Main Organ for Gaseous Exchange

2.2

Main Organ for Gaseous Exchange

Stomata

The pores located on the lower epidermis of the leaf.
Each stoma is guarded by a pair of guard cells that controls the opening and closing of the stoma by changing their shapes.
The guard cells contain chloroplasts to carry out photosynthesis.

The Mechanism of Stomatal Opening and Closing

The mechanism of stomatal opening and closing depends on the conditions of the guard cells whether turgid or flaccid.
The condition of the guard cells depends on
- The potassium ion (K+ ) uptake by the cells.
- The sucrose concentration in the sap of the guard cells.

Uptake of Potassium Ions by Guard Cells

The accumulation or elimination of potassium ion (K+ ) in the guard cells changes the solute potential.
This increases or decreases the water potential in the guard cells.
Water is diffused out or into the guard cells through osmosis.
This condition determines whether the guard cells are turgid or flaccid.

Sucrose Concentration in the Guard Cell Sap

During day time or in the presence of light, photosynthesis takes place and produces dissolved sugar (sucrose).
During night time or in the absence of light, sugar in the guard cells converts into starch.

The Opening of Stoma
Uptake of Potassium Ions by Guard Cells	Sucrose Concentration in the Guard Cell Sap
The potassium ions enter the guard cells. The solute potential in the guard cells increases. The water potential in the guard cells decreases. The water molecules from the epidermal cells diffuse into the guard cells by osmosis. The guard cells become turgid and curve outwards. The stoma opens.	In the presence of light, photosynthesis occurs. The concentration of sucrose in the guard cells becomes high. The water potential in the guard cells decreases. The water molecules from the epidermal cells diffuse into the guard cells by osmosis. The guard cells become turgid and curve outwards. The stoma opens.

The Closing of Stoma
Uptake of Potassium Ions by Guard Cells	Sucrose Concentration in the Guard Cell Sap
The potassium ions move out from the guard cells. The solute potential in the guard cells decreases. The water potential in the guard cells increases. The water molecules diffuse out from the guard cells to the epidermal cells by osmosis. The guard cells become flaccid. The stoma closes.	In the absence of light, photosynthesis does not occur. The sucrose concentration in the guard cells becomes low. The water potential in the guard cells increases. The water molecules diffuse 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

Fresh plants:
- When the plant obtains enough water, the guard cells become turgid.
- The inner cell wall of the guard cells is thick and less elastic as compared to the outer cell wall.
- The thin and more elastic outer cell wall causes the guard cells to curve outwards and the stoma to open.
Wilted plants:
- When the plant lacks water, the guard cells become flaccid.
- The thin and more elastic outer cell wall causes the guard cells to lose turgidity and the stoma to close.