Gas exchange in plants

Categories: Photosynthesis and respiration; physiology

All living organisms continually produce gases via metabolic and cellular activities, and the vast majority of living things are in one way or another in intimate contact with a gaseous medium. In most instances, therefore, there is ample opportunity for all organisms to exchange gases with the environment. The gaseous balance in plants is quite complex because plant cells carry on both respiration and photosynthesis. Plants respire in much the same way as animals; oxygen is used to oxidize carbohydrates, and carbon dioxide and water are produced as waste products. The photosynthetic process requires an input of carbon dioxide and water. These two reactants are used to produce carbohydrates, and oxygen is released as a waste product. Under normal conditions, photosynthetic rates are higher than respiration rates; thus, there is a net increase in oxygen production, accompanied by a net increase in the usage of carbon dioxide. On balance, therefore, plants use carbon dioxide and produce oxygen.

Stomata and Guard Cells

The gases move into and out of the plants through specialized openings located along the lower surface of the leaf. These openings, called stomata, are of optimum size, shape, and distribution for the efficient diffusion of gases. Each stoma (or stomate) is surrounded by two specialized structures called guard cells. These two cells are attached together at each end of both cells. The lateral edges of the two cells are not attached to each other, but, when flaccid, the sides of the guard cells do touch each other and effectively close the stomate. Specialized structural components prevent the guard cells from increasing in diameter as expansion occurs. Hence, when guard cells take up water, expansion takes place only along the longitudinal axis. Because the ends of the cells are connected to each other, the expanding of the cells forces the sides apart and results in the opening of the stomate.

Role of Water

The opening of stomata is dependent on how well hydrated the plant is. The water initially comes from the soil. The water enters the root by osmotic processes, then moves across the root and into the xylem tissues, which transport it up the stem to the leaves. From the xylem in the leaves, the water moves into the palisade and spongy parenchyma cells, which make up the bulk of the leaf tissue. The water then moves into the subsidiary cells that immediately surround the guard cells.

When the leaf is exposed to light, the process of photosynthesis begins. As the photosynthetic reactions proceed in the guard cells, the residual carbon dioxide is converted to carbohydrates. The disappearance of carbon dioxide from the cytosol of the guard cell results in an increase in the cellular pH. As the pH rises, the activity of the enzymes that convert starch and sugars to organic acids increases. The higher concentration of organic acids results in a higher concentration of hydrogen ions. The hydrogen ions of the guard cells are then exchanged for potassium ions in the subsidiary cells. This increased concentration of potassium, combined with the higher levels of organic acids, lowers the osmotic potential of the guard cells, and, since water moves from regions of high osmotic potential to regions of lower osmotic potential, water will move from the subsidiary cells into the guard cells. This movement of water increases the turgor pressure (inner pressure) of the guard cells and causes them to swell. Thus, the stomata open.

Oxygen Out, Carbon Dioxide In

Once the stomata open, the intercellular free space around both the palisade and spongy parenchymas is put into continuous contact with the outside atmosphere. As the water within the parenchyma cells moves across the cellular membranes, it evaporates into the free space and diffuses through the stomata into the atmosphere. Oxygen produced during photosynthesis exits the plant in much the same manner as the water vapor.

Carbon dioxide, however, follows the reverse path. It enters through and across cell membranes into the parenchyma tissues. In each case, the gas involved moves down a concentration or pressure gradient. The pressure of water vapor and oxygen is higher inside the leaf’s free space than in the atmosphere, whereas the partial pressure of carbon dioxide is greater in the atmosphere than within the free space. Thus, the impetus is for the former two gases to move out of the plant and for the latter to enter it.

This exchange will take place as long as the stomata remain open and the pressure gradient is in the right direction. As a general rule, stomata close in the dark. Without an input of solar energy, the light-mediated reactions of photosynthesis stop. In the absence of these reactions, the carbon dioxide level increases, thus decreasing the pH. The lower pH activates the enzymatic conversion of organic acids to sugars and starch. This causes the potassium ions to move from the guard cells into the subsidiary cells. As a result, the osmotic potential of the guard cells is raised, the water moves out, the cells become flaccid, and the stomata close.

