6.1.1 stimuli leading to a response

• what is a stimulus

  a change in an organisms internal or external environment
• why is it important that organisms can respond to stimuli

  organisms increase their chance of survival by responding to a stimuli
• what is a tropism

  growth of a plant in response to a directional stimulus, positive tropism=towards a stimulus, negative tropism=away from a stimulus
• summarise the role of growth factors in flowering plants

  specific growth factors (hormone-like growth substances) e.g auxins such as IAA move via phloem or diffusion from growing regions e.g shoot/root where they are produced, to other tissues where they regulate growth in response to a directional stimuli (tropisms)
• explain phototropism in flowering plants

  cell in tip of shoot/root produce IAA

  IAA diffuses down shoot/root evenly initially

  IAA moves to shaded side of shoot/root so concentration increases

  in shoots this stimulates cell elongation whereas in roots this inhibits cell elongation

  so shoots bend towards light whereas roots bend away from light
• explain gravitropism in flowering plants

  cells in tip of shoots/root produce IAA

  IAA diffuses down shoot/root evenly initially

  IAA moves to lower side of shoot/root so concentration increases

  in shoots this stimulates cell elongation whereas in roots this inhibits cell elongation

  so shoots bend away from gravity whereas roots bend towards gravity
• describe how IAA affects cells in roots and shoots

  in shoots, high concentration of IAA stimulates cell elongation

  in roots, high concentration of IAA inhibits cell elongation
• describe the simple responses that can maintain a mobile organism in a favourable environment

  taxes (tactic response), directional response, movement towards or away from a directional stimulus

  kinesis (kinetic response), non-directional response, speed of movement or rate of direction change, changes in response to a non-directional stimulus, depending on intensity of stimulus
• example of taxis

  woodlice moving away from light to avoid predators
• example of kinesis

  woodlice moving faster in drier environment to increase their chance of moving to an area with higher humidity to prevent drying out
• explain the protective effect of a simple (e.g 3 neurone) reflex

  rapid as only 3 neurones and few synapses (synaptic transmission is slow)

  autonomic (doesn't involve conscious regions of brain) so doesn't have to be learnt

  protects from harmful stimuli e.g escape predators/prevents damage to body tissues