Stimulus & Response
Stimulus – detectable change in internal or external environment of an organism that produces a response in an organism •Ability to respond to stimuli – increases chance of survival e.g. detect and move away from harmful stimuli •Surviving organisms have greater chance of raising offspring – passing alleles to next generation, therefore a selection pressure favouring organisms with more appropriate responses Stimuli – detected by receptiors, transform energy of stimulus into a form of energy that can be processed by the organism – leads to response Effectors – carry out the response
Nervous system – many different receptors and effectors, linked to a central coordinator, acts like a switchboard connecting information from each receptor with the appropriate effectors STIMULUS RECEPTOR COORDINATOR EFFECTOR RESPONSE Taxis – simple response whose direction is determined by direction of stimulus, a motile organism responds directly to environmental changes by moving whole body towards a favourable stimulus or away from an unfavourable one Positive taxis – movement towards stimulus
Negative taxis – away from stimulus
Eg. Single – celled algae move towards light – positive phototaxis Kinesis – form of response, not movement towards or away from stimulus, the more unpleasant the stimulus, the more rapidly it moves and the more rapidly it changes direction – increase in random movements, important when stimulus is less directional Eg. Woodlice – lose water from their bodies in dry conditions, move more rapidly in dry area and change direction more often, move less in moist areas – prevents drying out and increases chance of survival Tropism – growth movement of part of a plant in response to a directional stimulus, plants grow towards or away from stimulus e.g. plant shoots grow towards light (positive phototropism) and towards water (positive hydrotropism)...