Homeostasis, response, and two systems
Homeostasis (2.81) is the maintenance of a constant internal environment — the two named 4BI1 examples are body water content and temperature. Every coordinated response (2.82) needs a stimulus (a change), a receptor (detects it) and an effector (muscle or gland that responds). Two communication systems (2.86): nervous — electrical impulses, fast, short-lived, targeted; hormonal — chemicals in the blood, slower, longer-lasting, widespread.
The reflex arc and the synapse
The CNS is the brain + spinal cord (2.87). A reflex arc (2.90) runs: receptor → sensory neurone → relay neurone (spinal cord) → motor neurone → effector. At each synapse the impulse cannot cross the gap electrically, so it triggers release of a neurotransmitter (a chemical) that diffuses across and binds receptors on the next neurone, starting a new impulse (2.89). Reflexes bypass the brain, so they are faster — the survival advantage.
Hormones, thermoregulation and auxin
Hormones: adrenaline (adrenal glands, fight-or-flight), insulin (pancreas, lowers blood glucose), testosterone (testes), oestrogen/progesterone (ovaries); Paper 2B: ADH/FSH/LH (pituitary, 2.95B). Thermoregulation: heat → sweating + vasodilation; cold → vasoconstriction + shivering. Auxin gathers on the shaded side in one-sided light, so those cells elongate and the shoot bends towards light.
Drawn from real examiner reports.
Homeostasis needs the exact phrase
The mark-scheme definition needs two parts: maintenance and constant internal environment. "Keeping the body balanced", "controlling conditions", or naming only temperature all score zero. Safe form: "maintenance of a constant internal environment", plus the two named 4BI1 examples — body temperature and water content.
Do not confuse LH, FSH and oestrogen
Three menstrual-cycle confusions: (1) FSH does not trigger ovulation — it matures the egg/follicle; (2) LH does not thicken the uterus lining — oestrogen does; (3) oestrogen does not trigger ovulation — LH does (LH also triggers progesterone release). On Paper 1B, FSH/LH detail is emboldened (2.95B) and earns no marks; only Paper 2B rewards it.
June 2024 Paper 2B Q4(a)(ii): LH triggers ovulation and progesterone release; candidates confused it with FSH (egg maturation) or oestrogen. June 2023 Paper 1BR Q7(a)(ii): oestrogen repairs the uterus lining, progesterone maintains it.
Vessels dilate, they do not move
Three thermoregulation errors: (1) blood vessels do not move to the surface — in heat, vasodilation widens vessels near the skin, so more blood there loses heat by radiation; (2) vasoconstriction is the cold response (narrowing vessels to conserve heat), not the heat response; (3) sweating cools by evaporation removing heat, not merely by being wet.
June 2024 Paper 2B Q3(a) and Nov 2024 Paper 2B Q4(a)(iii): sweating cools by evaporation and vasodilation raises blood flow to the skin; errors included naming vasoconstriction for the heat response.
Do not omit the relay neurone
Reflex-arc errors: (1) naming only sensory and motor neurones, omitting the relay neurone in the spinal cord; (2) saying the impulse "jumps" the synapse — a neurotransmitter diffuses across and binds the next neurone; (3) the impulse is electrical; only the synapse signal is chemical. Order: receptor → sensory → relay → motor → effector.
June 2024 Paper 1BR Q6(a): the reflex-arc question needed receptor, sensory neurone, synapse/neurotransmitter, motor neurone and effector; the relay neurone and neurotransmitter step were most often omitted.
Cataracts: say impulses, not signals
For cataracts, explain that the clouded lens lets less light through and alters refraction of light onto the retina, so vision is impaired. A specific mark trap: the word signals is not accepted where the answer needs impulses — nerve cells carry electrical impulses to the brain. Use the precise term impulses.
June 2023 Paper 1BR Q10(b)(i): cataracts reduce light reaching/refracted onto the retina; "signals" was not accepted for "impulses".
Auxin gathers on the shaded side
In one-sided light, auxin moves to the shaded side of the shoot, not towards the light. Cells on the shaded side receive more auxin and elongate more, so the shoot bends towards the light (positive phototropism). Do not say auxin goes to the lit side, and do not say it just "makes the plant grow taller" — the bend comes from unequal elongation.
Nervous vs hormonal: pair the contrasts
Pair nervous vs hormonal: speed — nervous fast (ms), hormonal slower; duration — nervous short-lived, hormonal longer; transmission — nervous electrical, hormonal chemical; target — nervous specific effector, hormonal widespread.
Learn the eye accommodation chains
Near object: ciliary muscles contract → suspensory ligaments slacken → lens thicker/more convex. Far object: muscles relax → ligaments taut → lens thinner. Pupil: bright light constricts it, dim light dilates it.
Match the command word; answer what is asked
Watch the command word: describe = what happens, explain = the reason. Give a direction of change ("sweat rate increases", not "changes"). Answer only what the paper asks — emboldened Paper 2B hormone detail on a Paper 1 question earns nothing and wastes time.
Homeostasis (statement 2.81) — the maintenance of a constant internal environment. Body water content and body temperature are the two named examples. A vague answer such as "keeping the body balanced" scores zero.
Co-ordinated response (statement 2.82) always requires three elements:
Define homeostasis. (Two elements required.)
(a) State the meaning of the term homeostasis. (1 mark)
(b) Give two examples of what is maintained by homeostasis in the human body. (2 marks)
