Cambridge IGCSE Biology 0610: Coordination and response – Revision Notes

These revision notes cover topic 14, Coordination and response, of the Cambridge IGCSE Biology 0610 syllabus for examination in 2026, 2027 and 2028: sections 14.1 Coordination and response, 14.2 Sense organs, 14.3 Hormones, 14.4 Homeostasis and 14.5 Tropic responses. Supplement content is marked Extended only; it appears on Papers 2 and 4, not on the Core Papers 1 and 3. For full explanations and worked examples, use the coordination and response study guide.

Also useful: the existing practice questions on 14.1 and 14.2, the Cambridge IGCSE Biology hub, the printable 0610 checklist, and the free Core diagnostic and Extended diagnostic.

Key definitions

Term	What to write
Sense organ	A group of receptor cells responding to specific stimuli: light, sound, touch, temperature and chemicals
Reflex action	A means of automatically and rapidly integrating and coordinating stimuli with the responses of effectors (muscles and glands)
Synapse	A junction between two neurones
Hormone	A chemical substance, produced by a gland and carried by the blood, which alters the activity of one or more specific target organs
Homeostasis	The maintenance of a constant internal environment
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
Negative feedback (Extended only)	A change away from the set point triggers a response that reverses the change

14.1 Nervous system

• Electrical impulses travel along neurones.
• CNS = brain + spinal cord. PNS = nerves outside the brain and spinal cord.
• Role: coordination and regulation of body functions.

Reflex arc in five steps: receptor → sensory neurone → relay neurone → motor neurone → effector.

Neurone	Direction	Identifying feature
Sensory	Receptor → CNS	Cell body on a side branch
Relay	Inside CNS	Short, in brain or spinal cord
Motor	CNS → effector	Cell body at the CNS end, long fibre

Synapse (Extended only)

Structure: vesicles containing neurotransmitter, synaptic gap, receptor proteins on the next neurone.

Events in four steps: impulse → neurotransmitter released from vesicles into the gap → neurotransmitter diffuses across the gap → binds with receptor proteins on the next neurone → new impulse stimulated.

Synapses make impulses travel in one direction only.

14.2 Sense organs

Part of eye	Function
Cornea	Refracts light
Iris	Controls how much light enters the pupil
Lens	Focuses light on to the retina
Retina	Contains light receptors, some sensitive to different colours
Optic nerve	Carries impulses to the brain
Blind spot	No receptors, where the optic nerve leaves

Pupil reflex (Core): bright light → pupil diameter decreases; dim light → pupil diameter increases.

Iris muscles (Extended only):

Light	Circular muscles	Radial muscles	Pupil
Bright	Contract	Relax	Constricts
Dim	Relax	Contract	Dilates

Accommodation (Extended only):

Object	Ciliary muscles	Suspensory ligaments	Lens	Refraction
Near	Contract	Slack (low tension)	Fatter, more convex	More
Distant	Relax	Tight (high tension)	Thinner, less convex	Less

Rods and cones (Extended only): rods are more sensitive and give night vision; three kinds of cone absorb light of different colours for colour vision. Cones are concentrated at the fovea, which gives the most detailed colour image. Rods are spread over the rest of the retina.

14.3 Hormones

Gland	Hormone
Adrenal glands	Adrenaline
Pancreas	Insulin; glucagon (Extended only)
Testes	Testosterone
Ovaries	Oestrogen

• Adrenaline (“fight or flight”), Core: increased breathing rate, heart rate and pupil diameter.
• Adrenaline, Extended only: increases blood glucose concentration and heart rate (control of metabolic activity).

	Nervous	Hormonal
Speed of action	Fast	Slower
Duration of effect	Short	Longer

14.4 Homeostasis

Core: homeostasis is the maintenance of a constant internal environment; insulin decreases blood glucose concentration.

Blood glucose in steps (Extended only)

Too high: detected → pancreas secretes insulin → liver converts glucose to glycogen → blood glucose falls to set point.

Too low: detected → pancreas secretes glucagon → liver converts glycogen to glucose → blood glucose rises to set point.

Type 1 diabetes treatment (Extended only): insulin injections, monitoring blood glucose, controlling carbohydrate in the diet, and planning exercise.

Skin and temperature (Extended only)

Skin structures to identify: hairs, hair erector muscles, sweat glands, receptors, sensory neurones, blood vessels, fatty tissue.

Too hot: sweating (evaporation cools); vasodilation of arterioles supplying skin surface capillaries (more heat lost); hairs lie flat.

Too cold: shivering (respiration releases heat); vasoconstriction of those arterioles (less heat lost); hair erector muscles contract so hairs trap insulating air.

Always: fatty tissue insulates; the brain detects changes and coordinates responses.

