders for cuttings, and in tissue culture. Ethene is a gas that controls cell division and ripening, used to ripen fruit during transport and storage. Gibberellins are used to end seed dormancy, promote flowering and produce larger or seedless fruit. Higher-tier answers should tie each use back to the growth process the hormone controls.
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
Whenever a question hands you a homeostatic situation, structure the answer as receptor → coordination centre → effector, and describe the correction in both directions of negative feedback — what happens when the level is too high and when it is too low. Answers that only describe one direction cap at half marks.
For the nervous system, write the reflex pathway in full and in order, and stress that it does not involve the conscious brain — that phrase is the discriminating point. For the endocrine system, be ready to compare it with the nervous system on transmission (blood versus neurones), speed (slow versus fast) and duration (long versus short). Name hormones and their glands precisely: FSH and LH come from the pituitary, oestrogen and progesterone from the ovary; muddling the sources is a frequent, avoidable loss.
Use 'vasodilation' and 'vasoconstriction' correctly and never say vessels move. In plant-hormone questions, explain the mechanism — auxin accumulating on the shaded or lower side — and remember it promotes growth in shoots but inhibits it in roots. On required practicals 7 and 8, the marks are in the control variables and the fair comparison, so state them explicitly rather than just quoting a result.