How plants respond to the environment- mechanisms

Phototropic response mechanism and leaf abscission 

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Phototropic response mechanism

  • Blue light increases activity of enzymes phototrophin 1 & 2 on the light side, these phototrophins become phosphorylated
  • Less phototrophin activity on the shaded side causes auxin redistribution, and so the auxin IAA (Indoleacetic acid) increases activity on the shaded side
  • This causes cell elongation in the zone of elongation
  • IAA auxin binds to IAA auxin receptors on the cell surface membrane in the zone of elongation, causing ATPase enzyme to break down ATP
  • Energy from ATP is then used to power proton pump, and so subsequently protons are pumped from cell into the cell wall, decreasing the pH
  • This leaves the optimum pH for expansins to loosen the bonds in cellulose, hydrogen ions also disrupt hydrogen bonds in cellulose
  • Cell walls become less rigid and can expand as the cell takes in water
  • As cells on the shaded side have a higher concentration of IAA, they stretch more than the cells in the light
  • This causes the shoot to move towards the light
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Leaf Abscission

  • As leaf ages, the rate of auxin production declines (auxin inhibits leaf abscission by acting on the abscission zone- part of the leaf stem that breaks when leaf is shed)
  • This drop in auxin concentration causes an increase in ethene production, making cells in the abscission zone more sensitive to ethene
  • Ethene increases the production of cellulase (enzyme that digests cellulose)
  • This digests the cell walls in the abscission zone, causing the leaf to fall
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