Oxygenic photosynthesis

The net overall chemical reaction of oxygenic photosynthesis (by plants, algae, and cyanobacteria) is shown in reaction (1):

(1)

where {CH₂O} stands for a carbohydrate (sugar). The photochemical reaction in photosynthesis belongs to the type known as oxidation-reduction, with carbon dioxide (CO₂) acting as the oxidant (electron acceptor) and water acting as the reductant (electron donor). The unique characteristic of this particular oxidation-reduction is that it is energetically unfavorable; that is, it converts chemically stable materials into chemically unstable products. Light energy is used to make this “uphill” reaction possible (Fig. 2). A considerable part of the light energy used for this process is stored as chemical energy. See also: Carbon dioxide; Oxidation-reduction

Temporal phases of photosynthesis

Photosynthesis is a complex multistage process that consists of nearly a hundred physical processes and chemical reactions. To make this complex process more understandable, it is useful to divide it into four temporal stages. Each phase is based roughly on the time scale in which it occurs. These phases are (1) photon absorption and energy transfer processes in antennas (or antenna chlorophylls, that is, molecules that collect light quanta); (2) primary electron transfer in photochemical reaction centers; (3) electron transport and adenosine triphosphate (ATP) formation; and (4) carbon fixation and export of stable products (Fig. 2). See also: Adenosine triphosphate (ATP)

Sites of photosynthesis

The photosynthetic process in plant cells and algae takes place within pigment-bearing subcellular organelles called chloroplasts (which are cell plastids) [Fig. 3]. In the leaves of the higher terrestrial plants, these organelles are usually flat ellipsoids measuring about 5 μm in diameter and 2.3 μm in thickness. Ten to 100 of them may be present in the average parenchyma cell of a leaf. Under the electron microscope, all chloroplasts show a layered structure with alternating lighter and darker layers of roughly 0.01 μm in thickness. These layers are membranes, called thylakoid membranes (thylakoid stands for a membrane sac), which contain proteins. These proteins bind all of the chlorophyll. The thylakoid membranes are the sites of the first three phases of photosynthesis. In algae, the number and shape of the chloroplasts are much more variable. See also: Cell plastids; Leaf; Parenchyma; Plant anatomy

Fig. 3 A typical chloroplast: (a) schematic diagram of a higher plant chloroplast; (b) electron micrograph of a chloroplast. (Credit: Based on original images provided by L. Taiz and E. Zeiger)

The photochemical apparatus is less complex in cyanobacteria. These cells are prokaryotes and therefore lack a nucleus and other organelles, including chloroplasts and mitochondria. The early phases of photosynthesis take place on thylakoid membranes, which extend throughout the interior of the cell. See also: Prokaryote

Two photosystems

Two photochemical events cooperate to carry out oxygenic photosynthesis (Fig. 2). Experiments suggest that plants contained two pigment systems. One (called photosystem I, or PSI; sensitizing reaction I) is primarily composed of chlorophyll a; the other (called photosystem II, or PSII; sensitizing reaction II) is also composed of chlorophyll a, but includes most of chlorophyll b or other auxiliary pigments (including the carotenoids and the phycobilins). Efficient photosynthesis requires the absorption of an equal number of quanta in PSI and in PSII. This ensures that the excitation energy within both systems is absorbed by the antenna system and partitioned to each photosystem, where the energy drives the chemical reactions. The PSII reaction is the one most closely associated with O₂ evolution. The final result of this set of reactions is the oxidation of water to O₂ and the reduction of a plastoquinone (an oxidation-reduction catalyst). Current evidence suggests that light absorbed by the major part of the accessory pigments is ultimately transferred to a special chlorophyll a molecule in the PSII reaction center, which is in a favorable position to act as an energy trap.

The two photochemical events (PSI and PSII) take place on four large protein complexes embedded in the thylakoid membrane (Fig. 4). Note that the membrane has an inherent asymmetry in that the protein complexes are oriented in a particular way in the membrane. This orientation is essential to the proper functioning of photosynthesis.

