6.1.1 Survival and response

• ● 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

  
• Stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector → response

  
• 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