Survival and Response

• What is a stimulus?

  A change in an organism's 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 stimulus
• Summarise the role of growth factors in flowering plants

  • Specific growth factors (hormone-like growth substances) eg. Auxins (such as IAA) move (via phloem or diffusion) from growing regions eg. shoot / root tips where they’re produced
  • To other tissues where they regulate growth in response to directional stimuli (tropisms)
• Describe how indoleacetic acid (IAA) affects cells in roots and shoots

  • In shoots, high concentrations of IAA stimulates cell elongation
  • In 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. ↑)

  4. In shoots this stimulates cell elongation whereas in roots this inhibits cell elongation

  5. So shoots bend towards light whereas roots bend away from light

  
• Explain the protective effect of a simple (eg. 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 eg. escape predators / prevents damage to body tissue