Fig. 4 A diagram of the four major protein complexes of the thylakoid membrane. (Copyright © McGraw-Hill Education)

Figure 5 shows the Z scheme, which delineates the way in which the two photosystems cooperate to carry out the electron transfer reactions involved in photosynthesis. It is an energetic diagram, in that the energy of the component, which is measured as the midpoint redox potential E_m , is shown on the y axis and the progress of the reaction is shown on the x axis. The two vertical arrows in the diagram represent energy input to the system due to photon absorption. See also: Photon

Fig. 5 The Z scheme of photosynthesis, representing electron flow and chemical reactions in the two light steps in photosynthesis. Y_z (a tyrosine residue, also referred to as Z) is the electron donor to the oxidized chlorophyll a P680 of PSII, and Mn (a manganese-containing protein) is the charge accumulator that leads to O₂ evolution. Pheo (pheophytin) and A₀ (a type of chlorophyll a) are the primary electron acceptors of PSII and PSI, respectively. Q_A and F_X (an iron-sulfur center) are the stable electron acceptors of PSII and PSI, respectively. Q_B is another bound plastoquinone, whereas PQ is plastoquinone, Cyt is cytochrome, and PC is plastocyanin. A₁ is an electron acceptor of PSI, F_A and F_B are iron-sulfur centers, F_d is ferredoxin, and FNR is ferredoxin NADP⁺ reductase. The reduction-oxidation (redox) potential E_m is shown at the left on the y axis (given in units of eV). (Copyright © McGraw-Hill Education)

Photophosphorylation

When illuminated in the presence of adenosine diphosphate (ADP) and inorganic phosphate (P_i), the inner cytoplasmic membranes from photosynthetic bacteria, cyanobacterial cells, and chloroplasts from green plants and algae use light energy to synthesize adenosine triphosphate (ATP). About 42 kilojoules (kJ) of converted light energy in this reaction is stored in each mole of the high-energy phosphate, ATP. This photophosphorylation is coupled with energy-releasing steps in photosynthesis, such as the electron flow from PSII to PSI. When phosphorylation is associated with noncyclic electron flow from H₂O to NADP⁺, it is called noncyclic photophosphorylation. In addition, under certain conditions, electrons in PSI, instead of going to NADP⁺, may return to an intermediate (such as a cytochrome, plastoquinone, or plastocyanin) and thus close the cycle. This type of cyclic electron flow, mediated by added cofactors as well as ADP and inorganic phosphate, also leads to the production of ATP and has been termed cyclic phosphorylation; it has been shown to exist under certain experimental conditions in vivo. See also: Nicotinamide adenine dinucleotide (NAD)

Accessory antenna pigments

Besides chlorophyll a (which is present in nearly all oxygenic photosynthetic organisms), there are other chlorophylls, including chlorophyll b in the green algae and higher plants. In addition, chlorophyll c replaces chlorophyll b in brown algae, whereas most of the chlorophyll a in the marine cyanobacterium Acaryochloris marina is replaced by chlorophyll d. There are also nonchlorophyllous pigments belonging to two groups: the carotenoids and the phycobilins. The carotenoids (named because of their similarity to the orange pigment of carrots) are a variable assortment of pigments found in all photosynthetic higher plants and algae. The phycobilins, or vegetable bile pigments, are chemically related to animal bile pigments. They are either red (phycoerythrin) or blue (phycocyanin). All of these pigments are associated with specific proteins to form so-called antenna pigment proteins. The structures of many of these antenna complexes are known, and their absorption spectra have been analyzed (Fig. 6). See also: Carotenoids; Phycobilin

Fig. 6 Absorption spectra of three photosynthetic pigments: chlorophyll a, chlorophyll b, and the carotenoids. (Copyright © McGraw-Hill Education)

Carbon dioxide fixation

The light-dependent conversion of radiant energy into chemical energy as adenosine triphosphate (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 (for example, glucose polymers or sucrose) and form the base for the nutrition of all living things, as well as serving as the starting material for fuel, fiber, animal feed, oil, and other compounds used by humans. Collectively, the biochemical processes by which CO₂ is assimilated into organic molecules are known as the photosynthetic dark reactions, which are so named because light is not required (in contrast to the photosynthetic light reactions). Notably, CO₂ fixation by photosynthetic organisms is an important mechanism by which this “greenhouse” gaseous molecule is removed from the atmosphere during carbon cycling on Earth. Approximately 100 pentagrams of carbon (1 pentagram equals 10⁹ metric tons) as CO₂ is assimilated annually into organic molecules by photosynthesis (about half of this amount is assimilated by photosynthetic marine algae).

