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m j..... !:- .~--) c.J ,-~ ""'\ -- the leaves, provide the raw materials for synthetic operations. Sunlight, transformed into chemical bond energy and reducing power through the light reactions of photosynthesis, provides operating power. In combination, these provide all the essential components necessary for maintenance, growth. and reproduction of a plant, The plant in Figure 4 is represented vertically as composed of ~oots, stem, leaves, and their associated stomata. Leaves also contain the chloroplasts, or cellular components responsible for photosynthesis, The stomata are small, nUme~ous pores within the leaf exterior through which gas exchange occurs, Photosynthesis p~ovides the energy and reducing power required to transform carbon dioxide into glucose and fructose, the basic starting points for plant metabolism. Once transported through the plant system, these sugars provide the organic substrates and reaction energy necessary to carryon all required metabolic functions. Photosynthesis also provides energy-rich compounds that are required to actively transport selected components across semipermeable membranes, creating osmotic gradients. When salts, such as potassium, are actively and p~eferentially moved into intracellular compartments permeable to water, water flows into those compartments in an attempt to equalize cross-membrane concentrations of solute and water, If the compartments are confined by some resistant structure, such as a cell wall, pressure builds up within the cell until the pressure exerted equals the p~essure potential applied by the water, The resulting turgor pressure provides ridigity to the plant; asymet~ical turgor pressures a~e responsible for plant bending. Turgor pressure thus functions in structure and orientation, Osmotic pressu~e is responsible for many intra- and intercellular exchanges of material, These phenomena combine to regulate the rate of transpiration in plant systems, The external openings of the stomata are surrounded by specialized guard cells. In the presence of light, chloroplasts within these cells produce the chemical ene~gy necessary to selectively absorb potassium from outside the cell, thereby increasing the osmolarity of the guard cell cytoplasm. Water enters the cell. inc~easing pressure against an unevenly elastic cell wall, This results in increased turgor pressure as the cell pushes against the cell wall, leading to a separation of adjacent guard cells in areas of cell wall inelasticity. This. in turn, exposes the stomatal chambe~ and membrane surfaces, across which gaseous exchanges occur, Water vapors exit the chambe~ in ~esponse to the water vapo~ partial pressure differential, and carbon dioxide likewise enters. The significance of this is that transpiration occurs only during the day when the guard cells are open, This process takes place at the expense of a small amount of photosynthetic energy. Further, the active transport of salts requires energy 13.