These revision notes provide an in depth summary of this specific chapter of AQA Biology in the A Level Specification -
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Biology revision – Response to stimuli (Unit 14)
Survival and response
Stimulus and response
- Stimulus is a detectable change in internal or external environment of an organism
that leads to a response in their organism
- Ability to respond to stimuli is characteristic of life and increases chances of survival
for an organism
- Those organisms that survive have a greater chance of raising offspring and of
passing their alleles to the next generation
- Thus, always a selection pressure favouring organisms with more appropriate
responses
- Stimuli are detected by receptors – receptors are specific to one type of stimulus
- A co-ordinator formulates a suitable response to a stimulus
- Coordination may be at the molecular level or involve a large organ such as the brain
- A response is produced by an effector
- Response may be at molecular level or involve behaviour of a whole organism
- One means of communication in large, multicellular organisms occurs via chemicals
called hormones, which is a relatively slow process found in both plants and animals
- As well as hormones animals have another, rapid, means –nervous system
- Nervous systems usually have many different receptors and control effectors
- Each receptor and effector is linked to a central coordinator of some type
- The coordinator acts like a switchboard, connecting info from each receptor with the
appropriate effector
- The sequence of events can thus involve either chemical control or nerve cells and
may be summarised as:
Stimulus Receptor Coordinator Effector Response
- Towards stimulus = positive response Away from stimulus = Negative response
Taxes
- Taxis is simple response whose direction is determined by direction of the stimulus
o Thus, mobile organisms respond directly to environmental changes by moving
whole body either towards favourable stimulus or away from unfavourable one
- Taxes are classified according to whether movement is towards stimulus or away and
also by the nature of the stimulus
- Some examples are:
o Single-celled algae will move towards light
Increases survival chance – being photosynthetic, require light to
manufacture their food
o Earthworms will move away from light
Increases survival chance as it takes them into the soil, where they’re
better able to conserve water, find food and avoid predation
o Some species of bacteria will move towards a region where glucose is more
highly concentrated
Increases their chances of survival as they use glucose as food source
Kineses
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