Control in cells and organisms

  • Created by: Laura
  • Created on: 14-04-14 15:13
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  • Control in cells and organisms
    • A stimulus is a detectable change in the internal or external environment. This produces a response. The ability to respond to stimuli increases the chances of survival for an organism
      • There is always a selection pressure favouring organisms with more appropriate responses
    • Stimuli are detected by cells or organs known as receptors. Receptors transform the energy of a stimulus into some form of energy that can be processed by then leads to a response.
    • The response is carried out by one or more of a range of different cells, tissues, organs and systems. These are known as effectors.
    • Each receptor and effector are linked to a central coordinator of some type. The coordinator acts as a switch board connecting each receptor with the appropriate effector.
    • Taxes
      • A taxis is a simple response whose direction is determined by the direction of the stimulus. If they move towards the stimulus it is a positive taxis and if it is away it is a negative taxis.
    • Kinesis
      • Kinesis does not move away from the stimulus instead the more unpleasant the stimulus the more rapidly it moves and the more rapidly it changes direction
    • Tropisms
      • A tropism is a growth of part of a plant in response to a directional stimulus. Plants grow in response to three things; light = phototropism, water = hydrotropism and gravity = geotropism.
      • Plant growth factors
        • Collection of chemicals found in plants, known as plant hormones. One group is called auxins. One in particulaar is IAA which causes cells to elongate.
        • IAA is usually made in the shoot which causes elongation. If only one side of the shoot is getting the light the IAA moves down the side of the shoot which is in shade. This causes one side to grow taller than the other. This causes the shoot to bend towards the light


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