Coordination, homeostasis and plant responses — Edexcel International GCSE Biology
How organisms respond to change using a stimulus, receptor and effector, what homeostasis is, and how plants respond to light and gravity.
Revision notes
3 short notes, in the order of the specification. Each one in short:
A stimulus is a change in the environment. A co-ordinated response needs a stimulus, a receptor that detects it and an effector, a muscle or gland, that carries out the response. In animals a coordination centre such as the brain or spinal cord links receptor and effector using nerves or hormones. Plants respond mainly by changing how they grow.
Homeostasis is the maintenance of a constant internal environment. Examples are the control of body water content and body temperature, which change when we sweat, drink, exercise or move between hot and cold places. It matters because enzymes and cells work best in a narrow range of conditions, so reactions slow down or stop if conditions move too far.
A tropism is a plant growth response to a stimulus. Shoots show positive phototropism and negative geotropism; roots show negative phototropism and positive geotropism. Auxin, a plant hormone made in the shoot tip, moves to the shaded side when light comes from one side. It makes those cells elongate more, so the shoot bends towards the light.
Exam questions
6 exam-style questions (18 marks), each with its mark scheme.
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9 cards: flip them, mark what you knew, and practise the rest.
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Quick check questions
What three parts are needed for a co-ordinated response?
Show answer
A stimulus, a receptor and an effector.
What is homeostasis?
Show answer
The maintenance of a constant internal environment.
Give two examples of homeostasis.
Show answer
Control of body water content and control of body temperature.
Which way do roots respond to gravity?
Show answer
Roots grow downwards: positive geotropism.
Where does auxin move to when light shines on a shoot from one side?
Show answer
To the shaded side, where it makes cells elongate.
Frequently asked questions
What is the difference between a receptor and an effector?
A receptor is a cell or organ that detects a stimulus, while an effector is a muscle or gland that carries out the response. For example, temperature receptors in the skin detect a hot object, and a muscle in the arm contracts to pull the hand away. The brain and receptors are never effectors.
Why is homeostasis important?
Homeostasis is important because enzymes and cells work best in a narrow range of conditions. If body temperature or body water content moves too far from normal, reactions in cells slow down or stop. Homeostasis keeps the internal environment constant by bringing conditions back to a set level, keeping them within narrow limits.
How does auxin make a shoot bend towards light?
Auxin makes a shoot bend towards light by causing unequal growth. It is made in the shoot tip, and when light shines from one side it moves to the shaded side. There it makes the cells elongate more, so the shaded side grows faster than the lit side and the shoot bends towards the light.
What is the difference between phototropism and geotropism?
Phototropism is a plant growth response to light, while geotropism, also called gravitropism, is a growth response to gravity. Shoots show positive phototropism and negative geotropism, so they grow up towards light for photosynthesis. Roots show positive geotropism and negative phototropism, so they grow down into the soil to take in water and ions.
Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026