Tropisms

Question 1

What are the different types of tropisms?

Answer 1

Tropisms: Directional growth response, direction of the response is determined by the direction of the external stimulus

Phototropism – unidirectional response to light

Geotropism – response to gravity

Chemotropism – response to chemicals

• +ve chemotropism e.g.: Pollen stimulates pollen tube to grow down to ovary so fertilisation can occur

Thigmotropism – response to touch

Question 2

What is phototropism?

Answer 2

• Phototropism occurs due to the movement of auxins across a shoot (or root) if exposed to sunlight.
• If plants are exposed to light which is even on both sides, the plant will grow upwards
• If plants are exposed to light stronger on one side than the other, the shoot will bend towards the light
• Shoots are positively phototropic, roots are negatively phototropic

Question 3

1. What occurs when the shoot tip is removed, and light is shone on one side?
2. What occurs when the tip is covered with an opaque cover?
3. What occurs when an impermeable, thin barrier or mica is placed on the side of the shoot facing the light?

Answer 3

1. There is no response (plant doesn’t grow), suggesting the tip detects a stimulus and/or produces the hormone
2. There is no response, suggesting the tip has to detect the stimulus

3. The shoot bends towards the light, meaning the hormone must pass down the shaded side

Question 4

1. What occurs when mica is placed on the shaded side?
   * 2.* What occurs when a gelatine block, permeable to chemicals and not electrical signal is placed across the diameter of the shoot

Answer 4

1. There is no response. Confirming that the hormone moves down the shaded side
2. The shoot bends toward the light. Implying there are no electrical signals, purely chemical (and hormonal)

Question 5

What is the effect of unilateral light on auxins?

Answer 5

• Light causes auxins to laterally move across the shoot, meaning a greater [AUXIN] accumulates on the shaded side.
  
  • Stimulating cell elongation by increasing the plasticity of cell walls on the shaded side causing the shoot to bend towards the light.

Question 6

How do plants grow in the dark?

Answer 6

• Plants grow quicker in the dark than in light
  
  • This is because the plant is then encouraged to grow straight upwards rapidly to reach light to photosynthesise
• Gibberelins are responsible for extreme elongation of plants in the dark
  
  • As the plant is exposed to light the gibberellin concentration falls
  • During an illuminated phase, upwards growth is hindered, resources are then used for synthesising leaves, strengthening stems and overall growth

Question 7

How are plants geotropic?

Answer 7

• Shoots are negatively geotropic, and roots are positively geotropic (grow towards the stimulus, gravity)
• Helps anchor the plant - so the plant is stable and won’t fall over

Question 8

Describe a practical procedure to investigate phototropism

Answer 8

1. Germinate and grow seedlings in different light conditions
   
   • Dark, all-round light, unilateral light
   • Observe and measure patterns of growth
     
     • use time-lapse photography to observe changes
2. Germinate and grow seedlings in unilateral light with different colour filters to see which light wavelengths trigger a phototropic response
3. Repeat Darwin and Boysen-Jensne responses
   
   • Cover coleoptiles with foil, remove tips, add mica etc.

Question 9

Describe a practical procedure to investigate geotropism

Answer 9

1. Investigate in shoots and roots by using a clinostat
2. Rotate the clinostat at 4rph and grow the shoot straight, observe the root and shoot growth
3. Place seeds in a petri dish, align the petri dish perpendicular to the surface
4. Witness a geotropic response (roots grow down, shoots grow up)

Question 10

What is the mechanism by which auxins stimulate cell elongation?

Answer 10

• Increases cell wall plasticity so the cell wall can stretch easier when the rest of the cell elongates
  
  • Prevents stress/damage to the cell

IAA mechanism for cell elongation: acid growth hypothesis

1. IAA is synthesised in the meristem, diffuses away from the tip and binds to receptor sites on the plasma membrane
2. This causes protons to be actively (uses ATP) pumped into the spaces in the cellulose cell wall
3. The protons activate proteins called expansins
   
   • They cause the cellulose fibres to loosen to allow stretching of the cell wall

Once the cell matures, auxins are destroyed so the action of expansins stops, cell wall is rigid and fixed, doesn’t grow