Gaseous Exchange in Plants

 Plants carryout gaseous exchange for

 • Photosynthesis, and

•Cellular respiration

     Photosynthesis is carried out by chloroplast containing cells in the presence of sunlight. Cellular respiration is carried out by all plant cells all the time. Since plants are inactive, their energy requirements are low, so cellular respiration proceeds slowly. Photosynthesis is a very vigorous process, especially in bright sunlight. Thus, actively photosynthesizing plants take in the absence of photosynthesis, plants take in oxygen and give out carbon dioxide like animals. Besides the gases involved in these two processes, water vapour also escaped from plants during transpiration. 
 
      Plants do not have special gaseous exchange structures like complex animals. Instead, gases enter and leave the plant body through

 • Microscopic openings called stomata on the surfaces of green aerials parts of plants;

 • tiny openings called lenticels on old stems and roots; and •root hairs in young roots.
Stomata

 Stomata are found in the epidermal layer in green aerial parts of plants, especially leaves. They occur mainly on the Lowe surface e of dicotyledonous leaves, although on monocotyledonous leaves they are found on both surfaces.

     Intercellular air spaces found throughout the lead are linked to stomata as a leaf is a very thin flattened organ, individual leaf cells are either in direct contact with intercellular air spaces or very close to such air spaces. This ensures that the rate of gaseous exchange meets the high metabolic demands of an actively photosynthesizing leaf.

 Note : As a leaf is led than 1 mm thick, its cells are less than 0.5mm from intercellular air spaces. This presents no problem for diffusion.

      The gases that enter the intercellular spaces of the lead dissolve in the moisture on the walls of the cells lining these spaces. These walls are kept moist by a continuous stream of water that reaches the leaves from the root.

 Opening and closing of stomata

    The opening and closing of stomata control the flow of gases in and out of the leaves. this control is necessary to prevent excessive loss of water as vapour from the plant body via transpiration. Usually stomata are open during the day and closed at night.

       Each stomata or stomatal pore is flanked by two bean shaped guard cells, the only epidermal cells with chloroplast s. The walls of the guard cells next to the pore are thicker than those adjacent to the epidermal cells the thicker walls cannot stretch as much as the thinner walls.

      Changes in the solute concentration of the guard cells cause water to flow in and out of them by osmosis. When the solute concentration of the guards cells is high, water flows into them from the surrounding epidermal cells. As a result, the volume and turgidity of the guards cells increase. The thin walls strectch more than the thicker walls, causing the guard cells increase. The thin walls stretch more than the thicker walls causing the guards cells to become more curved, and so open the stoma. When the solute Concentration of the guard cells is low, hence their volume, decrease, I.e. the guard cells becomes flaccid. As the walls of the guard cells are elastic, they return to their original position. This causes the guard cells to straighten up and close the stoma.

      Recent studies have shown that guard cells can actively pump in ions, especially potassium, from the surrounding cells, thereby increasing their solute concentration. this active transport mechanism needs energy (ATP) which is probably supplied by photosynthesis in the guards cells. When active transport of Ione into the guard cells stops, the ions in them diffuse out, causing water to flow out also. The guard cells thus become flaccid.

 Lenticels

 These are the air pores found in the bark of stem and roots. They appear as scars or small protrusions on the surface of stems and roots.

    Lenticels are formed when stems and roots undergo secondary thickening. Usually a lenticels develops below a stoma, where the cork cambium, instead of forming compact rows of cork cells, produces irregularly shaped cork cells which are loosely arranged with a lot of intercellular spaces. As these cork cells increase in number and size, the epidermis ruptures to form an opening or lenticels thorough which air can diffuse in and out of the plant. The intercellular spaces in the cork cambium and cortex ensure that the living plant cells in the stem and root are in contact with or close to the air from outside.

, Note : Most of the cells in a woody stem, however, are composer of dead cells.

Root hairs

 These provide a large surface area for the absorption of water, mineral salts and oxygen. Oxygen , present in the soil air, dissolves in the soil moisture and diffuses into the root hairs. From here, it diffuses into the other root cells. The carbon dioxide produced by the root cells diffuses out of the root into the soil via the root hairs.