14.5 Tropic responses

	Shoot	Root
Gravitropism	Grows away from gravity	Grows towards gravity
Phototropism	Grows towards light	Grows away from light

Auxin (Extended only), four statements: made in the shoot tip; diffuses through the plant from the tip; unequally distributed in response to light and gravity; stimulates cell elongation.

Shoot lit from one side: auxin collects on the shaded side → those cells elongate more → shoot bends towards light.

Shoot lying horizontally: auxin collects on the lower side → lower cells elongate more → shoot bends upwards.

Answer frames for common question types

“Describe a reflex arc for this response.” Name all five parts in order and fit each to the example: which receptor detects the stimulus, where the relay neurone is (spinal cord or brain), and which muscle or gland is the effector.

“Explain the pupil reflex” (Extended). Give the stimulus (change in light intensity), the muscles (circular and radial, one contracts while the other relaxes), the change in pupil diameter, and the result (more or less light reaching the retina).

“Explain how the eye focuses on a near object” (Extended). Four linked points in order: ciliary muscles contract → suspensory ligaments slacken → lens becomes more convex → light is refracted more and focused on the retina. Reverse every point for a distant object.

“Describe how blood glucose is controlled” (Extended). Say what is detected, which hormone the pancreas secretes, what the liver does, and that the concentration returns to the set point. Use “insulin” for a rise and “glucagon” for a fall.

“Explain how a mammal cools down” (Extended). Pair each response with how it causes heat loss: sweating with evaporation; vasodilation of arterioles with more blood flowing near the skin surface.

“Explain why the shoot bends” (Extended). Auxin is made in the tip, diffuses down, collects on one side (shaded or lower), and stimulates cell elongation there, so that side grows longer and the shoot curves.

Must-know distinctions

Don’t confuse	With	The difference
Glucagon	Glycogen	Glucagon is a hormone; glycogen is the storage carbohydrate in the liver
Circular muscles	Radial muscles	Circular contract in bright light; radial contract in dim light
Ciliary muscles	Suspensory ligaments	Only the muscles contract; ligaments change tension
Vasodilation	“Capillaries moving”	Arterioles widen; capillaries do not move
Rods	Cones	Rods: sensitive, night vision; cones: colour vision, at the fovea
Blind spot	Fovea	Blind spot has no receptors; fovea has most cones
Nervous	Hormonal	Fast and short vs slower and longer lasting

Quick self-test

1. Name the two parts of the CNS.
2. Put these in order for a reflex arc: motor neurone, receptor, effector, relay neurone, sensory neurone.
3. Name two types of effector.
4. State the function of the cornea.
5. Name the gland that secretes testosterone.
6. State two differences between nervous and hormonal control.
7. In dim light a pupil widens from 2.0 mm to 6.5 mm; in bright light it returns from 6.5 mm to 2.0 mm. Calculate the percentage decrease in diameter when it returns.
8. (Extended) Which iris muscles contract in dim light?
9. A person’s blood glucose rises from 4.8 to 7.2 mmol per dm³ after a meal. Calculate the percentage increase and name the hormone the pancreas secretes in response.
10. (Extended) State the role of the brain in temperature control.
11. (Extended) Where is auxin made?
12. (Extended) A shoot is laid horizontally in the dark. Which side receives more auxin, and which way does the shoot bend?

Answers

1. Brain and spinal cord.
2. Receptor → sensory neurone → relay neurone → motor neurone → effector.
3. Muscles and glands.
4. Refracts light.
5. The testes.
6. Nervous control is faster; its effect is shorter lasting.
7. (6.5 − 2.0) ÷ 6.5 × 100 = 69.2%.
8. The radial muscles.
9. (7.2 − 4.8) ÷ 4.8 × 100 = 50%; insulin.
10. It detects changes in temperature (including impulses from skin receptors) and coordinates responses such as sweating and shivering.
11. In the shoot tip.
12. The lower side; the lower side elongates more, so the shoot bends upwards.

Where marks are usually lost

• Stating that the suspensory ligaments contract or relax. Ligaments are not muscles.
• Writing that capillaries dilate, constrict or move closer to the skin surface.
• Writing “glycogen” when the answer is “glucagon”, or the reverse.
• Saying insulin “turns glucose into glycogen”; the liver does the conversion.
• Saying auxin is destroyed by light or moves towards light; it is unequally distributed and collects on the shaded side.
• Leaving out “cell elongation” when explaining how auxin causes bending.
• Giving only muscles as effectors; the syllabus names muscles and glands.
• Describing the pupil reflex as “the eye gets smaller” instead of “the pupil diameter decreases”.
• Comparing nervous and hormonal control on anything other than speed of action and duration of effect.

Official syllabus

Cambridge IGCSE Biology 0610 syllabus for examination in 2026, 2027 and 2028 (Version 3), published by Cambridge University Press & Assessment (Cambridge International Education). Topic 14, sections 14.1 to 14.5 Coordination and response.