Stimulus
A stimulus is any change in the surroundings or inside the body that can be detected and can produce a response.
Practice This ConceptMain explanation
Teacher explanation
A stimulus may be light, sound, heat, touch, pain, gravity, water, or even a chemical change. The body or plant senses that change and then reacts in a suitable way. In humans, a stimulus is usually detected by a receptor, and in plants it is detected by cells that respond by growth or movement. The important idea is simple: stimulus first, response after that.
Example
When you touch a hot vessel, the heat acts as a stimulus and your hand moves away quickly. In a plant, sunlight falling from one side acts as a stimulus and the shoot bends towards it.
Simple analogy
Stimulus is the signal, response is the reply.
Common confusion
Many students think stimulus and response mean the same thing. They do not. Stimulus is the change that is noticed, while response is the action taken after noticing it.
Exam tip
In answers, always write the change first, then the body part that detects it, and finally the response. This keeps the sequence correct.
Answer writing and exam use
1-mark use
Write the exact meaning of stimulus in one clean line.
2-mark use
Define stimulus and add one example or condition.
3-mark use
Explain stimulus, show the method or example, and mention the common mistake.
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