GASEOUS EXCHANGE IN PLANTS
Oxygen is required by plants for the production of energy for cellular activities. Carbon (IV) oxide is required as a raw material for the synthesis of complex organic substances. Oxygen and carbon (IV) oxide are obtained from the atmosphere in the case of terrestrial plants and from the surrounding water in the case of aquatic plants. Gaseous exchange takes place mainly through the stomata.
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Structure of Guard Cells
The stoma (stomata – plural) is surrounded by a pair of guard cells. The structure of the guard cells is such that changes in turgor inside the cell cause changes in their shape. They are joined at the ends and the cell walls facing the pore (inner walls) are thicker and less elastic than the cell walls farther from the pore (outer wall). Guard cells control the opening and closing of stomata.
Mechanism of Opening and Closing of Stomata
In general stomata open during daytime (in light) and close during the night (darkness). Stomata open when osmotic pressure in guard cells becomes higher than that in surrounding cells due to increase in solute concentration inside guard cells. Water is then drawn into guard cells by osmosis.
Guard cells become turgid and extend. The thinner outer walls extend more than the thicker walls. This causes a bulge and stoma opens. Stomata close when the solute concentration inside guard cells becomes lower than that of surrounding epidermal cells. The water moves out by osmosis, and the guard cells shrink i.e. lose their turgidity and stoma closes.
Proposed causes of turgor changes in guard cells.
Accumulation of sugar.
Guard cells have chloroplasts while other epidermal cells do not. Photosynthesis takes place during daytime and sugar produced raises the solute concentration of guard cells. Water is drawn into guard cells by osmosis from surrounding cells. Guard cells become turgid and stoma opens.
At night no photosynthesis occurs hence no sugar is produced. The solute concentration of guard cells falls and water moves out of the guard cells by osmosis. Guard cells lose turgidity and the stoma closes.
pH changes in guard cells occur due to photosynthesis.
In day time carbon (IV) oxide is used for photosynthesis. This reduces acidity while the oxygen produced increases alkalinity. Alkaline pH favour conversion of starch to sugar. Solute concentration increases inside guard cells; water is drawn into the cells by osmosis. Guard cells become turgid and the stoma opens.
At night when no photosynthesis, Respiration produces carbon (IV) oxide which raises acidity. This favour conversion of sugar to starch. Low sugar concentration lead to loss of turgidity in guard cells and stoma closes.
Explanation is based on accumulation of potassium ions
In day time (light) adenosine triphosphate (ATP) is produced which causes potassium ions to move into guard cells by active transport. These ions cause an increase in solute concentration in guard cells that has been shown to cause movement of water into guard cells by osmosis. Guard cells become turgid and the stoma opens.
At night potassium and chloride ions move out of the guard cells by diffusion and level of organic acid also decreases. This causes a drop in solute concentration that leads to movement of water out of guard cells by osmosis. Guard cells lose turgor and the stoma closes. Process of Gaseous Exchange in Root Stem and Leaves of Aquatic and Terrestrial Plants