Tropisms

• Define tropism:

  Directional plant responses that occur in response to a stimulus.
• Where do plants grow?

  In their root tips and shoot tips. Growth hormone factors are made here.
• What are the 3 main types of plant growth:

  1. Cell division.
  2. Cell elongation.
  3. Cell differentiation.
• Where is IAA produced?

  In root tips and shoot tips.
• What does IAA cause in a plant?

  In shoots, it stimulates cell elongation, and in roots it inhibits cell elongation.
• How does IAA move throughout a plant?

  It diffuses from cell to cell. It is transported long distances via the phloem.
• Describe positive phototropism when sunlight is directly above the shoot tip:

  • IAA is made in cells in the shoot tip.
  • It is evenly distributed.
  • Cells elongate at the same rate.
  • The shoot grows upwards.
• Describe positive phototropism when sunlight is facing one part of the shoot tip:

  • Uneven light causes IAA to diffuse to the shaded side of the shoot.
  • Higher concentration on the shaded side causes cells to elongate more than on the light side.
  • Shoot bends towards the light.
• Describe geotropism in shoots:

  • High IAA concentration at the bottom of the shoot where it stimulates cell elongation.
  • Low IAA concentration on the top of the shoot inhibits cell elongation.
  • Shoot bends upwards- positive geotropism.
• Describe geotropism in roots:

  • High IAA concentration at the bottom of the root inhibits cell elongation.
  • Low IAA concentration at the top of the root leads to elongation of cells.
  • Shoot bends downwards- negative geotropism.
• Tip remove from shoot of plant -> Then, tip replaced on one side of cut shoot -> Then, growth curvature occurred without a directional light source.

  Explain why this happened using your knowledge of IAA (3)

  • Tip produces IAA.
  • IAA diffuses into shoot.
  • More elongation of cells on one side than the other.
• Shoot tips placed on agar -> Shoot tips removed and agar cut into smaller blocks -> Each block placed on one side of cut shoot -> Shoot kept on dark and degree of curvature is measured.

  Using this process and a calibration curve, describe how you could compare IAA concentration in shoot tips from 2 different plant species (5)

  • Repeat process for each species several times and calculate a mean degree of curvature.
  • Use calibration curve to determine IAA concentration for degree of curvature and compare results.

  Controls:

  • Temperature.
  • Number of shoot tips.
  • Type of agar.
  • Time shoots are left in the dark.
• Explain why lids were kept on Petri dishes in an experiment investigating the effect of changing IAA concentration in agar blocks on change in shoot length (2)

  To prevent evaporation which would alter IAA concentration.