Coordination and Response
This domain tests nervous and hormonal coordination, homeostasis and plant responses. You should be able to trace a reflex arc, explain negative feedback, and interpret tropism experiments.
Nervous system
A stimulus is detected by a receptor. A reflex arc passes through a sensory neurone, relay neurone and motor neurone to an effector. Reflexes are rapid and automatic because they often bypass conscious brain processing. Synapses use neurotransmitters to pass impulses between neurones.
Hormones and homeostasis
Hormones are chemical messengers carried in the blood; they act more slowly and for longer than nerve impulses. Insulin lowers blood glucose by stimulating the liver to store glycogen. Adrenaline raises heart rate. Negative feedback reverses deviations. Plant shoots show positive phototropism because auxin accumulates on the shaded side and stimulates cell elongation.
Worked example
After a sugary meal, blood glucose rises. The pancreas detects this and secretes insulin. Insulin causes liver and muscle cells to take up glucose and store it as glycogen. Blood glucose falls, and insulin secretion decreases. This is negative feedback because the response opposes the original increase.
Traps
- Reflex actions are not 'decided' by the brain before they happen.
- Hormones travel in the blood, not along nerve cells.
- Auxin promotes elongation; it does not cause the shoot to bend away from light.