Photosynthesis is the biological process by which the energy of sunlight is absorbed and used to power the formation of organic compounds from carbon dioxide and water. Although primarily associated with green plants, photosynthesis also occurs in algae and a limited number of bacteria. This process ultimately supplies the energy required by all living organisms for their continued survival. THE PHOTOSYNTHETIC PROCESS The photosynthetic process is divided into two stages: absorption of light energy and carbon fixation. Light Absorption Chlorophyll-pigment molecules absorb, or trap, light energy from the Sun. The electron-transport system of photosynthetic cells subsequently converts this absorbed energy into a biologically useful form in two high-energy molecules, \TATP\t (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate). Carbon Fixation Carbon fixation, or carbon assimilation, involves using the energy contained in the \TATP\t and NADPH to drive reactions that form organic compounds, such as starch and sugars, from the simple molecules of carbon dioxide and water. The organic compounds formed sustain all life either directly, in the case of the photosynthetic organism itself or any organism that consumes it, or indirectly, in the case of larger animals that occupy the higher positions in various food chains. Splitting of Water One important outcome of the photosynthetic reaction is that light energy splits the water molecule so that one of its component atoms, oxygen, is released as molecular oxygen and serves to replenish the atmospheric oxygen supply, which would otherwise be depleted rapidly by respiration processes of organisms and by burning substances. Photosynthetic processes also result in consumption of the carbon dioxide produced by respiration. These two reactions are summarized as follows: respiration: organic compounds + oxygen --> carbon dioxide + water photosynthesis: carbon dioxide + water --> organic compounds + oxygen Thus, photosynthetic processes are basically the opposite of those of respiration. HISTORY Early Theories The Greek philosopher \TAristotle\t may have been the first to attempt to explain the processes of photosynthesis and food production. He believed that plants could obtain from the soil all the components that they require for growth. The cycle was complete when organisms perished and became reincorporated into the soil. This view was not seriously challenged until the 17th century with the experiments of Johannes Baptista van \THelmont\t, a Belgian physician. He carefully measured the weight increase of a willow planted in soil, to which he periodically added only rainwater. The plant increased in weight by 77 kg (169 lb), and the soil decreased in weight by 57 gm (2 oz). He deduced that it was water, and not substances in the soil, that provided plants with their growth material. Later, in the early 18th century, Stephen Hales conjectured that light and air might be significant factors in the growth of plants. The classical experiments (1771) of Joseph \TPriestley\t laid the foundations of the modern photosynthetic theory. He found that the composition of air inside a closed glass container changed after a candle had burned in it or after a small animal had breathed in it. This air then was unable to support further burning or breathing. However, this "fixed air" contained "phlogistic matter"--later found to be carbon dioxide--that could be "dephlogisticated" by plants. Although his interpretation was wrong, Priestley nevertheless had discovered that plants use a component of the atmosphere in their life processes. Jan Ingen-Housz in 1779 and Jean Senebier in 1796 refined Priestley's ideas. They observed that plants could restore the "fixed air" only in the presence of light; the plants made the air noxious if kept in darkness. The role of light in photosynthesis thus was firmly established. Antoine Laurent \TLavoisier\t had previously described the chemical composition of air; his findings led Ingen-Housz to recognize that plants utilize carbon dioxide to obtain carbon in order to build organic molecules, and that they release oxygen into the atmosphere. That plants need water in the process was demonstrated by careful quantitative experiments of Nicholas Theodore De Saussure. With the formulation of the theory of energy conservation in 1845 by Julius Robert Mayer, the function of light as the energy source for the photosynthetic process began to be understood. Elucidation of the Photosynthetic Process Once the basic elements of photosynthesis were defined, research was begun on the details of the process. George G. Stokes and Henry C. Sorby described the chemical structure of the light-absorbing pigment chlorophyll. By 1913, Richard