Survival & Response

• Stimulus - A change in the environment that triggers a response in the body.
• Reaction pathway:

  Stimulus -> Receptor -> Coordinator -> Effector -> Response
• Why do organisms need to respond to the environment:

  • Avoid hostile environments, competition and predators
  • Keep in favourable environment (correct humidity etc)
  • Grow towards/away from a stimulus
  • Dispersing to find new mates (pollen, organisms)
• Taxis v Kinesis:

  • Both are a response to stimuli through the movements of the whole organism
  • Taxis is random and kinesis is directional
• Responses to stimuli - behavioural patterns:

  • Instinctive behaviour is an innate response to stimuli
  • Innate behaviour is genetically determined (not learned)
  • Members of a species which inherit alleles for a particular behaviour produce same response to stimulus
• Responses to stimuli:

  • Kinesis (random) - increases/decreases activity in response to stimulus
  • Taxis (directional) - movement of organisms towards/away stimulus
• Types of taxis:

  • Positive taxis - towards the stimulus
  • Negative taxis - away from the stimulus
• Reflex action - a rapid, automatic response to a stimulus that does not involve the conscious part of the brain
• Reflex actions allow:

  1. Protection against damage to the body tissues
  2. Help escape from predators
  3. Enable homeostatic control
  4. Finding food, mates or suitable environmental conditions
• Reflex arc:

  Stimulus -> Receptor -> Sensory neurone -> CNS -> Relay neurone -> Motor neurone -> Effector
• Indoleacetic acid (IAA) -A plant growth factor that promotes or inhibits cell elongation
• IAA:

  • IAA is produced in tips of roots and shoots
  • Diffuses to growing region
  • Causes cell elongation in the shoots
  • Inhibits cell elongation in the roots
  • IAA transported to more shaded parts of roots and shoots
  • IAA moves to underside of roots and shoots
• In the shoots - Promotes cell elongation

  • IAA synthesised in shoot tips
  • IAA diffuses into growing (elongating) region
  • Light causes movement of IAA on shaded side
  • Causes directional growth as greater elongation on shaded side
  • Shaded side grows faster and causes shoot to bend towards light
• In the roots - Inhibits cell elongation

  • IAA synthesised in root tips
  • IAA diffuses into the growing region
  • Light causes movement of IAA from light side to shaded side, towards gravity
  • Higher concentration of IAA on the shaded side
  • Cell elongation inhibited on the shaded side
  • Directional growth away from light and towards gravity
• Similarity between taxis and tropism:

  • Directional response to a stimulus
• Difference between taxis and tropism:

  • Whole organism moves in taxis and tropism is a growth response
• Seeds will respond to light and so may affect experiment results
• Why lids are kept on Petri dishes in IAA experiments:

  • Reduces evaporation
  • That may affect IAA concentration