Key Processes of Exchange
Gas Exchange in Plants
All organisms that rely on photosynthesis
to manufacture energy require copious amounts of carbon dioxide and a means of disposing of oxygen. In order to carry on cellular respiration
, all cells need oxygen and a means of disposing of carbon dioxide. Obviously, plants are in a very precarious situation, one that must be balanced and properly monitored by each plant cell. Additionally, unlike most animals, plants have no specific organs designed for gas exchange. However, there are several important adaptive reasons they can get along without them:
The plant divides up the task of gas exchange among its parts; that is, each section of the plant (indeed, each cell) takes care of removing whatever excess gas it needs to and brings in whatever gases are needed on its own. Although xylem and phloem are present for liquid and mineral exchange, neither has a significant part in the exchange of gases.
The transpirative parts of the plant (roots, stems, leaves) have adapted to lose water and precious gases at a much lower rate than those of animals (ex.- C4 and CAM plants, use of stoma)
All living cells in a photosynthetic organism are located near the surface, facilitating gas exchange by limiting drastically the distance that gases must travel to reach the surface of the plant.
Plant cells tend to be loosely packed, allowing for gases to rapidly diffuse and move throughout the plant. The diagram below demonstrates this fact:
The exchange of oxygen and carbon dioxide in leaves and fleshy stems (as well as the loss of water vapor in transpiration) occurs through pores called stomata (singular = stoma). Normally, stomata will open when struck by light (as occurs in the morning) and close during the night. It is interesting to note the differences in CAM plants in this regard. CAM plants open the stoma at night at close them during the day. The most likely cause for stoma activity is a change in the turgor, or water pressure, of the guard cells. The inner wall of each guard cell is much like a rubber band, able to expand and contract quite easily. When turgor develops within the two guard cells flanking each stoma, the thin outer walls bulge out and force the inner walls into a crescent shape. This opens the stoma. When the guard cells lose turgor, the elastic inner walls regain their original shape and the stoma closes. In this way, it is primarily water loss or gain that influences the actions of the guard cells, and, by necessity, the stomata themselves.
Roots and Stems
As anyone who has ever hit a tree trunk at any appreciable speed can attest, the thick bark surrounding woody plants is quite dense and impregnable. This thick layer of dead cells is covered with suberin, a waxy coating that is both air proof and waterproof. Roots also, have this same waxy coat. Suberin, by design, is non-permeable to both oxygen and carbon dioxide. This would seem to cut off gas exchange for a plant. However, the bark of woody stems and the roots of most plants are perforated by tiny opening called lenticels. This openings allow air to enter the stem or root and come in contact with the living cells, thus proliferating gas exchange. In most herbaceous plants, the stems are green and almost as important for photosynthesis as the leaves. These stems use stomata rather than lenticels for gas exchange.