Control and Coordination
What the exam tests
Control and Coordination tests the nervous system, reflex action, the human brain, hormones and plant movements. Expect questions on the reflex arc, the role of insulin and thyroxine, and tropisms.
Core rules
- A neuron has a cell body, dendrites and an axon. The synapse is the gap across which a chemical signal passes.
- A reflex action is a fast, automatic response. The reflex arc is receptor → sensory neuron → spinal cord → motor neuron → effector. The brain is informed but does not initiate the response.
- The brain has the cerebrum (thinking, memory), cerebellum (balance and coordination) and medulla (involuntary actions such as breathing).
- Hormones are chemical messengers secreted by endocrine glands into the blood.
- Pituitary is the master gland; thyroid secretes thyroxine, which needs iodine; pancreas secretes insulin, which lowers blood glucose; adrenal secretes adrenaline for fight-or-flight; testes and ovaries secrete sex hormones.
- Plant hormones: auxin promotes cell elongation and phototropism; gibberellin promotes growth; cytokinin promotes cell division; abscisic acid inhibits growth and closes stomata.
- Tropisms are directional: phototropism to light, geotropism to gravity, hydrotropism to water.
Worked example
A person touches a hot pan and pulls the hand away before feeling pain. Explain. Heat receptors in the skin generate a nerve impulse. The impulse travels along a sensory neuron to the spinal cord, where it passes to a motor neuron without waiting for the brain. The motor neuron stimulates the arm muscle to contract. The brain receives the information at the same time but the withdrawal has already occurred, which is why pain is felt slightly later.
Traps that cost marks
- Saying the brain controls reflex actions consciously.
- Confusing insulin with thyroxine; insulin controls glucose.
- Writing that auxin causes geotropism only; it also mediates phototropism.
- Forgetting that abscisic acid inhibits growth, while gibberellin promotes it.