Edexcel IGCSE Biology · Spec 2.80-2.82
Homeostasis
What homeostasis is, the stimulus-receptor-effector-response pathway, and negative feedback.
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Homeostasis
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Explained
Keeping the inside steady
Homeostasis is the maintenance of a constant internal environment. Cells and the enzymes inside them need steady conditions to work, so the body holds temperature, blood glucose and water content within narrow limits regardless of what is happening outside.
It is worth learning as a general definition rather than as a list of examples, for a reason the mark schemes make explicit below.
The pathway
Every homeostatic response follows the same five stages, in order.
- Stimulus: a change in a condition, such as the body getting too warm.
- Receptor: a cell or organ that detects the change.
- Coordinator: usually the brain, which processes the information and decides what to do.
- Effector: a muscle or a gland, because those are the only two things that can act.
- Response: what the effector does, which reverses the original change.
If a question describes a situation and asks you to explain the response, naming the five stages in order and identifying what plays each role is usually the whole answer.
Negative feedback
The word negative does not mean bad. It means the response acts in the opposite direction to the change, cancelling it out.
Too hot, so the body cools itself. Too cold, so it warms itself. Blood glucose too high, so insulin lowers it; too low, so glucagon raises it. In every case the correction reverses the change and brings the condition back towards normal.
Because the correction only starts once a change has been detected, the condition oscillates gently around the ideal rather than sitting exactly on it. That is why a graph of blood glucose over a day is a wavy line rather than a flat one, and being able to say so is often the final mark on a graph question.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme asks what homeostasis means and offers four acceptable answers: the maintenance of constant internal conditions in the body, the control or regulation of internal conditions, the same for the internal environment, and keeping conditions in the body the same or within a narrow range.
Beside them is a single instruction: ignore just examples, such as temperature or water.
That is the trap in this topic. Temperature control and water balance are the things you have studied, so they come to mind first, and neither of them defines the word. A definition has to describe the whole idea; an example describes one instance of it.
The same point appears in a report on another paper, which records an answer gaining no marks because it referred specifically to thermoregulation rather than to homeostasis in general.
Elsewhere a multiple choice question asks which action is an example of homeostasis, and the mark scheme explains each wrong option the same way: lifting a weight is not restoring internal balance, releasing saliva is not, and withdrawing your hand is not. The test to apply is whether the response returns an internal condition to its normal level.
What is being controlled
Three examples are examinable: body temperature at about 37 degrees Celsius, blood glucose concentration, and the water and salt content of the blood.
Each uses the same pathway with different components. For temperature the receptors are in the skin and the brain, and the effectors are sweat glands, blood vessels and muscles. For blood glucose the receptor and effector are both the pancreas, and the response is a hormone. For water balance the receptors are in the brain and the effector is the kidney.
Recognising the same five stages behind three different systems is what turns three topics into one.
Spec 2.80-2.82
What you need to know
- Explain what homeostasis is
- Describe the stimulus-response pathway
- Explain negative feedback
Active recall
Quick check
Answer each question before opening the answer.
Define homeostasis.
The maintenance of a constant internal environment in the body.
What is negative feedback?
A control mechanism where a change from the set point is detected and corrected back towards it.
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