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DESCRIPTION OF THE OPENING AND CLOSING OF THE
STOMATA:
Photosynthetic theory:
For stomata to Open:
The guard cells have
chloroplasts which carry out photosynthesis in the presence of sunlight.
Photosynthesis
manufactures glucose which increases the osmotic pressure in the guard
cells as compared to the epidermal cells.
Water moves into the guard
cells by osmosis hence increasing its turgidity.
The inner walls of the
guard cells are thicker than the hence the outer walls stretch more than
the inner wall causing the inner wall to bulge outwards. In this process,
the stomata opens.
For stomata to close:
In absence of light (at
night), no photosynthesis takes place in the guard cells of the leafs.
The glucose in the cells
that was manufactures in the day is converted into starch; which lowers
the osmotic pressure of the guard cells than that of the epidermal cells.
The epidermal cells
withdraws water from the guard cells through osmosis, making the guard
cell flaccid.
The thinner outer wall
shrinks and the curvature of the inner wall reduces; then the stomata
closes.
Starch-sugar inter-conversion theory:
During opening:
Photosynthesis occurs
during the day due to the presence of light. This lowers the
concentration of Carbon dioxide which is a raw material for the above
process. This reduces the acidity of the guard.
This condition favours
conversion of starch to glucose (sugar); which then increases the guard
cells’ osmotic pressure; water from the nearby epidermal cells will move
by osmosis to the guard cell making it more turgid.
The thinner outer walls
stretches more causing the guard cells to bulge out hence opening the
stomata.
During closing:
At night when there is no
light, no photosynthesis takes place that means the level of carbon
dioxide in the guard cells increases increasing acidity.
Acidic condition promotes
conversion of glucose to starch; and the osmotic pressure of the guard
cells reduces than that of the neighbouring cells hence loses water
through osmosis.
The cell thus become
flaccid and the stomata has no choice than to close.
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