Exchange of Gases in plants

Exchange of gases in plants, Diffusion, Opening and Closing of Stomata, structure of stomata, endosmois, exosmosis

Exchange of gases in plants

Exchange of gases is an important phenomenon that is essential for physiological processes like photosynthesis and respiration. Oxygen is released and carbon dioxide is absorbed in Photosynthesis and in respiration of plants carbon dioxide is released and oxygen and absorbed. Although, like animals plants do not posses any special respiratory organs but this exchange of gases take place by the process of diffusion.

Diffusion is the movement of molecules or ions of gas, liquid, or solute from an area of greater concentration to an area of lighter concentration.

All living cells of root, stem and leaves respire all through day and night. In higher plants, the respiration takes place through stomata present on leaves. The older stems exhibit the gaseous exchange through lenticels present on the bark. In lenticels the cells are loose and thin walled and have many air spaces in them. They are often projected above the outer covering of stem.

Opening and Closing of Stomata

Stomata are the minute pores found on the epidermis of leaves. These help in the exchange of gases during the time of photosynthesis and respiration. It also helps in transpiration; this kind of transpiration that takes place with the help of stomata is called stomatal transpiration.

Structure of stomata

In the stoma two guard cells bound a minute elliptical pore.

Guard cell : characters

1.      These guard cells are basically modified epidermal cells which are in shape of kidney beans.

2.       The wall of guard cells near the pore is thick but the outer wall is thin, elastic and semi-permeable.

3.      Guard cells are filled with chloroplast

4.      Sometimes the immediate cells surrounding the guard differ from epidermal cell and they are called subsidiary or accessory cells.

5.      In monocots the guard cells are dumb-bell in shape.

Mechanism of opening and closing of stomata

Opening and closing of stomata occur due to turgor change in guard cells.

Opening of guard cells

Due to endosmosis   -------------à increase in turgor of guard cells    -----------------à causes stretching and bulging of outer thin walls  --------------à results in pulling apart of inner thicker wall ---------à this creates an opening or pore in the guard cells of stomata

Due to endosmosis the turgor pressure of guard cells increases which leads to stretching and bulging of outer thin walls. When the outer thin wall swell, it stretches out and in turn pulls the inner thick wall. This pulling back of iner wall creates a pore between the guard cells.

Closing of guard cells

Turgor pressure of guard cells decreases due to exosmosis    ------------à inner wall of guard cell sags     -----------à the pore created closes.

When the turgor pressure of guard cells decreases due to exosmosis the inner wall of guard cells drop back in its position thus closing the pore.

What causes endosmosis and exosmosis?

During day the starch of guard cells is converted into sugar. This sugar increases the osmotic content of guard cells which lead to endosmosis and finally opens the stomata.

During night, the sugar is converted back to starch reversing the whole process and thus closing the stomata.