Willstatter and A. Stoll had published the empirical formulas of chlorophylls a and b; they went on to hypothesize that chlorophyll combines with carbon dioxide and water and that light decomposes the complex into oxygen and formaldehyde. This theory was superseded in the 1920s, when Otto H. Warburg and Warbus Negelein demonstrated that photosynthesis consists of several distinct steps and that a variety of molecules are involved in the photosynthetic reaction. The nature of biological electron-transport molecules in many systems began to be elucidated in the 1930s. In 1937, Robert Hill showed that extracted chloroplasts still could release oxygen from water in the presence of a suitable hydrogen acceptor; as a result of this experiment, he established the concept of a light-activated electron-transporting photosynthetic chain that releases oxygen from water. Since Hill's research, a tremendous amount of work has been done to elaborate on this basic scheme. It is clear now that two light reactions in higher plant photosynthesis exist and act in series. Both involve chlorophyll in a reaction center. When the first of these reaction centers absorbs a light quantum, the chlorophyll becomes oxidized and is capable, in turn, of oxidizing water by removing hydrogen atoms and releasing oxygen. The electron removed from the chlorophyll during this first light reaction passes down a chain of electron-transport proteins to the second reaction center chlorophyll, at which site absorption of the second light quantum causes the electron to leave the chlorophyll and eventually reduce NADP to its energy rich form, NADPH. In addition, \TATP\t is produced during the electron-transport process and is used, together with the NADPH, to drive a complex series of enzyme-catalyzed reactions that incorporate the carbon of carbon dioxide into various complex organic products. SITE OF PHOTOSYNTHESIS IN PLANTS Chloroplast Chloroplasts, in which the entire process of photosynthesis occurs, are the tiny green organelles inside the cells of green plant tissue. They were first observed by Hugo von Mohl in 1837, but their role in photosynthesis was not fully understood until the experiments of Julius von Sachs nearly 30 years later, which showed that starch was produced only in those chloroplasts which were exposed to light. Chloroplasts generally are 4 to 6 microns in length, 1 to 2 microns in width, and somewhat discoid or ellipsoid. The chloroplast envelope that surrounds them is composed of two lipoprotein membranes, each 60 angstroms thick. Inside this envelope is a complex array of membranes (lamellae) in a granular fluid known as the stroma. The lamellae are paired to form disklike structures called thylakoids, which tend to be stacked to form ordered structures termed grana. The grana are connected sporadically by unstacked thylakoids. Molecules involved in light absorption and the formation of \TATP\t and NADPH are membrane-bound and associated with the thylakoids; those molecules associated with carbon fixation are located in the stroma. Electron microscopic techniques have revealed two different types of particles within the thylakoid membranes, and presumably these particles contain the chlorophyll and proteins of the electron-transport system. Chloroplasts also produce protein molecules by means of their own \TDNA\t and ribosome complement, which provide the necessary genetic information. Chlorophyll The major light receptors in higher plants are the chlorophyll molecules. These molecules are similar in structure to the heme molecule of red blood cells, having a porphyrin ring to which an atom of magnesium is attached. They are green because they have very strong red and blue absorption bands in the visible region of the light spectrum where solar energy has its maximum output. The cells do not absorb green bands well, which therefore are reflected. Two types, chlorophylls a and b, have slightly different absorption bands and allow a greater portion of the visible light quanta to be absorbed. Other red, yellow, and orange pigments, termed carotenoids and xanthophylls, aid in the absorption of light energy that falls between the bands of the chlorophylls. Carotenoids and xanthophylls pass the absorbed light energy to chlorophylls for conversion to chemical energy. Photosystems I and II The pigments are arranged into large units, each containing as many as 600 chlorophyll molecules. In each unit, however, only one chemically reactive chlorophyll is present, termed the reaction center chlorophyll. The remainder are termed antennae pigments. The two light reactions of higher plant photosynthesis are independent units, the end products of the first utilized by the second. The first is carried