PHOTOSYNTHESIS

the process by which green plants and certain other organisms transform light energy into chemical energy. During photosynthesis in green plants, light energy is captured and used to convert water, carbon dioxide, and minerals into oxygen and energy-rich organic compounds. It would be impossible to overestimate the importance of photosynthesis in the maintenance of life on Earth. If photosynthesis ceased, there would soon be little food or other organic matter on Earth. Most organisms would disappear, and in time the Earth's atmosphere would become nearly devoid of gaseous oxygen. The only organisms able to exist under such conditions would be the chemosynthetic bacteria, which can utilize the chemical energy of certain inorganic compounds and thus are not dependent on the conversion of light energy. Photosynthesis also is responsible for the "fossil fuels" (i.e., coal, oil, and gas) that power industrial society. In past ages, green plants and small organisms that fed on plants increased faster than they were consumed, and their remains were deposited in the Earth's crust by sedimentation and other geological processes. There, protected from oxidation, these organic remains were slowly converted to fossil fuels. These fuels not only provide much of the energy used in factories, homes, and transportation, but they also serve as the raw material for plastics and other synthetic products. Unfortunately, modern civilization is using up in a few centuries the excess of photosynthetic production accumulated over millions of years. Requirements for food, materials, and energy in a world where human population is rapidly growing have created a need to increase both the amount of photosynthesis and the efficiency of converting photosynthetic output into products useful to people. One response to these needs-the so-called "Green Revolution"-has achieved enormous improvements in agricultural yield through the use of chemical fertilizers, pest and plant disease control, plant breeding, and mechanized tilling, harvesting, and crop processing. This effort has limited severe famines to a few areas of the world despite rapid population growth, but it has not eliminated widespread malnutrition. A second agricultural revolution, based on plant genetic engineering, may lead to increases in plant productivity and thereby partially alleviate malnutrition. Since the 1970s, molecular biologists have possessed the means to manipulate a plant's genetic material (DNA) to achieve improvements in disease and drought resistance, product yield and quality, frost hardiness, and other desirable properties. In the future, such genetic engineering may result in improvements in the process of photosynthesis. the process by which green plants and certain other organisms transform light energy into chemical energy. Photosynthesis in green plants harnesses the energy of sunlight to convert carbon dioxide, water, and minerals into organic compounds and gaseous oxygen. In addition to the green plants, photosynthetic organisms include certain protists (such as euglenoids and diatoms), cyanophytes (blue-green algae), and various bacteria. The process in photosynthetic protists and cyanophytes resembles that in green plants; it differs in the photosynthetic bacteria in that compounds other than water serve as a reactant and oxygen is not produced. All photosynthetic organisms-with the exception of a minor group of bacteria, the halobacteria-contain the light-absorbing pigment chlorophyll, which plays a key role in the transfer of energy from light to chemical compounds. Photosynthesis is the fundamental process that maintains life on Earth. Living cells convert food into energy and structural components. Almost all organisms derive this food, directly or indirectly, from the organic compounds formed within plants during photosynthesis. The stored energy in these compounds is essential for growth, repair, reproduction, movement, and other vital functions. Without photosynthesis, not only would replenishment of the fundamental food supply halt but the Earth would eventually become devoid of oxygen. Just as the organic molecules in the bodies of living organisms contain energy converted by photosynthesis from the energy of the Sun, so do the molecules of fossil fuels. The energy provided by coal, oil, and gas comes from photosynthesis carried on by plants of earlier times and preserved down through the ages, to be released by combustion in modern industrial processes. Most of the energy released both by the burning of fossil fuels and by the metabolism of living cells is given off as heat and must be replaced by the continued input of radiant energy from the Sun. The principal organic products of plant photosynthesis are carbohydrates. Formation of the simple carbohydrate glucose is shown by the equation The molecules of glucose produced are usually