Survival and 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 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
• Describe how indoleacetic acid (IAA) affects cells in roots and shoots

  • Shoots = High concentration of IAA stimulates cell elongation
  • Roots = High concentrations of IAA inhibits cell elongation
• Explain Gravitropism in flowering plants:

  1. Cells in tip of shoot / root produce IAA
  2. IAA diffuses down shoot / root (evenly initially)
  3. IAA moves to lower side of shoot / root (so concentration increases)
  4. In shoots this stimulates cell elongation whereas in roots this inhibits cell elongation
  5. So shoots bend away from gravity whereas roots bend towards gravity
• Explain phototropism in flowering plants:

  1. Cells in tip of shoot / root produce IAA
  2. IAA diffuses down shoot / root (evenly initially)
  3. IAA moves to shaded side of shoot / root (so conc. increases)
  4. In shoots this stimulates cell elongation whereas in roots this inhibits cell elongation
  5. So shoots bend towards the light whereas roots bend away from the light
• What is Taxis?


  • A directional response
  • Movement towards or away from a directional stimulus
• What is Kinesis?


  • Non-directional response
  • Speed of movement or rate of direction change changes in response to a non-directional stimulus
  • Depending on intensity of stimulus
• Explain the protective effect of a simple reflex:

  • Rapid as only three neurons and few synapses (synaptic transmission is slow)
  • Autonomic (doesn't involve conscious regions of the brain) so doesn't have to be learned
  • Protects from harmful stimuli e.g. escape predators / prevent damage to body tissue