2.2 - Main Organ for Gaseous Exchange

The Necessity of Gaseous Exchange in Plants

Plants carry out photosynthesis to produce food. Enough source of carbon dioxide and sunlight are two important elements needed for optimum photosynthesis to take place. The exchange of photosynthesis gases (oxygen and carbon dioxide) happens through stomata. The opening and closing of the stomata are controlled by a pair of guard cells by changing their shapes. Guard cells also contain chloroplasts for photosynthesis.

The Mechanism of Stomatal Opening and Closing

The opening of stoma

• Uptake of potassium ions by guard cells

  • Potassium ions (K+) enter the guard cells. The solute potential in the guard cells whereas the water potential decreases. Water from the epidermal cells diffuses into the guard cells by osmosis. As a result, the guard cells become turgid and curve outwards. The stoma opens.

• Sucrose concentration in the guard cell sap

  • The presence of light allows photosynthesis to occur and produce sucrose in the plants. Therefore, the concentration of sucrose in the guard cells increases whereas the water potential decreases. Water from the epidermal cells diffuses into the guard cells via osmosis. The guard cells become turgid and curves outwards. The stoma opens.

The closing of stoma

• Uptake of potassium ions by guard cells

  • Potassium ions (K+) exit the guard cells. The solute potential in the guard cells decreases whereas the water potential increases. Water diffuses out of the guard cells via osmosis. As a result, the guard cells become flaccid. The stoma closes.

• Sucrose concentration in the guard cell sap

  • The absence of sunlight prevents photosynthesis process. Therefore, the concentration of sucrose in the guard cells decreases whereas water potential increases. Water diffuses out from the guard cells to the epidermal cells by osmosis. The guard cells become flaccid. The stoma closes.

The Effect of Water Deficiency in Plants on Stomatal Opening and Closing

Figure 3 - When there is enough water supply, the guard cells become turgid. The inner cell wall of the guard cells is thicker and less elastic compared to the outer cell wall. The outer cell wall which is thinner and more elastic allows the cell to bend outwards and the stoma to open. The irony is that when the stoma opens, water is loss to the outer environment in the form of water vapor. The rate of water loss increases when the opening of stoma is large.

Figure 4 - When water supply is limited, the guard cells become flaccid. The outer cell wall which is thinner and more elastic allows the stoma to close. When the stoma closes, this prevents further loss of water.