out by photosystem II, the reaction center chlorophyll being termed P 680 (680 nanometer light-absorbing pigment), and the second is carried out by photosystem I, the reaction center chlorophyll of which is termed P 700 (700 nm light-absorbing pigment). MECHANISM OF TRAPPING LIGHT Light is absorbed by a pigment of a photosynthetic unit and is transferred extremely rapidly, with the loss of very little energy, to the reaction center chlorophyll by a process called resonance transfer. The absorbed energy then causes an electron from the reaction center chlorophyll to leave the chlorophyll molecule--that is, the chlorophyll becomes oxidized--and to be transferred to an electron-accepting species, which becomes reduced. This electron transfer occurs against a chemical potential gradient and requires an external source of energy, in this case the light energy, to drive it. The electron transfer produces a "high-energy state," which can be used for the generation of high-energy molecules. When photosystem II absorbs a quantum of light, the immediate products are an oxidized P 680 chlorophyll and a reduced acceptor, termed the primary acceptor or sometimes Q. The oxidized P 680 is such a highly oxidizing species that it is capable of oxidizing water so that molecular oxygen is released. The reaction is catalyzed by a complex manganese-containing enzyme. The four electrons produced by this reaction are used to reduce once again the oxidized P 680. Each electron, residing on the reduced primary acceptor after the light reaction, is passed along a chain of biological electron carriers and arrives at the reaction center chlorophyll of photosystem I. It seems likely that at least three major components, plastoquinone, cytochrome f (a c-type cytochrome), and plastocyanin (a blue copper-containing protein), are involved in the electron-transfer sequence. As electrons pass along these components, energy is released and is used to drive the formation of \TATP\t from its precursors, ADP (adenosine diphosphate) and inorganic phosphate. The process is termed photophosphorylation and is presumably similar to the process of oxidative phosphorylation, which occurs when \TATP\t is produced during respiration in mitochondria. It seems that the electron transfer creates a gradient of hydrogen ions and a difference in potential across the membrane in which the components are located. These components subsequently are discharged and are considered speculatively to drive the formation of the \TATP\t. On reaching the reaction center chlorophyll of photosystem I, a second quantum of light is absorbed and causes the electron to reduce another electron acceptor, ferredoxin, against a chemical potential gradient. Ferredoxin is a protein whose active site contains iron and sulfur. It is an extremely powerful reducing agent once it has accepted an electron, and it is used to reduce NADP to NADPH. An alternative fate of the electron on the ferredoxin is to return to the oxidized reaction center chlorophyll of photosystem I by way of a further chain of electron-transport molecules. The details of this chain of carriers is thought to involve a b-type cytochrome together with cytochrome f and plastoquinone. Energy is also released as the electron passes along this chain and can be used to drive the formation of \TATP\t from ADP and inorganic phosphate. This is termed cyclic phosphorylation because the electron passes around a continuous loop of carriers. In noncyclic phosphorylation, on the other hand, the electron passes along the electron-transport chain from photosystem II to photosystem I. By varying the ratio of noncyclic (producing \TATP\t and NADPH) to cyclic (producing only \TATP\t) phosphorylation, the correct ratio of NADPH to \TATP\t is produced in order to drive the reactions of carbon fixation. THE PATHWAY OF CARBON FIXATION Melvin \TCalvin\t and his coworkers elucidated the pathway of carbon fixation in the late 1940s. This pathway, also known as the Calvin cycle, utilizes the \TATP\t and NADPH produced by the light reactions. The key reaction of this cycle is that of carbon fixation itself, which involves the addition of a carbon dioxide molecule to a sugar, ribulose-1,5-diphosphate (RUDP), producing two molecules of 3-phosphoglycerate. The remaining reactions of the cycle involve the regeneration of ribulose-1,5-diphosphate. For each molecule of carbon dioxide that is fixed, three molecules of \TATP\t and two molecules of NADPH are required to drive the reaction. The cycle is also known as the reductive pentose phosphate pathway because it effectively reverses the oxidative pentose phosphate pathway of metabolism by coupling certain steps to \TATP\t cleavage. The stable end products of the Calvin cycle