Homeostasis — AQA GCSE Biology
Test yourself on Homeostasis with AQA GCSE practice questions.
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Homeostasis explained
Homeostasis keeps the internal environment within narrow limits so cells work properly.
Read the full explanation
It is regulation, not a fixed state: receptors detect a change, a coordination centre processes the information, and effectors respond to restore the optimum. The change may come from inside the body, such as a rise in blood glucose after a meal, or from outside, such as cold air lowering body temperature. The response is usually a negative feedback loop: the correction cancels the original change. For example, if body temperature falls, receptors in the skin and hypothalamus detect it, the coordination centre triggers effectors, and shivering and reduced sweating raise the temperature back towards 37 °C.
Homeostasis maintains optimal conditions for enzyme action and all cell functions.
Enzymes are proteins that catalyse metabolic reactions, and their activity depends on temperature and pH. If conditions drift too far from the optimum, enzyme molecules change shape, the active site no longer fits the substrate, and the rate of reaction falls; extreme changes denature the enzyme permanently. Homeostasis therefore protects enzyme action by holding core temperature near 37 °C and controlling pH and substrate levels. Because enzymes drive processes such as respiration, digestion and protein synthesis, maintaining optimum conditions for enzymes also maintains all cell functions. For example, a high fever can slow or stop enzyme-controlled reactions, which is why body temperature is regulated so carefully.
In the human body, these include control of: • blood glucose concentration • body temperature • water levels.
The human body regulates several key internal conditions. Blood glucose concentration is controlled by insulin and glucagon from the pancreas: insulin lowers glucose by promoting uptake into cells and storage as glycogen, while glucagon raises it by releasing glucose from glycogen. Body temperature is controlled by responses such as sweating, vasodilation, shivering and vasoconstriction, which balance heat loss and heat gain. Water and ion levels are controlled by the kidneys, which adjust the volume and concentration of urine; ADH from the pituitary gland controls how much water is reabsorbed. Each example follows the same pattern: a receptor detects the change, a coordination centre processes it, and effectors restore the optimum by negative feedback.
These automatic control systems may involve nervous responses or chemical responses.
Automatic control systems keep internal conditions steady without conscious thought. They work through two complementary routes. Nervous responses use electrical impulses travelling along neurones: a receptor detects a stimulus, a coordination centre processes the information, and an effector responds. This is fast and short-lived, suiting rapid changes such as touching something hot. Chemical responses use hormones carried in the blood plasma: a gland secretes a hormone, which travels to a target organ with matching receptors and triggers a response. This is slower but longer-lasting, suiting sustained regulation such as blood glucose concentration. Both routes share the same three-part framework of receptor, coordination centre and effector, so the key skill is identifying which route a scenario describes and justifying that choice using speed, duration and transmission method.
All control systems include: • cells called receptors, which detect stimuli (changes in the environment) • coordination centres (such as the brain, spinal cord and pancreas) that receive and process information from receptors • effectors, muscles or glands, which bring about responses which restore optimum levels.
Homeostasis keeps internal conditions within narrow limits despite external change. Every control system has three linked parts. Receptors are cells that detect stimuli, which are changes in the environment; examples include temperature receptors in skin and chemoreceptors in the pancreas detecting blood glucose. Coordination centres receive and process information from receptors; the brain, spinal cord and pancreas are named examples. Effectors are muscles or glands that bring about responses restoring optimum levels. Muscles contract; glands secrete substances such as hormones. For instance, a rise in blood glucose is detected by pancreatic receptors, processed by the pancreas, and effectors respond: insulin-secreting cells release insulin, and liver and muscle cells store glucose, lowering it. Negative feedback reverses the change, so optimum levels are restored.
Your focus
- Explain homeostasis as regulation of internal conditions in response to change.
- Describe the roles of receptors, coordination centres and effectors in a homeostatic response.
- Apply negative feedback to a named internal or external change.
Show all 15 objectives
- Explain how temperature and pH affect enzyme activity and active site shape.
- Describe why homeostasis is needed to maintain optimum conditions for enzymes.
- Link enzyme-controlled reactions to the maintenance of all cell functions.
- Describe how blood glucose concentration is controlled by insulin and glucagon.
- Explain how body temperature is regulated by named responses.
- Explain how the kidneys and ADH control water levels in the body.
- State that automatic control systems may involve nervous responses or chemical responses.
- Describe the key features of nervous and chemical responses, including transmission route, speed and duration.
- Compare nervous and chemical responses and justify which is operating in a given scenario.
- Identify receptors, coordination centres and effectors in a named control system.
- Describe how information passes from a stimulus to a response.
- Explain how effector responses restore optimum levels by negative feedback.
Homeostasis exam tips
Marking Points
- Homeostasis is the regulation of internal conditions of a cell or organism.
- The purpose is to maintain optimum conditions so that enzymes and cells can function effectively.
- Responses occur in response to both internal changes, such as blood glucose concentration, and external changes, such as temperature.
- Control involves receptors detecting a stimulus, a coordination centre processing it, and effectors producing a response.
- Negative feedback restores conditions towards the optimum by reversing the original change.
- Enzymes are biological catalysts that control the rate of metabolic reactions.
- Enzyme activity depends on temperature and pH, with an optimum for each enzyme.
- Conditions away from the optimum reduce enzyme activity, and extreme conditions denature enzymes by altering the active site.
