Response to a Stimuli

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  • response to stimuli
    • sensory reception
      • stimulus are detectable changes in the internal and external environment of an organism that produces a response in the organism
        • Stimuli are detected by Receptors (cells or organs)
          • Receptors transform stimuli energy to form another form of energy in order to lead to a response.
            • Responses are carried out by effectors (muscles or glands)
            • Receptors and effectors communicate via the use of hormones. this is a slow process in both plants and animals.
              • Responses are carried out by effectors (muscles or glands)
              • A more rapid means of communication in animals is via the nervous system.
                • stimuli--->receptor--->coordinator--->effector--->response.
      • the ability to respond to stimuli increases the organisms survival rate.
        • stimulus are detectable changes in the internal and external environment of an organism that produces a response in the organism
          • Stimuli are detected by Receptors (cells or organs)
            • Receptors transform stimuli energy to form another form of energy in order to lead to a response.
              • Receptors and effectors communicate via the use of hormones. this is a slow process in both plants and animals.
                • A more rapid means of communication in animals is via the nervous system.
                  • stimuli--->receptor--->coordinator--->effector--->response.
        • e.g. detect and move away from harmful stimuli like predators, extreme environment (e.g. high temperature) or detect and move towards stimuli e.g. a source of food.
          • These organisms that survive can breed successfully & pass their alleles to the next generation.
            • therefore this states that there is selection pressure i.e.. organisms with better responses to stimuli are more likely to be selected
      • Responses
        • Taxes
          • This is a simple response. A motile organism responds directly to environmental changes by moving their body...
            • Example of Taxis,
              • Single-celled algae move towards light (positive photo-taxis). This increases survival as being able to photosyntheses effectively as there's more food
              • Earthworms move away from light (negative photo-taxis) which increases it chance of survival as it takes them into the soil where they're better able to conserve water, find food & avoid predators.
              • Some species of bacteria move towards highly concentrated glucose regions (positive chemo-taxis) which also increases chance of survival as glucose is a source of food.
            • towards a favourable stimulus
              • POSITIVE TAXIS
            • or away from a unfavourable stimulus
              • NEGATIVE TAXIS
        • Kineses
          • This is a response which brings about an increase in random movement from the organism (either away or towards the stimuli).
            • the more unpleasant the stimulus, the more rapidly the organism moves & changes direction to allow it to go back into more favourable condtions
              • Kinesis response is important when the stimulus is less directional e.g. temperature & humidity. These don't produce a clear gradient from one extreme to another.
          • Example of Kinesis
            • Woodlice in dry conditions lose water from their bodies. This effect creates rapid, random movement (inc. changes their direction more).
              • This increases their chance of moving into a different, more favourable area of conditions (i.e. moist area).
                • Once reached this area they would slow down and change direction less meaning they spent more time in these favourable conditions, thus increasing their chance of survival
        • Tropisms
          • growth movement part of a plant in response to a Directional stimulus, so parts move...
            • towards (positive --tropism) stimulus
            • or away (negative -- tropism) from stimulus
          • Plant SHOOTS grow towards light (positive photo-tropism), therefore the leaves are in the most favourable condition to capture light for photosynthesis
          • Plant ROOTS grow towards water (positive hydro-tropism) so that the roots can develop where there's the most water available in the soil.
            • Plant ROOTS grow away from light (negative photo-tropism) & towards gravity (positive geo-tropism). Both increase the probability that the roots will grow into the soil where roots are better able to absorb water & mineral ions
          • Plant ROOTS grow away from light (negative photo-tropism) & towards gravity (positive geo-tropism). Both increase the probability that the roots will grow into the soil where roots are better able to absorb water & mineral ions

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