External Influences

Environmental conditions can affect stomatal openings. Drought conditions, which induce water stress, can affect gas exchange because the lack of water moving through the plant causes the guard cells to lose turgor and close the stomata. When the temperature becomes too warm, stomata also tend to close. In some instances, the higher temperature causes water to leave the leaf more rapidly, which leads to water stress.

In other cases, the increase in temperature causes an increase in cellular respiration that, in turn, increases carbon dioxide levels. Internal high carbon dioxide concentrations both reverse the carbon dioxide pressure gradient and cause the stomata to close.

The percentage of relative humidity can drastically affect the rate of water evaporating from the leaf surface. As the humidity increases, the higher water content of the air decreases the rate of water loss from the leaf because the water pressure gradient no longer favors evaporation from the leaf surface. The amount of solar radiation can also influence gas exchange. As the amount of light increases, the stomata open faster and wider, resulting in a more rapid rate of gas exchange. Wind currents will also increase gas exchange rates: As the wind blows across the leaf, it carries water vapor away and, in a sense, reduces the humidity at the leaf surface. Because of this lower humidity, the water evaporates from the leaf surface more rapidly.

Ecological Impact

The exchange of gases between living plants and the atmosphere is critical to the survival of all living organisms. Without the release of the oxygen produced during photosynthesis, the atmosphere would contain very little of this necessary gas. Furthermore, the vast majority of organisms on earth depend on the organic materials supplied by plants. The carbon dioxide taken from the atmosphere is photosynthetically fixed into the more complex carbon molecules that eventually serve as food not only for the plants but also for all those organisms that consume plants. The amount of carbon dioxide fixed in this fashion is tremendous: It is estimated that an average of approximately 191 million metric tons of carbon dioxide is fixed daily.

The flux of oxygen out of and carbon dioxide into the plant is possible only because of the opening of the stomata, which in turn is dependent on the flow of water through the plant. Studies have shown that for every kilogram of grain (such as corn) produced, as much as 600 kilograms of water will transpire through the stomata in a process called transpiration. This represents tremendous water loss and raises the question: What is the selective advantage of transpiration that outweighs its wastefulness?

The most logical explanation is that water loss by transpiration is the price plants pay to absorb carbon dioxide, essential to the life of the plant. There is the additional possibility that transpiration may serve some purpose beyond opening the stomata, such as mineral transport. Plant cell growth appears to be partially dependent on the existence of turgor pressure within the cell. Hence, the transpirational flow of water through the plant could supply the turgidity necessary for plant cell growth. Transpiration may also serve the same purpose in plants that perspiration does in humans—that is, to cool the leaf surface through water evaporation.

Gas exchange and transpiration in plants are very dynamic and interrelated processes. A thorough knowledge of both processes and of the interaction between them could one day lead to increasing maximum crop production while decreasing the amount of water required for the process.

Bibliography

Campbell, Neil A., and Jane B. Reece. Biology. 6th ed. San Francisco: Benjamin Cummings, 2002. An introductory college-level textbook for science majors. The chapter “Transport in Plants” provides a clear, concise, and somewhat detailed description of water movement through plants. The well-written text, combined with superb graphics, furnishes the reader with a clear understanding of transpiration. List of suggested readings at the end of the chapter. Includes glossary.

Hopkins, William G. Introduction to Plant Physiology. 2d ed. New York: John Wiley, 1999. Well-illustrated textbook with a practical approach to the study of plant physiology. Emphasizes the roles of plants within their environment and ecosystem. The second chapter covers plant respiration and gas exchange.

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. 6th ed. New York: W. H. Freeman/Worth, 1999. An introductory college-level textbook for science students. Chapter 23, “The Shoot: Primary Structure and Development,” provides an excellent discussion of leaf structure. The profusely illustrated text furnishes an excellent pictorial study of stomatal arrangement within the leaf. Includes glossary.

Stern, K. R. Introductory Plant Biology. 8th ed. Boston: McGraw-Hill, 2000. An introductory college-level textbook. Chapter 9, “Water in Plants: Soils,” provides a very good general discussion of the control of transpiration. Very readable text accessible to the nonscience student. Suggested readings at the end of each chapter. Includes glossary.

Willmer, Colin M., and David Fricker. Stomata. 2d ed. New York: Chapman & Hall, 1995. Upper-level science textbook on the function of stomata in gas exchange. Discusses structure and function of the stomata along with theoretical aspects of gas exchange and environmental factors affecting the process.