Responding to stimuli

Responding to stimuli

1. Responding to change. All living organisms must be able to detect changes in their enviroment and respond appropriately. Changes in the enviroment are called stimuli. A stimulus may be either external or internal

2. Sensitivity; the ability to respond appropriately to stimuli, is one of the characteristic features of life. Each organism has its own specific type of sensitivity that improves its chances of survival

3. In large multicellular organsims stimuli are usually detected in sense organs which contain specialised cells called receptors that are particulary sensitive to specific stimuli

4. Structures that bring about a particular response to a stimulus are called effectors. The sense organs and effectors may be in different parts of the body. Responses usually involve the coordinated ctions of many different parts of the body. To achieve this coordination, one part of the body must be able to pass information to another

5. Tropisms; are responses to directional stimuli that help maintain the roots and shoots of flowering plants in a favourable enviroment. The plant body parts repond to a stimulus by growing either towards or away from it. If the directional stimulus is light, the response is called phototropism, if gravity, it is called geotropism, and if water, it is called hydrotropism

6. Kineses and taxes are both forms of simple behavioural responses which help to keep and individual animal, such as a woodlouse in a favourable enviroment. A kinesis is a random movement in which the rate is related to the intensity of the stimulus but not is direction. There are two main types; Orthokinesis; invloves changes in speed of movement, Klinokinesis; involes changes in the rate of turning.

7. A taxis is a movement is response to a directional stimulus. Movements towards the stimulus are positive; those away from the stimulus are negative