9.3 Growth in Plants

Question 1

Name the plant hormone which is responsible for phototropic responses in plants, and where is is produced. (2)

Answer 1

Auxin, produced at the apical meristems

Question 2

What affect do auxins have on cells in the shoot, and what is the stimuli for this effect? (2)

Answer 2

Cell elongation, stimuli of light

Question 3

What is the affect of light on the location of auxins? (1)

Answer 3

Auxins move towards the shady side of the shoot

Question 4

How do protein pumps in the cell membranes of cells of the shoot control the movement of auxins? (2)

Answer 4

• the pumps actively transport auxins in/out of cells, allowing them to move from one cell to another
• auxins can only move out via active transport (and in by diffusion), so most of the transport is due to the protein pumps.

Question 5

What is the auxin’s mechanism of action in shoots? (6/9)

Answer 5

1. Activating proton pumps in the plasma membrane
   
   • auxins bind to auxin receptors
     
     • promoting transcription of specific genes
   • expression of genes causes the secretion of H+ ions into the cell wall
     
     • decreases the pH
   • causes cellulose fibres within cell wall to loosen (breaking bonds)
2. Increases expression of expansins (cell wall loosening proteins)
   
   • increases elasticity of cell wall
     
     • influx of water causes cell to increase in size as cell wall is more flexible

Question 6

How does light influence gene expression? (4/5)

Answer 6

1. Light is absorbed by photoreceptors (phototropins)
   
   • conformation changes after absorbing light of certain wavelengths
2. Phototropins bind to receptors within the cell
   
   • transcription of specific genes is controlled by these receptors
   • likely that these genes are involved in coding PIN3 proteins (transport auxins)