C₃ photosynthesis

The essential details of C₃ photosynthesis can be seen in Figure 7. The entire cycle can be separated into three phases—carboxylation, reduction, and regeneration. For the purposes of understanding, it is easiest to start with three molecules of CO₂ because the smallest intermediate in the cycle consists of three carbons. During the initial carboxylation phase, the three molecules of CO₂ are combined with three molecules of the five-carbon compound ribulose 1,5-bisphosphate (RuBP) in a reaction catalyzed by the enzyme RuBP carboxylase/oxygenase (Rubisco) to form three molecules of an intermediate, unstable enzyme-bound six-carbon compound. These unstable molecules are hydrolyzed further into six molecules of the three-carbon compound phosphoglyceric acid (PGA). These products of the carboxylation phase, that is, the six (three-carbon) PGA molecules, are phosphorylated by six molecules of ATP (releasing ADP to be used for photophosphorylation via the light reactions) to form six 1,3-bisphosphoglycerate (1,3-BP) molecules. The resulting compounds are reduced (that is, in the reduction phase of the C₃ cycle) by the NADPH formed in the photosynthetic light reactions to form six molecules of the three-carbon compound phosphoglyceraldehyde (PGAL). PGAL is isomerized to form another three-carbon compound, dihydroxyacetone phosphate (DHAP). PGAL (the aldehyde) and DHAP (the ketone) are energetically equivalent, reduced compounds and can be considered to be the products of the reductive phase of the C₃ photosynthetic cycle. PGAL and DHAP together form the triose phosphate (TP) pool of the chloroplast. The chloroplast TP pool is primarily composed of PGAL (the isomerase responsible for PGAL:DHAP interconversion favors PGAL formation).

Fig. 7 Schematic outline of the C₃ (Calvin-Benson-Bassham) carbon dioxide assimilation cycle (including the three phases of carboxylation, reduction, and regeneration), showing the partitioning of assimilated carbon into starch within the chloroplast [via the phosphorylated 6-C intermediates, fructose-6-phosphate (F6P) and glucose-6-phosphate (G6P)], and the assimilate efflux [through the three-carbon triose phosphate (TP) transporter] across the chloroplast inner envelope to the cytoplasm (in exchange for inorganic phosphate) leading to sucrose synthesis. For the reactions of the C₃ cycle, shown in the chloroplast, the relative numbers of molecules involved in each specific step are shown to the left of the substrate and/or product. (Copyright © McGraw-Hill Education)

The rest of the C₃ photosynthetic cycle (the regeneration phase) involves enzymatic steps that allow regeneration of RuBP, the initial carboxylation substrate. One molecule of PGAL is made available for combination with DHAP isomerized from a second PGAL (requiring a second “turn” of the Calvin-Benson-Bassham 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 continue the cycle.

It should be noted that the enzyme RuBP carboxylase/oxygenase (Rubisco) [Fig. 8] that incorporates CO₂ into an organic compound also allows O₂ to react with RuBP—hence the "oxygenase" in the name. This reaction initiates the process called photorespiration, which results in the release of one previously incorporated molecule of CO₂ for every two molecules of O₂ that are allowed to react. Due to its low catalytic efficiency, Rubisco can be up to half of the soluble protein in C₃ chloroplasts, and it is likely the most abundant protein found in nature. Structurally, Rubisco is a large and complex enzyme, comprising eight large polypeptide subunits and eight small subunits. See also: Enzyme; Photorespiration

Fig. 8 Colorized structural model of Rubisco. (Copyright © McGraw-Hill Education)

The net product of two “turns” of the cycle, that is, a six-carbon sugar (G6P or F6P), 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). This partitioning of newly formed photosynthate leads to two distinct pools; starch is stored in the photosynthesizing “source” leaf cells, and sucrose is available either for immediate metabolic requirements within the cell or for export to “sinks”, including developing reproductive structures, roots, or other leaves. Factors within the photosynthesizing cell, such as energy requirements in different compartments (mitochondria, cytoplasm, and chloroplasts), along with energy needs of the plant (for example, increased sink requirements during different developmental stages), and external environmental factors (for example, light intensity and duration) ultimately regulate the partitioning of the newly formed photosynthetic product (PGAL) into starch or sucrose. See also: Plant metabolism

C₄ photosynthesis

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). Further work demonstrated that plants having the Kranz anatomy utilized an additional CO₂ assimilation route, which is now known as the C₄-dicarboxylic acid pathway (Fig. 9). Carbon dioxide enters a mesophyll cell, where it is combined (in the form of bicarbonate) with the three-carbon compound phosphoenolpyruvate (PEP) via the enzyme PEP carboxylase to form a four-carbon acid, oxaloacetate, 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 and the CO₂ reassimilated via the C₃ cycle. To complete the cycle, the resulting three-carbon compound, pyruvic acid, moves back into the mesophyll cell and is transformed into PEP (at the cost of 2 ATP molecules) via the enzyme pyruvate phosphate dikinase located in the mesophyll chloroplasts. The net effect of this cycle is to increase the CO₂ concentration around Rubisco, thereby reducing photorespiration via the competing oxygenase activity of this enzyme.

