Investigating Phototropism and Geotropism 5.5.2

Question 1

What area of the plant is positively phototropic?

Answer 1

Shoots, as they grow towards light

Question 2

Why do plants grow towards light?

Answer 2

To maximise the amount of light they can absorb for photosynthesis

Question 3

Describe Darwin’s experiment

Answer 3

Discovered that removing the tip of a coleoptile stopped the phototropic response to a unidirectional light source from occurring.
To ensure this was not caused by wounding of the plant, he covered the tip with a cap instead to block the light - which also stopped the phototropic response.
This shows that the tip of the shoot was responsible for detecting light

Question 4

Describe Boysen - Jensen’s experiment

Answer 4

Found that if he replaced the cut tip back on top of the shoot and inserted a gelatine block as a barrier in between - the phototropic response was restored.
This showed that the stimulus for growth was a chemical hormone and travelled through the gelatine block
He inserted a mica barrier (impermeable to chemicals) below the tip, first on the lit side then the shaded side. On the lit side, the phototropic response occurred. On the shaded side, the phototropic response did not occur
This confirmed that the hormone for growth was produced at the tip, before travelling down the chute on the shaded side. Causing growth on the shaded side, not inhibiting growth on the lit side.

Question 5

Describe Paal’s experiment

Answer 5

Paal cut off the tip and replaced it off centre in the dark
The side of the shoot that the tip was placed on grew more than the other side - causing the shoot to curve
This showed that, in the light, the phototropic response was caused by a hormone diffusing through the plant tissue and stimulating growth

Question 6

Describe Went’s experiment

Answer 6

Went placed the cut tip on a gelatine block, allowing the hormones from the tip to diffuse into this block.
The block was then placed on the shoot, off centre and in the dark.
As in Paals experiment, the side of the shoot the block was placed on grew more than the other side, causing a curve.
The greater the concentration of hormone present in the block, the more the coleoptile curved

Question 7

What is Indole-3-acetic acid (IAA)?

Answer 7

Auxin

Question 8

What is auxin?

Answer 8

A specific growth factor found in plants

Question 9

Where is IAA synthesised?

Answer 9

In the growing tips of roots and shoots (i.e. in the meristems, where cells are dividing)

Question 10

What does IAA coordinate?

Answer 10

Phototropism’s by controlling elongation

Question 11

How does IAA cause elongation?

Answer 11

The IAA molecules activate proteins in the cell wall known as expansions, which loosen the bonds between cellulose microfibrils, making the cell walls more flexible. The cell can then elongate

Question 12

What does the concentration of IAA determine?

Answer 12

The rate of cell elongation

Question 13

What occurs if the concentration of IAA is not uniform on either side of a root or shoot?

Answer 13

Then uneven growth can occur

Question 14

What happens if there is a higher concentration of IAA on the shaded side of the shoot?

Answer 14

Causes a faster rate of cell elongation causing the shoot to bend towards the light

Question 15

How does gravity modify the distribution of IAA?

Answer 15

So that it accumulates on the lower side of the shoot.

Question 16

What is the impact of high concentrations of IAA in the shoot In geotropic response?

Answer 16

Negative geotropism

Question 17

What is the impact of high concentrations of IAA in the root in geotropic response?

Answer 17

Lower rate of cell elongation therefore the root bends downwards
Positive geotropism

Question 18

What is the apparatus required for investigating the effect of IAA on root growth?

Answer 18

• Seedlings (same age and species)
• Cutting tile
• Scalpel
• Light source
• Lightproof container
• Blocks of agar (all same volume)
• Pen
• Test tubes
• Water

Question 19

What is the method for investigating the effect of IAA on root growth?

Answer 19

• Use scalpel to cut 1cm section from root tip of each seedling
• Mark root tips at 2mm marks
• Divide these tips into three groups and place them in test tubes of water

Group A receives treatment one:
- Remove ends of root tips using scalpel
- Transfer root cutting with end removed to an agar bloc
- Uniform light source present

Group B receives treatment 2:
- Transfer intact root tips to an agar block
- Light proof container is placed over the seedlings

Group C receives treatment 3:
- Transfer intact root tips to agar block
- Apply directional light source to one side of root tips

Leave all roots in treatment conditions for 3 hours
Use 2mm marker lines to determine if growth has occurred
Note if the growth has been even on both sides

Question 20

What are the results for investigating the effect of IAA on root growth?

Answer 20

Group A = roots grow evenly as tips has been removed; no IAA synthesised
Group B = roots grow slightly less than A but evenly as there is equal concentration of IAA on both sides of tip
Group C = the cells on illuminated side grow longer than shaded side, as greater concentration IAA on shaded side = greater inhibition of cell elongation on shaded side

Question 21

What are the limitations of the method used for investigating the effect of IAA on root growth?

Answer 21

• 2mm marks may get smudged
• Plants will still have differences despite being of the same species
• Evenness of growth can be hard to determine using the naked eye