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Photosynthetic Carbon Metabolism Secondary article Article Contents . C3 Photosynthesis (Calvin Cycle) Archie R Portis, Jr, University of Illinois, Urbana, Illinois, USA . Regulation . C4 Photosynthesis Photosynthetic carbon metabolism encompasses the processes by which plants and other photosynthetic organisms use the energy captured from light in the form of NADPH and ATP to convert atmospheric carbon dioxide into carbohydrates. Carbohydrates produced by photosynthesis form the base for the nutrition of all other life as well as serving as the starting materials for fibres, oils and many other natural compounds used by people. C3 Photosynthesis (Calvin Cycle) Although studied for over a century, the details of how CO2 was converted into carbohydrates were not elucidated until the 1950s. The availability of radioisotopes (14CO2) and two-dimensional paper chromatographic techniques allowed Melvin Calvin and co-workers to map out a cyclic pathway involving a previously unknown pentose bisphosphate, ribulose 1,5-bisphosphate (Bassham, 1962). The first labelled product is a three-carbon acid and therefore the pathway is usually referred to as C3 photosynthesis to distinguish it from adaptations involving additional reactions (C4 and CAM photosynthesis) to be described later. However, it is often called either the Calvin cycle to honour Calvin, who received a Nobel prize in 1961 for the work, or the reductive pentose phosphate cycle to distinguish it from the oxidative pentose phosphate pathway, which uses many of the same enzymes. The Calvin cycle involves several metabolic intermediates and consists of a complicated series of enzymatic reactions that occur within the chloroplasts, although some enzymes have isoforms in the cytosol. It is often schematically simplified by consideration of only three stages: (1) carboxylation of the five-carbon sugar ribulose 1,5-bisphosphate with CO2 to form two molecules of 3phosphoglycerate; (2) reduction of the 3-phosphoglycerate to triose phosphate using ATP and NADPH generated through light capture by the thylakoid membranes; (3) regeneration of the starting 5-carbon sugar from triose phosphate (also requiring ATP at the final step). It is easier to grasp the cyclic nature of the process if we start with three CO2 molecules, which are converted into one triose phosphate with the following stoichiometry: 3 CO2 þ 9 ATP4 þ 6 NADPH þ 5 H2 O 3 ! triose phosphate ðC3 H5 O3 PO2 3 Þ þ 9ADP þ 2 þ þ6 NADP þ 8 HOPO3 þ 3 H The key step in C3 photosynthesis is the actual carboxylation reaction (Figure 1, Stage 1) catalysed by the enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (often called Rubisco) (E1), in which each of the three CO2 . CAM Plants molecules are combined with one ribulose bisphosphate to form a total of six molecules of 3-phosphoglycerate. The properties of this enzyme define many of the ways that plants have evolved to respond to their environment. For example, Rubisco is not very efficient in catalysing the carboxylation reaction because it has a lower maximum activity than most enzymes and the concentration of carbon dioxide in the atmosphere is less than that required for even half-maximal activity. Plants compensate by making a large amount of this one enzyme (up to 50% of their total soluble protein). A major problem that plants must contend with is the fact that Rubisco also allows Calvin Cycle Stage 2 Stage 1 (6) 3P-glycerate (3) CO2 (6) ATP (6) ADP E2 E1 (6) 1,3-BP-glycerate (6) NADPH (6) NADP E3 & Pi Triose P (3) Ribulose 1,5-BP (3) ADP E11 (5) Glyceraldehyde 3-P (3) ATP (3) Ribulose 5-P E10 E9 E4 Ribose 5-P (2) Dihydroxyacetone P (2) Xylulose 5-P E5 Fructose 1,6-BP Stage 3 E7 Sedoheptulose 7-P Pi E6 Pi Fructose 6-P E8 Sedoheptulose 1,7-BP E5 Erythrose 4-P E7 Xylulose 5-P Figure 1 Intermediates and enzymatic steps of C3 photosynthesis (Calvin cycle). The various