Principles of homeostasis and negative feedback (A-level only) — AQA A-Level Biology

Homeostasis is the maintenance of a relatively constant internal environment despite changes outside the body and in the body's own activity. The internal environment means the blood and the tissue fluid that bathe the cells: their temperature, pH, water potential and the concentrations of glucose and ions. Every control system has the same parts: a set point, receptors that detect a deviation from it, a coordinator, usually a centre in the brain or an endocrine gland, that receives that information, effectors that carry out a correction, and feedback of the new value to the receptors. Notice the word restricted rather than constant. Levels fluctuate around the set point rather than being held exactly, because a correction can only begin once a deviation has been detected.

Enzymes are globular proteins whose tertiary structure creates a complementary active site. Raising temperature increases the kinetic energy of enzyme and substrate molecules, so more successful collisions form enzyme-substrate complexes and the rate rises. Above the optimum, increased vibration breaks hydrogen and ionic bonds in the tertiary structure; the active site changes shape, substrate is no longer complementary and the enzyme is denatured. This denaturation is usually irreversible. Blood pH is maintained near 7.4 because a change in hydrogen ion concentration alters the ionic and hydrogen bonding that holds the tertiary structure, again altering active site shape. Outside the narrow pH range, metabolic reactions slow or stop. Core temperature and blood pH are therefore homeostatic variables kept within narrow limits by negative feedback.

Glucose matters twice over. First, it is the main respiratory substrate: it is oxidised in respiration to release energy for ATP synthesis, and some cells, notably brain cells, which cannot respire fatty acids, depend on it almost entirely. If blood glucose falls too low, those cells cannot produce enough ATP, so active transport and nerve function fail, causing confusion, unconsciousness and eventually death. Second, glucose is a solute, so it contributes to the water potential of blood. If blood glucose rises too high, the water potential of the plasma becomes more negative than that of the cells, so water leaves cells by osmosis down a water potential gradient and they dehydrate and shrink. In the kidney the excess glucose is not all reabsorbed, so glucose appears in the urine and water follows it osmotically, producing thirst and dehydration.

In negative feedback a change in a variable sets in motion a response that reverses that change. Receptors detect a deviation from the set point, the information passes to a coordinator, effectors produce a response in the opposite direction to the original change, the variable returns towards the set point, and when it is back within limits the receptors detect this and the corrective response is switched off. That last step matters, because a system that could not stop correcting would overshoot. Blood glucose is the standard example: a rise is detected by the beta cells of the islets of Langerhans, insulin is secreted, glucose is taken up and stored as glycogen, and the falling concentration then reduces insulin secretion. Positive feedback is the opposite.

A system with only one corrective mechanism can push a variable one way and then wait passively for it to drift back. A system with two mechanisms, each controlling a departure in a different direction, can actively pull the value back from either side, so corrections are faster and the variable stays within a narrower range, which is what a greater degree of control means. Blood glucose is regulated by insulin, which lowers it, and by glucagon, which raises it; the two are secreted by different cells of the islets of Langerhans and have opposite effects. Heart rate is raised by sympathetic and lowered by parasympathetic neurones. Body temperature is corrected by vasodilation and sweating when too hot and by vasoconstriction, shivering and a raised metabolic rate when too cold.

You will be given a trace, a flow diagram or a passage about an unfamiliar system and asked which kind of feedback it shows. Work it out rather than recall it. Identify the variable that changes, identify the response, then ask whether the response moves the variable back towards its starting value or further away from it. Negative feedback appears as oscillation around a steady level: the variable rises, a correction follows and it falls back. Positive feedback appears as an accelerating departure from the starting value, usually ending only when something else intervenes. On a graph, look for a return to the original value after a disturbance, which is negative, or a rise that becomes steeper, which is positive. Then justify the answer with the data: quote values, times and the direction of change, and name the receptor or effector if the information allows it.