Survival & Response

• Taxis


  Directional movement in response to external stimulus. Positive taxis are towards the stimulus.
• Kinesis


  Non-directional response to external stimulus. Rate of movement is affected by presence & intensity of stimulus. Speed & frequency of direction change can increase.
• Both taxis and kinesis maintain mobile organisms in optimum environment
• Organisms respond to temperature & humidity via kinesis - less directional stimuli; often no clear gradient from 1 extreme to another.
• Tropism


  Directional growth response to stimulus
• Plant growth factors


  Chemicals that regulate plant growth in response to directional stimuli. Produced in plant growing regions (apical meristems); Diffuse from cell to cell/phloem mass transport.
• Positive phototropism in shoots

  1. Indoleacetic acid (IAA) diffuses to the shaded side of the shoot tip

  2. IAA diffusing down the shaded side causes active transport of H+ ions into the cell wall

  3. Disruption to H-bonds between cellulose molecules & action of expansins make cells more permeable to water

  4. Cells on the shaded side elongate faster due to higher turgor pressure

  5. Shoot bends towards light
• Positive gravitropism in roots


  1. Gravity causes IAA to accumulate on the lower side of the root

  2. IAA inhibits elongation of root cells

  3. Cells on the upper side of the root elongate faster

  4. Root tip bends downwards
• Simple reflex arc


  • Receptor detects stimulus
  • Sensory neuron sends electrical impulse to relay neuron in CNS, which coordinates response
  • Motor neuron triggers response by effector
• Advantages of simple reflex arc: Rapid response to potentially dangerous stimuli since only 3 neurons involved (bypass the brain so no decision has to be made; Instinctive)