Stimuli and response

• Stimulus


  A detectable change in the environment
• Receptors


  Detect various stimuli
• Importance of flowering plants responding to stimulus

  • Increases their chance of survival
• Tropism


  The response of a plant to stimuli via growth, either growing away or towards a stimulus
• Positive tropism


  Growth of a plant towards a stimulus
• Negative tropism


  Growth of a plant away from a stimulus
• Phototropism


  The growth of a plant in response to light
• Positive phototropism


  Growth of a plant towards light
• Part of a plant that shows positive phototropism


  • Shoots, as they require light for photosynthesis
• Negative phototropism


  Growth of a plant away from the light
• Part of a plant that shows negative phototropism

  • Roots, as they don't carry out photosynthesis
• Gravitropism


  The growth of a plant in response to gravity
• Positive gravitropism


  Plants which grow downwards towards gravity
• Part of a plant that shows positive gravitropism


  • Roots
• Negative gravitropism


  Plants which grow upwards away from gravity
• Part of a plant that shows negative gravitropism


  • Shoots
• Growth factors


  Hormone like chemicals in plants which speed up or slow down plant growth
• Where growth factors are produced


  • Growing regions of the plant, shoot tips and root tips, contain meristem
• Example of a growth factor

  • Auxins
• IAA (Indoleacetic acid)


  A type of Auxin that controls cell elongation in shoots and inhibits growth of cells in roots
• High concentrations of IAA in shoots

  Elongates cells and results in growth
• High concentrations of IAA in roots

  Inhibits growth
• Where IAA is produced

  • In the tips of the roots and shoot, can diffuse to other parts of the plant
• IAA and phototropism in shoots

  IAA diffuses towards shaded side, causing more cell elongation, leading to the plant bending towards the light (positive phototropism)
• IAA and phototropism in roots

  High concentration of IAA on shaded region inhibits cell elongation, causing roots to elongate more on lighter side and bend away from light (negative phototropism)
• IAA and gravitropism in shoots

  IAA diffuses to lower side, causing cell elongation and upward growth (negative gravitropism)
• IAA and gravitropism in roots

  IAA concentration increases on lower side, inhibiting growth and causing downward bending (positive gravitropism)
• Taxes


  Specific type of movement where organisms move their entire body towards or away from a favourable stimulus
• Positive taxes


  When an organism moves towards a stimulus
• Negative taxes


  When an organism moves away from a stimulus
• Negative phototaxis in earthworms

  • Earthworms move away from light towards dark environments like soil to avoid dehydration, predators and find food
• Positive chemotaxis in bacteria


  • Bacteria move towards certain chemicals which aid in their survival
• Kinesis


  When an organism changes its speed of movement and rate of direction changes
• When an organism moves from an area with beneficial stimuli to an area with harmful stimuli


  The organism will increase the rate it changes direction to find favourable conditions
• When an organism is surrounded by negative stimuli

  The rate of turning decreases as the organism moves in a relatively straight line to increase its chances of finding a new location with favourable conditions