Taxis and Kinesis

• Why do organisms need to respond to their environment?


  • Food
  • Shelter
  • Suitable temperature
  • Adequate light
  • Gases (e.g. oxygen)
• Innate behaviours are developed with little influence from the environment
• Innate behaviours are inherited and often automatic
• Innate behaviours help to find mates, food and suitable living conditions
• Taxis


  A directional movement response where the animal moves towards the stimulus (positive taxis) or away from it (negative taxis)
• Positive phototaxis in algae


  • move towards the light
  • Increased rate of photosynthesis
• Negative phototaxis in worms


  • Move away from light
  • Increased chance of survival- conserve water, find food and avoid predators
• Positive chemotaxis in bacteria


  • Move towards an area of high glucose concentration
  • Increased rate of aerobic and anaerobic respiration
• Kinesis


  The number of turns is related to the intensity of the stimulus
• Woodlice favour damp environments, so they slow down and change direction less often
• Woodlice don't like dry environments, so they move rapidly and change direction lots
• Tropism


  Response of a plant to a directional stimulus
• Phototropism


  Light
• Gravitropism


  Gravity
• Hydrotropism


  Water
• Growth movement may be towards the stimulus (positive) or away from it (negative)
• Shoots experience negative gravitropism, which makes them grow up
• Roots experience positive gravitropism, which lets them grow down
• In the shoots, high concentrations of IAA promotes cell elongation, which causes it to bend and grow up
• In the root, high IAA concentration inhibits cell growth, so it bends and grows downwards
• 1. IAA is synthesised in the root tip and is transported along the root
  2. Gravity is detected by the root
  3. Gravity causes the IAA to accumulate on the lower side of the root
  4. IAA inhibits cell elongation on the lower side
  5. The roots grow and bend downwards
• Axins


  Cell growth
• Gibberelines


  Breaks dormancy
• Abscisic acid


  Promote dormancy
• Ethylene


  Fruit ripening
• Cytokinins


  Lateral growth