Auixns
Biology: Cambridge International A Level, topic 15.2.2
Auxins
Auxins control cell elongation in plants. They do this through stimulating hydrogen ion pumps.
Distribution of auxins
- Auxins can be transported over short distances (by diffusion or active transport) and longer distances (in the phloem) around the plant.
- When a plant detects directional stimuli, auxins are transported to different parts of the plant.
- This creates an uneven distribution of auxins.
- Where the distribution of auxins is uneven, a directional growth response occurs.
Expansins
- Auxins stimulate proton pumps in the cell wall.
- The transport of protons into the cell wall lowers the pH in the cell wall.
- This low pH activates proteins known as expansins.
- Expansins loosen cellulose. This makes cell walls less rigid. So, more water can enter the cell.
- As more water can enter cells, cells can swell and elongate.
Phototropism - shoots
- Shoots are positively phototropic. This means they grow towards light.
- If a shoot is exposed to an uneven light source, auxins are is transported to the more shaded part.
- A higher concentration of auxins in the shaded parts cause cells in the shaded area to elongate.
- Cell elongation causes the shoot to bend towards the light.
Phototropism - roots
- Roots are negatively phototropic. So, they grow away from light.
- If a root is exposed to an uneven light source, auxins are transported to the more shaded part.
- A higher concentration of auxins in the shaded parts inhibits cell elongation on the shaded part.
- The root bends away from the light.
Test yourself on Auixns
Which of the following statements about IAA are true?
What do auxins stimulate in the cell wall of plants?
What is the function of expansins?