A-LEVEL BIOLOGY AQA NOTES
survival and response
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- All multicellular organisms need to respond to changes in their environment (stimuli) in order to survive
- Tropisms are a directional growth response in plants, in which the direction of the response is determined by the direction of the external stimulus
- Plants respond to directional stimuli using specific growth factors, which move to regions where they are needed from growing regions
- Indoleacetic acid (IAA) causes elongation of shoot cells, while it also inhibits root cell elongation in order to cause positive geotropism & phototropism.
- Taxis is the movement of an animal towards or away from a stimulus
- In kinesis animals change the rate of movement (turning or speed) in order to move towards favourable conditions
- Taxis & kinesis are simple responses that can maintain a mobile organism in a favourable environment.
- Reflexes are rapid responses that don't require conscious thought.
- Reflexes can quickly protect the body from harm, as it does not involve many synapses, they use simple mechanisms and are localized to the part of the body where they occur