By the end of this lesson
- Define stimulus and response.
- Explain why responses improve survival.
- Compare response examples across organisms.
BIG QUESTION
How can an organism turn a change outside itself into an appropriate action?
Receptors detect stimuli; coordination systems process information and effectors produce responses.
A stimulus is a detectable change
A stimulusA change in the internal or external environment that is detected by a receptor. is a change in the internal or external environment that can be detected. Examples include light direction, temperature, chemicals, touch and water availability.
Detection requires a receptor or receptor cell sensitive to that form of energy or chemical signal.
Coordination links detection to action
Information is transmitted to coordinating cells or tissues, which influence an effectorA muscle or gland that carries out a response when it receives a nerve impulse or a hormone. such as a muscle, gland or plant growth region.
The resulting responseA change in an organism, or part of it, caused by a stimulus. may be rapid and reversible or slower and growth based.
Appropriate responses have value
Responses can obtain resources, avoid harm, maintain internal conditions or increase reproductive success. A response is not automatically conscious.
The same stimulus can produce different responses depending on context and organism.
Environmental responses
Different organisms use different effectors.
| Stimulus | Receptor/route | Response value |
|---|---|---|
| Bright light at a shoot | Light-sensitive tip/signalling. | Growth toward light supports photosynthesis. |
| Touching a hot object | Skin temperature/pain receptors. | Withdrawal limits tissue damage. |
| Low blood water content | Hypothalamic osmoreceptors. | ADH response conserves water. |
| Gravity acting on a root | Gravity sensing in root tip. | Growth downward supports anchorage and water uptake. |
Language that earns precision
- stimulus
- Detectable change in an organism’s internal or external environment.
- receptor
- Cell or structure that detects a stimulus.
- response
- Change in activity caused by a stimulus.
- effector
- Structure that carries out a response.
Common misconceptions
Claim: Only animals respond to stimuli.
Correction: Plants respond through growth, turgor and biochemical changes.
Claim: Every response is voluntary.
Correction: Many are automatic or involve growth.
Claim: A receptor is the same as a sense organ.
Correction: Sense organs contain receptors; individual receptor cells may also occur elsewhere.
Match the answer to the command word
Write stimulus → receptor → coordination → effector → response → benefit.
Name the specific change rather than writing only “the environment”.
Recall, apply, then justify
Explain how responding to environmental change can improve survival.
Hint
Use one complete pathway example.
Suggested marking points
- A change acts as a stimulus.
- A receptor detects it.
- An effector produces an appropriate response.
- The response obtains resources/avoids harm/maintains internal conditions.
Give one plant and one animal response, naming the stimulus.
Hint
Match stimulus and action twice.
Suggested marking points
- A named plant stimulus such as directional light/gravity.
- A matched plant growth response.
- A named animal stimulus such as heat/light/chemical change.
- A matched animal movement or physiological response.
Explain why a response that is useful in one context may be costly in another.
Planning hint
Biology involves trade-offs.
What a strong design includes
- Responses consume energy or resources.
- Stomatal closure conserves water.
- The same closure reduces carbon-dioxide uptake and photosynthesis.
- A rapid stress response can improve immediate survival.
- Prolonged activation can disrupt growth or homeostasis.
- Fitness depends on balancing costs and benefits in context.