Homeostasis — AQA GCSE Combined Science

Enzymes are proteins that catalyse reactions in cells, and each enzyme has an optimum temperature and pH at which its rate of action is highest. Homeostasis keeps these conditions close to the optimum so that metabolic reactions, including respiration and digestion, continue at a suitable rate. If temperature rises too far, enzyme molecules denature as the shape of the active site changes, so substrate molecules no longer fit and the rate falls. If pH moves away from the optimum, bonds in the enzyme are affected and activity decreases. Homeostasis also maintains conditions such as blood glucose concentration and water content so that cells can function. For example, sweating cools the body and insulin lowers blood glucose, protecting enzyme-controlled reactions.

Homeostasis keeps the internal environment of the body steady despite changes inside and outside it. In the human body, the automatic control systems that achieve this include control of blood glucose concentration, body temperature and water and ion balance. Each controlled factor has a normal set point, receptors that detect deviation, a coordination centre such as the brain or pancreas, and effectors such as muscles or glands that restore the set point. For example, if core temperature rises during exercise, sweating and vasodilation increase heat loss; if it falls, shivering and vasoconstriction conserve heat. Negative feedback reverses the original change, so the factor oscillates narrowly around its set point rather than drifting. This statement introduces the list of controlled factors that later statements develop in detail.

Blood glucose concentration is controlled by homeostasis. After a meal, absorbed glucose raises blood glucose; during exercise it falls. The pancreas detects these changes and releases hormones. When blood glucose is too high, insulin is secreted, causing liver and muscle cells to take up glucose and store it as glycogen. At Higher Tier (HT) only, students must also know that when blood glucose is too low, the pancreas secretes glucagon, causing liver cells to break down glycogen into glucose. HT students must understand that insulin and glucagon act as antagonistic hormones in a negative feedback loop to keep the concentration stable. This prevents osmotic damage and ensures a steady supply for respiration.

Body temperature is a key internal condition regulated by homeostasis. In humans, core temperature is maintained around 37 °C, the optimum for enzyme-controlled reactions. If temperature drops, kinetic energy decreases and reactions slow down; if it rises too high, enzymes denature. Body temperature control is an automatic system involving nervous and chemical responses. Receptors detect temperature changes and send signals to a coordination centre, such as the brain. The brain processes this information and sends impulses to effectors, which are muscles or glands. Effectors bring about responses, like sweating or shivering, to restore the optimum temperature. This ensures cells function efficiently despite external changes.

Water levels in the body are regulated by homeostasis to maintain optimum conditions for enzyme action and all cell functions. Water level control is an automatic system. Receptors detect changes in the water concentration of the internal environment. This information is sent to a coordination centre, such as the brain, which processes the signals and coordinates a response. Effectors then act to restore the optimum water level. For Combined Science, students only need to know that water levels are controlled by homeostasis; the specific mechanisms of water loss and kidney function are not required.

Automatic control systems keep internal conditions steady without conscious thought, using two complementary routes. Nervous responses are electrical: receptors detect a stimulus, a coordination centre processes it, and effectors act. This is fast, short-lived and targeted, as when a sudden noise triggers a reflex. Chemical responses use hormones carried in the blood plasma; they are slower, longer-lasting and act on cells with matching receptors. Insulin lowering blood glucose after a meal is a clear example. When you answer, name the stimulus, receptor, coordination centre and effector, and state whether the pathway is nervous or chemical.

This statement introduces the shared architecture of every automatic control system, whether nervous or chemical. Each system needs cells that detect a change, called receptors; a coordination centre that receives and processes the information, such as the brain, spinal cord or pancreas; and effectors that carry out the response, namely muscles or glands. Negative feedback then reverses the original change, returning the internal environment towards its set point. For example, a rise in blood glucose is detected by pancreatic cells, which release insulin; liver and muscle cells take up glucose, and the level falls. If the level drops too far, glucagon reverses the process. When describing any control system, identify each component and explain how the response counteracts the stimulus, rather than simply listing organs.

Receptors are specialised cells that detect stimuli, which are changes in the environment. A stimulus can be external, such as light intensity, or internal, such as a fall in blood glucose concentration. Each receptor is adapted to detect one particular type of stimulus: receptors in the eye respond to light, and receptors in the skin respond to temperature or touch. Detection works because the stimulus changes the receptor cell. In the nervous system, this generates electrical impulses sent along a neurone. In the endocrine system, such as the pancreas detecting glucose, it triggers hormone release. A useful method is to name the stimulus and the receptor that detects it.

Coordination centres receive information from receptors and process it to decide what the body should do. The brain processes information from many receptors and coordinates responses such as movement. The spinal cord carries information and coordinates rapid reflex actions. The pancreas is a coordination centre for blood glucose: it receives information about glucose concentration and releases hormones such as insulin (and glucagon for Higher Tier students). Processing means the centre interprets incoming signals—either electrical impulses in the nervous system or chemical levels in the blood—and sends instructions to effectors. Always link the centre to its specific input and output.

Effectors are the output side of every control system. A receptor detects a change away from the optimum; a coordination centre compares this with the set point; an effector then produces a response that restores the optimum. Two effector types are named: muscles and glands. Muscles respond by contracting or relaxing, changing movement or the diameter of passages. Glands respond by secreting a chemical, such as a hormone or enzyme. For example, when core temperature falls, skeletal muscle contracts rapidly as shivering, releasing extra heat; when blood glucose rises, the pancreas (a gland) secretes insulin. The response is corrective, not merely a change: it opposes the original stimulus, which is why it is called negative feedback.