Main Organ of Gaseous Exchange

Main Organ for Gaseous Exchange

2.2

Main Organ for Gaseous Exchange

Like other organisms, plants require oxygen gases to perform cellular respiration
In addition, plants also require carbon dioxide gas to perform 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	Sucrose concentration in the guard cell sap
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.	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 effects of water deficiency in plants on stomatal opening and closing

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.
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.