Responding to change, homeostasis and temperature control
Why organisms respond
The environment around an organism keeps changing — light, temperature, water availability, chemicals, touch. An organism that detects these changes and responds to them survives better: it can find food, avoid harm, and keep its internal conditions suitable for enzymes to work.
The three parts of a co-ordinated response
Every co-ordinated response needs all three:
| Part | What it does | Example (hand on hot pan) |
|---|---|---|
| Stimulus | the change that is detected | heat from the pan |
| Receptor | cell/organ that detects the stimulus | temperature receptor in the skin |
| Effector | muscle or gland that carries out the response | biceps muscle |
The response is what the effector does — a muscle contracts, or a gland secretes a hormone.
Receptors are grouped into sense organs: eye (light), ear (sound), skin (touch, temperature, pain), tongue and nose (chemicals).
Homeostasis
Homeostasis is the maintenance of a constant internal environment.
The two examples you must know:
- body water content
- body temperature
It matters because enzymes in your cells only work properly within narrow ranges. Too hot and enzymes are denatured (the active site changes shape so the substrate no longer fits). Too much or too little water and cells swell or shrink by osmosis.
The skin and temperature regulation
Core body temperature is held at about 37 °C. Temperature receptors detect the change; the skin is the main effector organ.
When you are too hot
- Sweating — sweat glands release sweat onto the skin surface. The water in it evaporates, and the energy needed for evaporation is transferred from the skin, cooling the body.
- Vasodilation — the small arterioles supplying the capillaries near the skin surface get wider. More blood flows close to the surface, so more energy is transferred to the surroundings by radiation. The skin looks red/flushed.
When you are too cold
- Sweating stops (or is greatly reduced), so no cooling by evaporation.
- Vasoconstriction — the arterioles supplying the surface capillaries get narrower. Less blood flows near the surface, so less energy is lost. The skin looks pale.
Common mistake: writing that "the blood vessels move up and down in the skin", or that "the capillaries dilate". The capillaries cannot change diameter — it is the arterioles supplying them that widen (vasodilation) or narrow (vasoconstriction). Say more/less blood flows near the skin surface, not the vessels move.
Common mistake: saying "sweat cools you down". Sweat only cools you when it evaporates — that word must appear.
Examiner tip. Read every word of the question stem before you write. If it asks about the response to being too cold, do not write a paragraph on sweating; if it says "explain", you must give a mechanism (evaporation transfers energy from the skin), not just "you sweat". Examiners repeatedly report marks lost because candidates spot a key word, rush into an answer, and describe the wrong half of the topic.