Fig. 9 Schematic outline of the C₄ carbon dioxide assimilation process in the mesophyll cells and bundle sheath cells of a type-1 NADP-ME plant. (Copyright © McGraw-Hill Education)

C₄ metabolism is classified into three types, depending on the primary decarboxylation reaction used with the four-carbon acid in the bundle sheath cells. The majority of C₄ species (exemplified by sugarcane, maize, crabgrass, and sorghum) belong to type 1 and employ NADP-malic enzyme (NADP-ME) for decarboxylation. NAD-malic enzyme (NAD-ME) C₄ plants belong to type 2 and include Amaranthus, Atriplex, millet, pigweed, and purslane; in contrast, C₄ plants categorized as type 3 use phosphoenolpyruvate carboxykinase (PCK) for decarboxylation and include Panicum grasses.

Crassulacean acid metabolism (CAM) photosynthesis

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; however, the uptake of CO₂, which occurs entirely through the stomata, is prevented. Therefore, many desert plants (including those in the Crassulaceae, Cactaceae, and Euphorbiaceae families) have evolved, apparently independently of C₄ plants, a similar strategy of concentrating and assimilating CO₂ by which the CO₂ is taken in at night when the stomata open; in general, water loss is low because of the reduced temperatures and correspondingly higher humidities. The biochemical understanding of the mechanisms involved in this process was first studied in plants of the Crassulaceae family; thus, the process has been called crassulacean acid metabolism (CAM). See also: Plant-water relations

In contrast to C₄ photosynthesis, where two cell types usually cooperate, the entire CAM 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) [Fig. 10].

Fig. 10 Scheme for the flow of CO₂ within a single crassulacean acid metabolism (CAM) cell over a day, showing the initial dark CO₂ fixation, malic acid storage in the vacuole at night, decarboxylation, and the C₃ cycle occurring the next day. (Copyright © McGraw-Hill Education)

Bacterial photosynthesis

Certain bacteria have the ability to perform photosynthesis. A general equation for bacterial photosynthesis is shown in reaction (2):

(2)

where A represents any of a number of reductants, most commonly S (sulfur). Because photosynthetic bacteria cannot use water as the hydrogen donor, they are incapable of evolving oxygen. They are therefore called anoxygenic photosynthetic bacteria. (Note that the prokaryotic cyanobacteria are excluded because their photosynthetic system closely resembles that found in eukaryotic algae and higher plants.) Anoxygenic photosynthetic bacteria can be classified in four major groups: (1) Proteobacteria, including nonsulfur purple bacteria (Rhodospirillaceae) and sulfur purple bacteria (Chromatiaceae); (2) green sulfur bacteria (Chlorobiaceae); (3) green gliding bacteria (Chloroflexi); and (4) Heliobacteria (Heliobacteriaceae). Like plants, algae, and cyanobacteria, anoxygenic photosynthetic bacteria are capable of photophosphorylation, which is the production of adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and inorganic phosphate (P_i) using light as the primary energy source.

Photosynthetic bacteria do not have specialized organelles such as the chloroplasts of green plants. Electron micrographs of certain photosynthetic bacteria show tiny spherical sacs, with double-layered walls, as a result of invaginations that form stacks of membranes. Other photosynthetic bacteria have invaginations that form thylakoids. These intracytoplasmic membranes, often called chromatophores, contain the photosynthetic apparatus and can be isolated easily by mechanical disruption of bacteria followed by differential centrifugation. Isolated chromatophores are often used for biochemical and biophysical studies of bacterial photosynthesis. See also: Chromatophore

The pigment bacteriochlorophyll (BChl) is a necessary component for bacterial photosynthesis. There are specialized BChl molecules in bacteria that engage in the primary chemical reactions of photosynthesis. In addition to these specialized molecules, there are 40–50 BChl molecules referred to as antenna pigments, whose sole function is to harvest light energy and transfer it to reaction center molecules. This is similar to the photosynthetic unit of plants, algae, and cyanobacteria. Each reaction center contains a special pair (dimer) of BChl molecules that engage in chemical reactions after they trap the absorbed light energy. They are also called the energy traps of bacterial photosynthesis. See also: Bacterial physiology and metabolism

Govindjee

Robert E. Blankenship

Gerald A. Berkowitz

Archie R. Portis, Jr.

R. J. Shopes