15.2.2

Auixns

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Auxins

Auxins control cell elongation in plants. They do this through stimulating hydrogen ion pumps.

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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.
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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.
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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.
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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.

Jump to other topics

1Cell Structure

2Biological Molecules

3Enzymes

4Cell Membranes & Transport

5The Mitotic Cell Cycle

6Nucleic Acids & Protein Synthesis

7Transport in Plants

8Transport in Mammals

9Gas Exchange

10Infectious Diseases

11Immunity

12Energy & Respiration (A2 Only)

13Photosynthesis (A2 Only)

14Homeostasis (A2 Only)

15Control & Coordination (A2 Only)

16Inherited Change (A2 Only)

17Selection & Evolution (A2 Only)

18Classification & Conservation (A2 Only)

19Genetic Technology (A2 Only)

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