Homeostasis and the co-ordination pathway
Homeostasis is the maintenance of a constant internal environment. Every response uses a fixed chain: a stimulus is detected by a receptor, which informs the co-ordinator (the CNS), which signals an effector (a muscle that contracts, or a gland that secretes). Two systems do this: the nervous system (fast electrical impulses, short-lived, precise) and the hormonal system (slower chemical hormones in the blood, longer-lasting, widespread).
The reflex arc and the synapse
A reflex arc is the fast, automatic pathway that produces a reflex action without conscious thought. Its order: receptor (detects the stimulus) → sensory neurone → synapse → relay neurone in the spinal cord → motor neurone → effector (a muscle that contracts). Neurones carry electrical impulses; at a synapse a neurotransmitter diffuses across and starts a new impulse. Bypassing the conscious brain makes a reflex fast and protective.
Auxin and plant tropisms
A tropism is a growth response whose direction depends on the stimulus direction. Shoots grow towards light and up, away from gravity; roots grow away from light and down, towards gravity. The stem mechanism uses the hormone auxin: made at the shoot tip, it collects on the shaded side, where it makes the cells elongate more, so that side grows faster and the shoot bends towards the light. Only auxin's role in the stem is needed at 4SD0.
Drawn from real examiner reports.
Near focus — ligaments slacken, not relax
For a NEAR object the ciliary muscles contract, the suspensory ligaments slacken (go loose), and the elastic lens becomes thicker / more convex, refracting light more onto the retina. Common errors are the reverse (muscles relaxing, ligaments contracting) and the vague 'lens gets bigger'. Say the ligaments 'slacken' (not 'relax') and the lens becomes 'thicker / more convex'.
Cooling: vasodilation, not vasoconstriction
When the body is too hot, sweat evaporates from the skin and evaporation removes heat — say it cools 'by evaporation', not just 'sweat controls temperature'. Vasodilation lets more blood flow near the skin surface, so more heat is lost. Classic errors: naming vasoconstriction when cooling is needed, and saying the vessels 'move to the surface' — they change width, not position.
Impulses, not "signals"; homeostasis exactly
Two precision traps. (1) Neurones carry electrical impulses; the words 'signals' or 'messages' are not credited when the required term is impulse. (2) Homeostasis must be defined as 'the maintenance of a constant internal environment' — vague answers such as 'keeping the body balanced' score nothing. A hormone is a chemical messenger made by a gland and carried in the blood.
Sensory vs motor neurone direction
Do not mix up the two neurones. A sensory neurone carries impulses from a receptor TO the CNS; a motor neurone carries impulses from the CNS TO an effector. A memory hook: sensory = towards the spinal cord/CNS. Drawing the impulse arrow the wrong way in the motor neurone (towards the CNS) is a common marked error — in the motor neurone the impulse travels away from the CNS.
Pupil reflex ≠ accommodation
Keep the two eye responses separate. The pupil reflex responds to light intensity: in bright light the iris muscles make the pupil smaller; in dim light the pupil widens. Accommodation is different — it changes the shape of the lens (not the pupil) to focus on near or distant objects. Confusing the pupil/iris response with the lens/accommodation response loses marks.
Auxin gathers on the shaded side
In a shoot lit from one side, auxin collects on the SHADED side, where it makes the cells elongate more, so the shaded side grows faster and the shoot bends TOWARDS the light. A common error is to say auxin gathers on the lit side, or that the lit side grows faster — both give the wrong bending direction. Remember: more auxin on the shaded side means faster growth there.
Use exact terms: impulse, slacken, widen
This topic is full of terminology traps, so use the exact word: impulse (not 'signal'), ligaments slacken (not 'relax') for near vision, and blood vessels widen or narrow (not 'move'). The precise term is often the difference between a mark and no mark.
On Paper 1, answer only what is asked
Some content is emboldened in the spec and NOT examined on Paper 1 (e.g. detailed FSH/LH roles). Answer only what the question asks — over-answering wastes time and can introduce contradictions. Match your answer to the command word and the marks available.
Write the pathway in order, with roles
For any 'describe the response' question, list the stages in order and give the ROLE of each part: stimulus → receptor (detects) → sensory neurone → CNS/relay → motor neurone → effector (muscle contracts / gland secretes) → response. Marks are awarded for the function.
The coordination pathway (all responses follow this chain):
Nervous vs hormonal communication (statement 2.86):
| Feature | Nervous system | Hormonal (endocrine) system |
|---|---|---|
| Signal carried by | Electrical impulses along neurones | Hormones (chemicals) in the blood |
| Speed | Very fast | Slower |
| How long the effect lasts | Short-lived | Longer-lasting |
| Where it acts | Precise (a specific muscle/gland) | Widespread (can affect many organs) |
The named hormones (statement 2.94) — source, role, effect:
| Hormone | Made by (source) | Role / effect |
|---|---|---|
| Adrenaline | Adrenal glands | "Fight or flight" — raises heart rate, breathing rate and blood glucose to prepare for action |
| Insulin | Pancreas | Lowers blood glucose (makes the liver store glucose as glycogen) |
| Testosterone | Testes | Male sex hormone — controls male secondary sexual characteristics |
| Oestrogen | Ovaries | Female sex hormone — controls female secondary sexual characteristics; repairs/thickens the uterus lining |
| Progesterone | Ovaries | Maintains the uterus lining during the menstrual cycle / pregnancy |
What three things does a co-ordinated response always require, and in what order does information flow?
(a) Define homeostasis and give one example of something it keeps constant. (2 marks) (b) A co-ordinated response requires three things. Name them in the order that information passes between them. (2 marks)
