Coordination, Response and Homeostasis
What this domain tests
Coordination, Response and Homeostasis examines nervous and hormonal control, reflex actions, sense organs, plant tropisms and maintenance of a stable internal environment.
Core rules
- A reflex arc: receptor → sensory neurone → relay neurone in the spinal cord → motor neurone → effector. Reflexes are rapid and automatic, protecting the body.
- Neurones carry electrical impulses; synapses are gaps where a chemical transmitter diffuses across to the next neurone.
- Hormones are chemical messengers carried in the blood; they act on target organs and often have slower, longer-lasting effects than nerves.
- Insulin lowers blood glucose by promoting uptake and storage as glycogen; glucagon raises it by converting glycogen to glucose.
- Negative feedback reverses a change: a rise in temperature triggers sweating and vasodilation; a fall triggers shivering and vasoconstriction.
- Plant tropisms: shoots are positively phototropic and negatively geotropic; roots are positively geotropic. Auxin redistributes and promotes cell elongation on the shaded or lower side.
Worked example
A person touches a hot object and withdraws their hand before feeling pain. Heat stimulates pain receptors in the skin; impulses travel along a sensory neurone to the spinal cord; a relay neurone passes the impulse to a motor neurone; the muscle contracts. The brain receives the impulse at the same time, but the response does not wait for conscious decision.
Traps
Do not say the brain is not involved at all; it is informed, but the reflex bypasses conscious control. Sensory neurones carry impulses to the central nervous system; motor neurones carry them away. Insulin and glucagon are antagonistic. In the eye, the pupil reflex is controlled by the iris, not the lens.