IGCSE Biology: nerves, hormones and homeostasis

Nervous and hormonal control compared

Cambridge 14.3.4 limits the comparison to speed of action and duration of effect.

Key ideas board by board

Cambridge 0610 Core covers the CNS (brain and spinal cord) and PNS, sensory, relay and motor neurones, the reflex arc, synapses as junctions between neurones, sense organs, the parts of the eye and their functions, the pupil reflex, hormones from the adrenal glands, pancreas, testes and ovaries, adrenaline's effects, homeostasis, insulin lowering blood glucose, and tropisms. Supplement adds synapse structure and events, the antagonistic iris muscles, accommodation, rods, cones and the fovea, glucagon, negative feedback with a set point, the liver's role in glucose control, Type 1 diabetes treatment, skin structure and temperature control, and how auxin works.

Edexcel 4BI1 covers responses needing a stimulus, receptor and effector, homeostasis of water content and temperature, geotropism and phototropism with auxin, the CNS, neurotransmitters at synapses, the reflex arc (using withdrawal from a hot object), the eye focusing and responding to light, the skin in temperature control, and adrenaline, insulin, testosterone, progesterone and oestrogen. ADH, FSH and LH (2.95B) are Paper 2 only. The Edexcel statements do not name glucagon or diabetes.

Worked answers: the processes in order

Synapse (Cambridge Supplement, Edexcel 2.89): an impulse arrives at the end of the first neurone and stimulates the release of neurotransmitter molecules from vesicles into the synaptic gap. The neurotransmitter diffuses across the gap and binds to receptor proteins on the next neurone, stimulating an impulse there. Because only the first neurone releases neurotransmitter, impulses travel in one direction only.

Too hot (Cambridge Supplement, Edexcel 2.93): temperature receptors detect the rise and the brain coordinates the response. Sweat glands release more sweat; its evaporation transfers heat energy from the skin. Arterioles supplying the skin capillaries dilate (vasodilation), so more blood flows near the surface and more heat is lost by radiation. Too cold: arterioles constrict (vasoconstriction), less blood reaches the surface, shivering muscles respire and release heat, hairs are raised by hair erector muscles to trap insulating air. Blood vessels do not move up or down in the skin; only the arterioles widen or narrow.

Blood glucose (Cambridge Supplement): after a meal glucose rises above the set point. The pancreas secretes insulin, the liver converts glucose to glycogen and stores it, and glucose falls back. If glucose falls too low, the pancreas secretes glucagon and the liver converts glycogen back to glucose. This is negative feedback: a change triggers a response that reverses it. In Type 1 diabetes the pancreas does not make enough insulin, so it is treated with insulin injections, monitoring blood glucose and controlling diet.

Accommodation (Cambridge Supplement, Edexcel 2.92): for a near object, ciliary muscles contract, suspensory ligaments slacken, the lens becomes fatter and refracts light more. For a distant object, ciliary muscles relax, suspensory ligaments are pulled tight, the lens becomes thinner and refracts light less.

Common mistakes

• Saying "messages" or "signals" travel along nerves. Write electrical impulses.
• Saying blood vessels "move closer to the surface". Arterioles dilate or constrict.
• Saying sweat cools you because it is cold. Evaporation of water transfers heat energy away from the skin.
• Confusing glucagon (hormone) with glycogen (storage carbohydrate). Spell them carefully.
• Saying insulin "turns glucose into glycogen". Insulin causes the liver to convert glucose to glycogen.
• Saying the iris muscles control the lens. The iris controls the pupil size; ciliary muscles and suspensory ligaments change the lens shape.
• Writing that auxin makes cells divide. Cambridge states auxin stimulates cell elongation.

Exam technique

These topics are sequences, and mark schemes give one mark per step in the right order. Learn each as a chain and write it as a chain, using linking words: "so", "which causes", "as a result". For tropism investigations, describe the set-up (seedlings in a box with light from one side, a control on a clinostat or with light from above), the result with direction of growth, and the explanation with auxin moving to the shaded side of the shoot and causing more elongation there.

For the eye, name structures exactly (cornea, iris, pupil, lens, retina, optic nerve, blind spot) and say what each does, using refracts for the cornea and focuses for the lens, as the Cambridge statement does.

How one-to-one lessons help

Students often know the parts but lose marks on order and wording. In lessons a tutor has the student explain each chain aloud, from stimulus to response, and records exactly where the sequence breaks. Then the student writes timed answers to past questions on reflexes, temperature control and glucose, and the tutor marks them line by line against the mark scheme for the right board and tier.

Self-check

1. Describe a reflex arc from stimulus to response, naming all three neurones.
2. Explain events at a synapse and why impulses travel one way.
3. Label the eye and give the function of each part; explain the pupil reflex.
4. Explain accommodation for a near and a distant object.
5. Explain temperature control when too hot and too cold.
6. Explain blood glucose control by negative feedback.
7. Explain phototropism in a shoot using auxin.