The response an organism makes might be the movement of the whole animal; an earthworm moving away from light, or movement of part of the animal; closing of the pupil of the eye in response to bright light or the secretion of sweat in response to increase in body temperature. In most plants the response is movement of part of the organism as seen when the tip of a plant grows in the direction of light or some flowers close their petals at night.
A response to an internal stimulus could be the secretion of enzymes in response to food, the production of antibodies in response to foreign organisms or the secretion of hormones in response to changes in blood glucose levels. It may also involve muscular activity, i.e. movement, for example, an increase in the heart rate in response to an increase in the carbon dioxide content of the blood.
The importance of response
Response to stimuli is important for the survival of organisms. The shoots of green plants grow in the direction of light so they can make food. The roots grow in the direction of gravity. This ensures that they grow towards moisture and also provide stability for the plant. Animals avoid unsuitable conditions by moving towards more suitable ones, for example, earthworms avoid the drying effects of the sun by moving towards dark, moist areas.
Humans maintain a constant body temperature by producing sweat when body temperature rises above normal temperature. The pancreas releases the hormone insulin in response to high blood glucose levels. This hormone stimulates the cells to absorb and use glucose, thereby bringing the glucose levels back to normal. These responses maintain ideal conditions for the enzymes which control metabolism.
A hunted animal avoids its predator by escaping when it sees, hears or smells it approaching.
An animal responds to changes in its internal environment which signal that it needs food. The hungry animal will then look for food and recognize it when it finds it. Without these responses it would soon die.
A male moth is attracted to a female of the same species by being able to detect and respond to chemical signals released by the female. A female bird is able to detect and respond to the courtship behavior of a compatible male. These responses ensure that reproduction takes place and the species survives.
Plants do not usually respond as actively as animals but there are some interesting exceptions; the sensitive plant (Mimosa pudica) folds its leaves quickly when touched. This makes it more difficult for it to be seen by animals which might eat it, and some insect-eating plants close certain specialized structures when insects rests on them, trapping the insects which they then digest.