linked with other molecules to form more complex carbohydrates. Other products of photosynthesis are formed by incorporating mineral elements into the process. The energy required to break the chemical bonds in the reactants and to create new bonds in the products is provided by light. The excess energy not used up in the chemical reactions is stored as chemical energy in the organic products formed. The rate of photosynthesis is dependent on the following environmental factors: light intensity, temperature, and the availability of carbon dioxide, water, and certain minerals. A shortage of any one of these factors can limit the rate of photosynthesis, and an increase in the particular rate-limiting factor will, up to a point, speed up the process. The rate also varies with the plant species and its physiological state. Photosynthesis is not a single process but consists of a number of photochemical and enzymatic reactions. In green plants, the intricate apparatus required for such complex processes is located in the chloroplasts, the cell organelles that contain the chlorophyll. The chloroplasts are crowded with multiple layers of membranes, the lamellae, composed of proteins and lipids. The protein matter includes some of the enzymes and coenzymes used in the photosynthetic process; the lipid portion contains two types of chlorophyll, along with other pigments that assist in absorbing light energy. The unique feature of photosynthesis consists in the coupling of enzymatic reactions to photochemical systems that can trap electrons set free by the impact of light. In essence, photosynthesis is an oxidation-reduction reaction between carbon dioxide and water, in which carbon dioxide is reduced and water oxidized when hydrogen atoms are transferred from the water to the carbon dioxide; light is the energy source that impels the reaction. Subreactions that form part of this complex process include photolysis (splitting by light) of water, photophosphorylation, and carbon dioxide fixation. The reactions of photosynthesis include a photochemical, or light-dependent, stage and an enzymatic, or dark, stage that involves enzymatic reactions. During the light stage the chlorophyll absorbs light energy, which excites some electrons in the pigment molecules to higher energy levels. These high-energy electrons leave the chlorophyll and are transferred along through a series of molecules, called electron carriers, that can gain electrons (reduction) or lose them (oxidation). During the tandem operation of two different systems of light reactions, electrons are split off from water by photolysis, thus oxidizing it; oxygen gas is released; energy is stored in the compound ATP (adenosine triphosphate) by photophosphorylation and the coenzyme NADP+ (nicotine adenine dinucleotide phosphate) is reduced to NADPH. During the carbon-reduction cycle, occurring in the dark stage of photosynthesis, carbon dioxide is fixed, reduced, and used to synthesize carbohydrates and other organic compounds. The cycle involves formation of intermediate compounds called sugar phosphates. The compound RuBP (ribulose-l,5-bisphosphate) combines with the carbon dioxide to form PGA (phosphoglycerate). The carbon is then reduced by further enzymatic reactions involving the energy contained in the ATP and NADPH that were formed during the light reactions. The sugars produced by these reactions are used to synthesize higher carbohydrates, proteins, and fats-the plant foodstuffs that are the end products of photosynthesis. Regulatory enzymes control the rate at which the various steps in the photosynthetic process take place. The manner in which plants convert light energy into chemical energy began to be understood somewhat in the 19th century. Investigation of the mechanism of photosynthesis is still going on, aided by the use of radioactive isotopes and fluorescence techniques and by the study of photosynthetic bacteria and algae. Additional reading Photosynthesis is discussed in Yash Pal Abrol, Prasanna Mohanty, and Govindjee (eds.), Photosynthesis: Photoreactions to Plant Productivity (1993); Frank B. Salisbury and Cleon W. Ross, Plant Physiology, 4th ed. (1992), chapters 10 and 11; R.P.F. Gregory, Biochemistry of Photosynthesis, 3rd ed. (1989); and D.O. Hall and K.K. Rao, Photosynthesis, 4th ed. (1987). Jerome A. Schiff (ed.), On the Origins of Chloroplasts (1982), studies the possible bacterial origins of chloroplasts; and Ralph A. Lewin (ed.), Origins of Plastids (1993), examines current research on chloroplast origins. Roderick K. Clayton, Photosynthesis: Physical Mechanisms and Chemical Patterns (1980), is a comprehensive review of research done during the 1970s, with ample discussion of background information. J. Kenneth Hoober, Chloroplasts (1984), is a concise, readable monograph on the structure and function of the photosynthetic organelle. James Alan Bassham