OCR · A-Level · Biology
Course version: H420 · Version from Sept 2025
Communication and homeostasis: (c) the principles of homeostasis
Homeostasis is the maintenance of a stable internal environment despite changes in the external environment.
Checked against the OCR H420 specification (opens in a new tab) on .
- Point 3 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Homeostasis is the maintenance of a stable internal environment despite changes in the external environment. It works through negative feedback: a receptor detects a change (a stimulus) away from the set point, a coordination centre compares it with the set point, and an effector produces a response that reverses the change, returning the value towards normal. For example, if core body temperature rises, receptors detect it, the coordination centre processes the information, and effectors such as sweat glands and skin blood vessels act to lower temperature. Negative feedback therefore counteracts the deviation. Positive feedback is different: it amplifies a change and is rare, occurring in specific events such as certain stages of childbirth. Homeostasis keeps conditions such as temperature, blood glucose and water potential within narrow limits so enzymes and cells function optimally.
What earns the marks
- Homeostasis maintains a stable internal environment within narrow limits despite external change.
- Negative feedback involves a receptor detecting a stimulus, a coordination centre, and an effector producing a response.
- The response reverses the original change, returning the value towards the set point.
- Positive feedback amplifies a change and is less common than negative feedback.
- Maintaining stable conditions allows enzymes and cells to function optimally.
Common mistakes and how to put them right
Describing negative feedback as increasing the original change.
Put it right: It counteracts and reverses the deviation.
Omitting the coordination centre.
Put it right: The pathway is stimulus, receptor, coordination centre, effector, response.
Treating positive feedback as the usual mechanism.
Put it right: Negative feedback is the main mechanism; positive feedback amplifies change and is rare.
Exam tips
- Trace the full feedback pathway in each option before deciding.
- Check that the stated response opposes the original stimulus for negative feedback.
- Reject options that confuse effectors with receptors.
Try the lesson
The start of the Communication and homeostasis: (c) the principles of homeostasis lesson, made from checked teaching notes for this specification point.
Have a go first
What is the role of negative feedback in homeostasis?
Listen
Homeostasis is the maintenance of a stable internal environment despite changes in the external environment. It works through negative feedback: a receptor detects a change (a stimulus) away from the set point, a coordination centre compares it with the set point, and an effector produces a response that reverses the change, returning the value towards normal. For example, if core body temperature rises, receptors detect it, the coordination centre processes the information, and effectors such as sweat glands and skin blood vessels act to lower temperature. Negative feedback therefore counteracts the deviation. Positive feedback is different: it amplifies a change and is rare, occurring in specific events such as certain stages of childbirth. Homeostasis keeps conditions such as temperature, blood glucose and water potential within narrow limits so enzymes and cells function optimally. A strong answer earns marks for these points. Homeostasis maintains a stable internal environment within narrow limits despite external change. Negative feedback involves a receptor detecting a stimulus, a coordination centre, and an effector producing a response. The response reverses the original change, returning the value towards the set point. Positive feedback amplifies a change and is less common than negative feedback. A common slip: Describing negative feedback as increasing the original change. Instead, it counteracts and reverses the deviation. Exam tip: Trace the full feedback pathway in each option before deciding.
Key words
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Stable internal environment
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Negative feedback pathway
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Positive feedback
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Optimal conditions for cell function
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See every definition
- Stable internal environment
- The maintenance of conditions within narrow limits despite changes in the external environment.
- Negative feedback pathway
- A receptor detects a stimulus, a coordination centre compares it with the set point, and an effector produces a response that reverses the change.
- Positive feedback
- A mechanism that amplifies a change and is rare, occurring in specific events such as certain stages of childbirth.
- Optimal conditions for cell function
- Keeping conditions such as temperature, blood glucose and water potential within narrow limits so enzymes and cells function optimally.
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