stimuli and responses

• • Taxis ( plural=taxes) - directional movement of whole organism directly towards or away from a stimulus.
• • Kinesis (plural=kineses) -  non-directional response to a stimulus
• Tropisms:

  • The growth of all or part of an organism in a particular direction in response to an external stimulus (sessile organisms - can't move, so mainly plants and fungi)
  • Types: phototropism (light), gravitropism/geotropism (gravity), hydrotropism (water)
• Responses to stimuli are crucial for survival as they allow organisms to:

  • Navigate towards beneficial resources like food.
  • Evade predators or harmful conditions.
• A taxis may either be positive or negative:

  • Positive taxes lead towards a favourable stimulus.
  • Negative taxes prompt movement away from adverse stimuli.
• Control of positive phototropism in shoots:

  1. IAA is produced in cells in the tip of the plant shoot.
  2. IAA is transported down the plant shoot.
  3. Light stimulates IAA to move from the light side of the shoot to the shaded side.
  4. IAA becomes concentrated and stimulates more cell elongation on the shaded side of the shoot.
  5. The shoot bends towards the light.
• Control of negative phototropism and positive gravitropism in plant roots:

  1. IAA is produced in cells in the tip of the plant root.
  2. IAA is transported along the plant root.
  3. Any light available stimulates IAA to move from the light side of the root to the shaded side.
  4. Gravity also pulls IAA from the upper side of the root to the lower side of the root.
  5. IAA becomes concentrated and inhibits cell elongation in the lower, shaded side of the root.
  6. The root bends away from any available light and downwards towards the pull of gravity.