6.1.1 Survival and response

Cards (11)

  • ● Organisms increase their chance of survival by responding to stimuli
    Stimulus = change in an organism's internal or external environment
    • Tropism = growth of a plant in response to a directional stimulus
    Positive tropism = towards a stimulus;
    negative tropism = away from stimulus
  • 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 produced)
    ● To other tissues where they regulate growth in response to directional stimuli (tropisms)
  • How indoleacetic acid (IAA) affects cell elongation in roots vs. shoots:
    ● In shoots, high concentrations of IAA stimulates cell elongation
    ● In roots, high concentrations of IAA inhibits cell elongation
  • Gravitropism (gravity) explained by IAA
    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 conc. †)
    4. In shoots this stimulates cell elongation whereas in roots this inhibits cell elongation
    So shoots bend away from gravity whereas roots bend towards gravity
  • Phototropism (light) explained by IAA
    1. Cells in tip of shoot / root produce IAA → IAA diffuses down shoot / root (evenly initially)
    2. IAA moves to shaded side of shoot / root (so conc. ↑)
    3. In shoots this stimulates cell elongation whereas in
    roots this inhibits cell elongation
    4. So shoots bend towards light
    whereas roots bend away from light
  • Taxes (tactic responses)
    Directional response
    Movement towards (positive taxis) or away from
    (negative taxis) a stimulus
    ● Eg. woodlice move away from light to avoid predators
  • Kineses (kinetic responses)
    Non-directional response
    Speed of movement or rate of direction change
    changes in response to a non-directional stimulus
    ● Depends on intensity of stimulus
    ● Eg. woodlice move faster when drier to raise chance of
    moving to higher humidity to prevent drying out
  • Stimulusreceptorsensory neuronerelay neuronemotor neuroneeffectorresponse
  • Reflex Importance
    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 tissues
  • Uneven growth due to the uneven distribution of IAA concentrations in other regions of plants