are starch and sucrose. Starch is a polysaccharide with a chemical structure similar to that of glycogen; it acts as the main energy reserve in plant cells and also is synthesized in the chloroplasts. Sucrose, a sugar, is the main form of translocated carbohydrate in plants and is the energy source equivalent to the sugar glucose in animals. Sucrose is not synthesized in the chloroplast but in the cytoplasm. Therefore, carbon atoms to be used to manufacture sucrose must leave the chloroplast in some form; this step is achieved by a specific transporting molecule in the chloroplast membrane, which exchanges a phosphate molecule from the cytoplasm for a dihydroxyacetone phosphate molecule from the chloroplast. The key enzyme in the cycle is ribulose-1,5-diphosphate carboxylase, formerly known as carboxydismutase. Besides its normal reaction with carbon dioxide, it also can catalyze the reaction of ribulose-1,5-diphosphate with oxygen. The chloroplast is able to remove the phosphate group of the phosphoglycolate to produce glycolate but is unable to metabolize the glycolate further. The fate of this glycolate involves enzymes that occur in almost all parts of the cell--in the cytoplasm, the peroxisomes, and the mitochondria. This process, known as photorespiration, leads to carbon dioxide production and oxygen uptake. Photorespiration is particularly wasteful when the carbon dioxide level in cells is much less than in the surrounding air; for instance, when photosynthesis is occurring in high light intensities or at high temperatures, or when the plant has to restrict the opening of its stomata--the tiny pores through which carbon dioxide enters and oxygen leaves the plant--in order to restrict water loss by transpiration. SPECIAL ADAPTATIONS Many species of plants have adapted in particular ways to their environment. For instance, plants inhabiting salty or dry habitats have developed photorespiration mechanisms. Many tropical and subtropical plants expend additional photosynthetically generated \TATP\t in order to increase the carbon dioxide concentration in the chloroplasts; such plants virtually show no photorespiration. They are called C = 4 (commonly represented by a C followed by a subscript number 4) plants because the initial product of carbon dioxide fixation is a four-carbon acid, as opposed to the three-carbon acid produced in other plants. Taxonomically, C = 4 plants are very diverse but have been studied in the greatest detail in such grasses as sugarcane and maize. Several genera are known that have both C = 4 and C = 3 species. Another adaptation, associated with slow growth but very efficient in conservation of water, is crassulacean-acid metabolism (\TCam\t). Carbon dioxide is fixed into C = 4 acids during the night, and during the day the stomata are closed and light energy is used to release carbon dioxide from the C = 4 acids. Cacti and succulents, which grow in deserts or other habitats where daytime humidity is very low, utilize this type of metabolism. CURRENT RESEARCH IN PHOTOSYNTHESIS Although much has been learned about many of the steps in the photosynthetic process, large gaps in understanding the entire phenomenon still remain. For example, only a small percentage of the light energy that falls on the plant actually is involved in synthesizing useful biological end products. A very active field of investigation at present is concerned with trying to maximize the efficiency of the process in various plant species so that a greater crop yield may be obtained from a fixed amount of land. This may possibly be achieved, for example, by exploiting the enzyme commonly called RuBisCo, which catalyzes the slowest step in photosynthesis. If this enzyme can be modified using genetic engineering techniques, it may be able to accelerate the catalytic process and create faster-growing plants. At present, research is also being devoted to the development of efficient artificial photosynthesis systems. Thanks to recent advances in delineating the structure of a photosynthetic reaction center, a variety of molecular components have been synthesized for possible use in artificial photosynthesis. This research has also proved extremely useful for elucidating basic natural photosynthetic principles. In 1990 researchers were able to insert genetic material into photosynthesis structures, slightly opening the door to the possibility of manipulating photosynthetic centers to create bigger and more vigorous plants. Peter Rich Bibliography: Briggs, W. R., ed., Photosynthesis (1989); Clayton, R. K., Photosynthesis (1981); Coombs, J., and Hall, D., eds., Techniques in Bioproductivity and Photosynthesis (1981); Gregory, R. P. F., Biochemistry of Photosynthesis, 3d ed. (1988).