The manufacture in light of organic compounds (primarily certain carbohydrates) from inorganic materials by chlorophyll- or bacteriochlorophyll-containing cells. This process requires a supply of energy in the form of light. In chlorophyll-containing plant cells and in cyanobacteria, photosynthesis involves oxidation of water (H₂O) to oxygen molecules, which are released into the environment. In contrast, bacterial photosynthesis does not involve O₂ evolution—instead of H₂O, other electron donors, such as H₂S, are used. This article will focus on photosynthesis in plants. Bacterial physiology and metabolism Chlorophyll Plant respiration

The light energy absorbed by the pigments of photosynthesizing cells, especially by the pigment chlorophyll or bacteriochlorophyll, is efficiently converted into stored chemical energy. Together, the two aspects of photosynthesis—the conversion of inorganic into organic matter, and the conversion of light energy into chemical energy—make it the fundamental process of life on Earth: it is the ultimate source of all living matter and of all life energy.

The net overall chemical reaction of plant photosynthesis is shown in the equation below, where {CH₂O} stands for a carbohydrate (sugar).

The photochemical reaction in photosynthesis belongs to the type known as oxidation-reduction, with CO₂ acting as the oxidant (hydrogen or electron acceptor) and water as the reductant (hydrogen or electron donor). The unique characteristic of this particular oxidation-reduction is that it goes “in the wrong direction” energetically; that is, it converts chemically stable materials into chemically unstable products. Light energy is used to make this “uphill” reaction possible.

Photosynthesis is a complex, multistage process. Its main parts are (1) the primary photochemical process in which light energy absorbed by chlorophyll is converted into chemical energy, in the form of some energy-rich intermediate products; and (2) the enzyme-catalyzed “dark” (that is, not photochemical) reactions by which these intermediates are converted into the final products—carbohydrates and free oxygen.

Experiments suggest that plants contain two pigment systems. One (called photosystem I, or PS I, sensitizing reaction I) contains the major part of chlorophyll a; the other (called photosystem II, or PS II, sensitizing reaction II) contains some chlorophyll a and the major part of chlorophyll b or other auxiliary pigments (for example, the red and blue pigments, called phycobilins, in red and blue-green algae, and the brown pigment fucoxanthol in brown algae and diatoms). It appears that efficient photosynthesis requires the absorption of an equal number of light quanta in PS I and in PS II; and that within both systems excitation energy undergoes resonance migration from one pigment to another until it ends in special molecules of chlorophyll a called the reaction centers. The latter molecules then enter into a series of chemical reactions that result in the oxidation of water to produce O₂ and the reduction of nicotinamide adenine dinucleotide phosphate (NADP⁺). Chromatophores from photosynthetic bacteria and chloroplasts from green plants, when illuminated in the presence of adenosine diphosphate (ADP) and inorganic phosphate, also use light energy to synthesize adenosine triphosphate (ATP); this photophosphorylation could be associated with some energy-releasing step in photosynthesis.

The light-dependent conversion of radiant energy into chemical energy as ATP and reduced nicotinamide adenine dinucleotide phosphate (NADPH) serves as a prelude to the utilization of these compounds for the reductive fixation of CO₂ into organic molecules. Such molecules, broadly designated as photosynthates, are usually but not invariably in the form of carbohydrates such as glucose polymers or sucrose, and form the base for the nutrition of all living things. Collectively, the biochemical processes by which CO₂ is assimilated into organic molecules are known as the photosynthetic dark reactions, not because they must occur in darkness, but because light—in contrast to the photosynthetic light reactions—is not required.

The essential details of C₃ photosynthesis can be seen in . Three molecules of CO₂ combine with three molecules of the five-carbon compound ribulose bisphosphate (RuBP) in a reaction catalyzed by RuBP carboxylase to form three molecules of an enzyme-bound six-carbon compound. These are hydrolyzed into six molecules of the three-carbon compound phosphoglyceric acid (PGA), which are phosphorylated by the conversion of six molecules of ATP (releasing ADP for photophosphorylation via the light reactions). The resulting compounds are reduced by the NADPH formed in photosynthetic light reactions to form six molecules of the three-carbon compound phosphoglyceraldehyde (PGAL). One molecule of PGAL is made available for combination with another three-carbon compound, dihydroxyacetone phosphate, which is isomerized from a second PGAL (requiring a second “turn” of the Calvin-cycle wheel) to form a six-carbon sugar. The other five PGAL molecules, through a complex series of enzymatic reactions, are rearranged into three molecules of RuBP, which can again be carboxylated with CO₂ to start the cycle turning again. The net product of two “turns” of the cycle, a six-carbon sugar (glucose-6-phosphate) is formed either within the chloroplast in a pathway leading to starch (a polymer of many glucose molecules), or externally in the cytoplasm in a pathway leading to sucrose (condensed from two six-carbon sugars, glucose and fructose).

Initially, the C₃ cycle was thought to be the only route for CO₂ assimilation, although it was recognized by plant anatomists that some rapidly growing plants (such as maize, sugarcane, and sorghum) possessed an unusual organization of the photosynthetic tissues in their leaves (Kranz morphology). It was then demonstrated that plants having the Kranz anatomy utilized an additional CO₂ assimilation route now known as the C₄-dicarboxylic acid pathway (). Carbon dioxide enters a mesophyll cell, where it combines with the three-carbon compound phosphoenolpyruvate (PEP) to form a four-carbon acid, oxaloacetic acid, which is reduced to malic acid or transaminated to aspartic acid. The four-carbon acid moves into bundle sheath cells, where the acid is decarboxylated, the CO₂ assimilated via the C₃ cycle, and the resulting three-carbon compound, pyruvic acid, moves back into the mesophyll cell and is transformed into PEP, which can be carboxylated again. The two cell types, mesophyll and bundle sheath, are not necessarily adjacent, but in all documented cases of C₄ photosynthesis the organism had two distinct types of green cells. C₄ metabolism is classified into three types, depending on the decarboxylation reaction used with the four-carbon acid in the bundle sheath cells:

1. NADP-ME type (sorghum):

2. NAD-ME type (Atriplex species):

3. PCK type (Panicum species):

Under arid and desert conditions, where soil water is in short supply, transpiration during the day when temperatures are high and humidity is low may rapidly deplete the plant of water, leading to desiccation and death. By keeping stomata closed during the day, water can be conserved, but the uptake of CO₂, which occurs entirely through the stomata, is prevented. Desert plants in the Crassulaceae, Cactaceae, Euphorbiaceae, and 15 other families evolved, apparently independently of C₄ plants, an almost identical strategy of assimilating CO₂ by which the CO₂ is taken in at night when the stomata open; water loss is low because of the reduced temperatures and correspondingly higher humidities. First studied in plants of the Crassulaceae, the process has been called crassulacean acid metabolism (CAM).

In contrast to C₄, where two cell types cooperate, the entire process occurs within an individual cell; the separation of C₄ and C₃ is thus temporal rather than spatial. At night, CO₂ combines with PEP through the action of PEP carboxylase, resulting in the formation of oxaloacetic acid and its conversion into malic acid. The PEP is formed from starch or sugar via the glycolytic route of respiration. Thus, there is a daily reciprocal relationship between starch (a storage product of C₃ photosynthesis) and the accumulation of malic acid (the terminal product of nighttime CO₂ assimilation).