Organism response to Stimuli

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  • Organisms Responding to Change
    • Stimulus
      • Detectable change to external environment or internal
        • Produces organism response
          • Caused by muscle contraction in animals
          • Caused by chemical production in plants
            • Individuals with most appropriate response survive & pass on alleles - natural selection
              • Caused by muscle contraction in animals
      • Receptor
        • Co-ordinator
          • Effector
            • Response
    • Responses
      • Help organisms to survive
        • Taxes
        • Kineses
        • Tropisms
    • Taxes
      • Simple Directional Response
      • Positive Taxes
        • Move towards stimulus
      • Negative Taxes
        • Move away from stimulus
    • Kineses
      • Response to general stimulus
      • In favourable conditions, move slowly & change direction frequently to stay in same region
      • Not directional as there is no clear gradient. e.g. temperature
    • Tropisms
      • Growth movement of a plant in response to a directional stimulus
        • Phototropism - light
        • Gravitropism - gravity
        • Hydrotropism - water
      • Plant Growth Factors
        • Chemicals that control plant response to the environment, as plants don't have a nervous system
          • Indoleacetic Acid (IAA)
      • Plant shoots
        • Positive phototropism & negative gravitropism
          • Move towards light & away from gravity
        • Cells in tip produce IAA
          • IAA diffuses evenly down the shoot, causing cells to elongate
            • Directional light causes IAA to diffuse down the dark side more than the light.
              • Greater concentration of IAA on the dark side, means cells elongate more than that of the light side.
                • Dark side gets longer & causes shoot to bend towards the light.
      • Plant roots
        • IAA causes growth to decrease.
          • If root grows horizontally, IAA diffuses to bottom side of root
            • Higher concentration of IAA on bottom side reduces growth, causing root to bend down into the soil.
              • Positive gravitropism


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