2.80-2.82 - Responses, homeostasis, receptors and effectors
Organisms do not survive by staying passive while conditions change around them. They detect changes, respond in ways that help them survive, and keep important internal conditions suitable for cells. This lesson sets up the core language for co-ordination and response: stimulus, receptor, effector, response and homeostasis.
Responding to change
A living organism can respond to changes in its environment. The change might be outside the body, such as light, sound, touch, chemicals, or temperature. It might also be a change in the conditions around cells inside the organism.
Stimulus
A stimulus is a change that can be detected by an organism.
A response is the action or change that follows a stimulus. Responses can help an organism get resources, avoid damage, find suitable conditions, or keep its cells working properly.
Some responses are movements of the whole organism. A single-celled organism may move toward a useful chemical or away from a harmful one. Other responses involve only part of an organism. A plant shoot can grow toward light, and an animal may move a body part away from a damaging stimulus. You do not need the detailed mechanisms in this lesson; the important idea is that organisms detect changes and respond.
Homeostasis
The cells inside an organism need suitable conditions for their chemical reactions. If internal conditions change too much, enzymes and other cell processes may not work properly. Organisms therefore control important internal conditions.
Homeostasis
Homeostasis is the maintenance of a constant internal environment.
"Constant" does not mean that every condition is frozen at one exact value. It means the internal environment is kept within suitable limits. Conditions can rise or fall a little, but responses bring them back towards a suitable range.
Edexcel names two examples of homeostasis in this cluster:
| Internal condition | What is being kept suitable |
|---|---|
| Body water content | The amount of water in body fluids, including the blood and the fluid around cells. |
| Body temperature | The temperature of the body so that cell reactions can work properly. |
This lesson does not teach the detailed mechanisms for controlling water content or temperature. For now, the required idea is that body water content and body temperature are internal conditions that organisms control.
Receptors and effectors
A response has to be co-ordinated: the organism must connect detecting a change with doing something about it. Edexcel's core sequence is:
Continue this lesson.
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