Why organisms respond
Living organisms need to detect changes in their surroundings and within their bodies. Producing a suitable response can improve the chance of survival.
For example, responses can help an organism to:
- locate food;
- avoid predators or other harmful conditions;
- remain in environmental conditions that are suitable for survival.
Key terms
| Term | Meaning |
|---|---|
| Stimulus | A change in the internal or external environment. |
| Receptor | A cell or specialised structure that detects a stimulus. |
| Coordinator | Processes information from receptors and determines an appropriate response. |
| Effector | Carries out the response to the stimulus. |
| Response | The change produced after a stimulus has been detected and processed. |
Stimuli may come from outside the organism or from changes occurring internally. For example, light can be detected by photoreceptor cells in the eye, while changes in blood glucose concentration can be detected by cells in the pancreas.