- Homeostasis maintains optimum temperature and pH so enzyme-controlled reactions continue at a suitable rate.
- Because enzymes control cell processes, maintaining optimum conditions for enzymes maintains all cell functions.
- Blood glucose concentration is controlled by insulin and glucagon acting on the liver and body cells.
- Insulin lowers blood glucose by increasing uptake into cells and converting glucose to glycogen for storage.
- Body temperature is controlled by responses including sweating, shivering, vasodilation and vasoconstriction.
- Water levels are controlled by the kidneys, which vary the volume and concentration of urine produced.
- ADH from the pituitary gland controls the amount of water reabsorbed by the kidney tubules.
- Nervous responses involve electrical impulses travelling along neurones, whereas chemical responses involve hormones transported in the blood plasma.
- Nervous responses are generally rapid and short-lived; hormonal responses are generally slower and longer-lasting.
- Both types of response use the same sequence: receptor detects stimulus, coordination centre processes information, effector produces response.
- A named example of a nervous response, such as the reflex withdrawal of a hand from a hot surface, contrasted with a named hormonal example, such as insulin lowering blood glucose concentration.
- Hormones act only on target organs or cells that possess complementary receptors, whereas nervous impulses travel along defined neurone pathways to specific effectors.
- A valid comparison must address both routes; describing only one route cannot earn comparison credit.
- Receptors are cells that detect stimuli, which are changes in the environment.
- Coordination centres receive information from receptors and process it; named examples are the brain, spinal cord and pancreas.
- Effectors are muscles or glands; muscles contract and glands secrete substances such as hormones.
- Responses brought about by effectors restore optimum levels, typically by negative feedback.
- A complete account links a named stimulus, receptor, coordination centre, effector and response in one example.
Examiner Tips
- 💡Use the sequence stimulus, receptor, coordination centre, effector, response when explaining any homeostatic example.
- 💡State that the response is a correction that reverses the change, which shows understanding of negative feedback.
- 💡Give one internal and one external example to cover both parts of the statement.
- 💡Link temperature and pH to active site shape when explaining why enzyme activity falls.
- 💡Use the term denatured precisely and state that the change is permanent.
- 💡Connect enzyme optimum conditions to one named process, such as respiration or digestion, to show the link to cell function.
- 💡For each example, name the receptor, coordination centre and effector to show the full control pathway.
- 💡Use the terms glycogen and glucagon carefully, and check which one is the storage molecule and which is the hormone.
- 💡When describing temperature control, state whether the response increases or decreases heat loss.
- 💡When a question asks you to compare, write paired statements about the same feature, for example speed of nervous versus speed of hormonal response.
- 💡Name a specific example for each route, such as a reflex arc for nervous control and insulin or adrenaline for chemical control, to show secure understanding.
- 💡Use the terms electrical impulse, neurone, hormone, blood plasma and target organ precisely; vague wording such as 'signals' loses clarity.
- 💡If asked to identify which system is involved in a scenario, justify your choice using at least two features such as speed and duration of the response.
- 💡Use the three-part framework (receptor, coordination centre, effector) to structure any control-system answer.
- 💡Name a specific example, such as skin temperature receptors, the pancreas and insulin-secreting cells, rather than staying abstract.
- 💡Link each response to restoring an optimum level, using the phrase 'negative feedback' where the change is reversed.
Common Mistakes
- Saying homeostasis keeps conditions constant; correct this by saying it keeps conditions within narrow limits around an optimum.
- Omitting the receptor, coordination centre or effector when describing a response; correct this by naming all three in sequence.
- Confusing the stimulus with the response; correct this by stating the change first and the corrective action second.
- Saying enzymes are killed by high temperature; correct this by saying they are denatured, because enzymes are molecules, not living cells.
- Stating that denaturation changes the substrate; correct this by stating that the active site changes shape so the substrate no longer fits.
- Claiming homeostasis keeps every condition perfectly constant; correct this by saying it keeps conditions near the optimum within narrow limits.
- Saying insulin converts glucose into glucagon; correct this by stating that insulin converts glucose into glycogen for storage.
- Confusing vasodilation with vasoconstriction; correct this by stating that vasodilation widens vessels to lose heat and vasoconstriction narrows them to conserve heat.
- Stating that the kidneys make water; correct this by stating that the kidneys regulate water loss by reabsorbing water and excreting the remainder in urine.
- Saying hormones travel along nerves: correct this by stating that hormones are secreted into the blood and carried in the plasma to target organs.
- Claiming nervous responses are always longer-lasting than hormonal ones: correct this by stating that nervous responses are typically rapid and short-lived, while hormonal responses are slower and longer-lasting.
- Treating the two systems as completely separate with no shared components: correct this by noting that both use receptors, coordination centres and effectors.
- Describing a response as chemical simply because a chemical is involved, such as a neurotransmitter: correct this by classifying the response according to whether the signal travels as an electrical impulse along neurones or as a hormone in the blood.
- Saying receptors detect 'the stimulus' without defining a stimulus as a change in the environment; correct by stating that a stimulus is any detectable change, such as a temperature change or a rise in blood glucose.
- Treating the brain as the only coordination centre; correct by including the spinal cord and pancreas as coordination centres that process information.
- Describing effectors as receptors or as 'things that detect change'; correct by stating that effectors are muscles or glands that bring about responses.