CCEA A-Level Biology Physiology, Coordination and Control: a complete overview of homeostasis, nerves, hormones and metabolism

What this module demands

Physiology, Coordination and Control is the systems module: how organisms keep their internal environment stable and coordinate responses. It runs from homeostasis and the kidney through nervous and hormonal control to the energy-transfer reactions of photosynthesis and respiration and the growth responses of plants. CCEA tests precise sequencing of processes and the ability to explain control as negative feedback.

This guide walks through the five dot points of the module, then sets out the exam patterns CCEA repeats. Each topic has a matching dot-point page with practice questions; this overview ties them together.

Homeostasis and the kidney

Homeostasis keeps the internal environment stable by negative feedback: a receptor detects a change and effectors reverse it. The kidney filters blood at the glomerulus by ultrafiltration, then selectively reabsorbs glucose, ions and most water along the nephron. Osmoregulation is controlled by ADH: when blood water potential falls, ADH makes the collecting duct more permeable, so more water is reabsorbed and concentrated urine is produced.

The nervous system

Neurones carry electrical impulses; the resting potential of about minus 70 millivolts is set by the sodium-potassium pump. An action potential is a rapid depolarisation as sodium ions enter, then repolarisation as potassium ions leave, travelling fast along myelinated axons by saltatory conduction. At a cholinergic synapse, calcium entry releases acetylcholine, which crosses the cleft and depolarises the next neurone before being broken down.

Hormonal control

Hormones are chemical messengers carried in the blood to target cells. The pancreas controls blood glucose: insulin lowers high glucose by promoting uptake and glycogen storage, while glucagon raises low glucose by breaking glycogen down. Type 1 diabetes is a lack of insulin; type 2 is reduced cell response to insulin. Hormonal control is slower and longer-lasting than nervous control.

Photosynthesis, respiration and plant coordination

In photosynthesis, the light-dependent reactions make ATP and reduced NADP and release oxygen, and the Calvin cycle fixes carbon dioxide into sugars. In aerobic respiration, glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation make ATP, with oxygen as the final electron acceptor; anaerobic respiration yields far less ATP. In plant coordination, auxin (IAA) controls tropisms by promoting elongation in shoots and inhibiting it in roots.

How this module is examined

A typical CCEA profile for Physiology, Coordination and Control:

• Process sequencing. Describing ultrafiltration and reabsorption, the action potential, synaptic transmission, and the stages of respiration and photosynthesis.
• Negative feedback. Explaining osmoregulation by ADH and blood-glucose control as feedback loops.
• Comparison. Distinguishing nervous and hormonal control, and insulin and glucagon.
• Data and graphs. Interpreting action-potential traces and rate-of-photosynthesis or respiration data.

Check your knowledge

A mix of recall and explanation questions covering the module. Attempt them under timed conditions, then check against the solutions.

1. Explain what is meant by negative feedback in homeostasis. (2 marks)
2. Describe how ADH changes the urine produced when you are dehydrated. (3 marks)
3. Explain how the resting potential of a neurone is maintained. (3 marks)
4. Describe how an impulse crosses a cholinergic synapse. (3 marks)
5. Describe what happens after a meal raises blood glucose. (3 marks)
6. State the products of the light-dependent reactions of photosynthesis. (3 marks)
7. State where each of the four stages of aerobic respiration occurs. (3 marks)
8. Explain how auxin causes a shoot to bend towards light. (3 marks)