- A stimulus is a detectable change in the internal or external environment of an organism that produces a response in the organism
- The ability for an organism to respond to a stimulus increases its chance of survival. These organisms which survive can therefore carry out natural selection.
- Stimulus are detected by receptors.
- Receptors are able to transfer energy of a stimulus into a form of energy that can be processed by the organism and lead to a response
- The response is carried out by Effectors
- Animals use their nervous system to communicate between receptors and effectors
- Each effector and receptor is connected to a coordinator
- The coordinator connects information from each receptor with the appropriate effector
Stimulus à Receptor à Coordinator à Effector à Response
- Taxis is a simple response by direction which is determined by a direction of the stimulus
- A motile organism will respond directly to the environment changes by moving its entire body either towards a favourable stimulus or away from an unfavourable one
- Taxes are classified according to whether the movement is:
- towards the stimulus (positive taxis)
- away from the stimulus (negative taxis
- Kinesis is an indirect movement towards/away from a stimulus determined by a more unpleasant environment causing a more rapid change in direction compared to a favourable one where changes in directions are reduced.
- Kinesis results in an increase in random movements
- Kinesis aims to return the organism back into a favourable environment.
- Kinesis is particularly important for less directional stimuli
- g. Humidity and Temperature do not have a fixed gradient from one extreme to another
- Tropism is a growth movement of a part of a plant in response to a directional stimulus
- Plants will grow towards and away from a stimulus
- Plant shoots grow towards light (positive phototropism)
- Plant roots grow away from light (negative phototropism) but towards gravity (gravitropism)
- Plant roots grow towards water (positive hydrotropism)
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