enzymes are described in the text and have the following EC numbers E1, 4.1.1.39; E2, 2.7.2.3; E3, 1.2.1.13; E4, 5.3.1.1; E5, 4.1.2.13; E6, 3.1.3.11; E7, 2.2.1.1; E8, 3.1.3.37; E9, 5.1.31; E10, 5.3.1.6; E11, 2.7.1.19. Light-regulated enzymes are highlighted in red. Green arrow, Stage 1; blue arrows, Stage 2; grey arrows, Stage 3. ENCYCLOPEDIA OF LIFE SCIENCES / & 2002 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net 1 Photosynthetic Carbon Metabolism oxygen to react with ribulose bisphosphate, producing the two-carbon acid phosphoglycolate, in addition to 3phosphoglycerate. The phosphoglycolate product is recycled back into 3-phosphoglycerate in a process called photorespiration (Ogren, 1984). In this process one CO2 is released for every two phosphoglycolate molecules converted. Therefore, the reaction of ribulose bisphosphate with oxygen rather than CO2 and the release of CO2 during photorespiration reduce the productivity of C3 plants under normal atmospheric conditions (21% O2 and 0.036% CO2). The oxygenase reaction increases faster with temperature than does the carboxylase, but at moderate temperatures about one O2 reacts for every three to four CO2 molecules captured. Reduction of six 3-phosphoglycerate molecules to triose phosphate (Figure 1, Stage 2) requires three enzymes. Phosphoglycerate kinase (E2) converts the 3-phosphoglycerate to 1,3-bisphosphoglycerate by phosphorylation with six ATP. Then glyceraldehyde phosphate dehydrogenase (E3) converts the six 1,3-bisphosphoglycerate molecules to glyceraldehyde 3-phosphate and inorganic phosphate by reduction with six NADPH. Triose phosphate is a collective term for glyceraldehyde 3-phosphate and dihydroxyacetone phosphate, which are rapidly interconverted by the enzyme triose phosphate isomerase (E4). Regeneration (Figure 1, Stage 3) of the three original ribulose bisphosphates requires only five triose phosphates, leaving one as the net product of the three initial carboxylation reactions. The regeneration process requires seven enzymes (E5 to E11) and proceeds in the following manner. Two of the triose phosphates (one glyceraldehyde 3-phosphate and one dihydroxyacetone phosphate) are joined by aldolase (E5) to form fructose 1,6-bisphosphate. Fructose 1,6-bisphosphate is irreversibly hydrolysed to fructose 6-phosphate and inorganic phosphate by fructose 1,6-bisphosphatase (E6). The enzyme transketolase (E7) transfers a two-carbon group from fructose 6-phosphate to the third triose phosphate (as glyceraldehyde 3-phosphate) to form erythrose 4-phosphate and xylulose 5-phosphate. Aldolase (E5) is also able to join the erthyrose 4-phosphate with the fourth triose phosphate (as dihydroxyacetone phosphate) to form sedoheptulose 1,7-bisphosphate, which is irreversibly hydrolysed to sedoheptulose 7phosphate and inorganic phosphate by sedoheptulose 1,7-bisphosphatase (E8). The enzyme transketolase (E7) can also transfer a two-carbon group from sedoheptulose 7-phosphate to the fifth and last triose phosphate (as dihydroxyacetone phosphate) to form ribose 5-phosphate and xylulose 5-phosphate. The three pentose phosphates are then converted to ribulose 5-phosphate. The two xylulose 5-phosphates are converted with ribulose phosphate epimerase (E9) and the one ribose 5-phosphate is converted with ribose phosphate isomerase (E10). Finally the initial three ribulose 1,5-bisphosphate molecules are regenerated irreversibly by phosphorylation with ATP, as catalysed by phosphoribulose kinase (E11). 2 The cyclic way in which CO2 is converted into triose phosphate means that the cycle is autocatalytic and plants are able to reach steady-state rates of photosynthesis after only a short delay while the intermediates rise to the appropriate levels. After this, the excess triose phosphate is available for conversion into the two major end products of photosynthesis. One pathway leads to the accumulation during the day of starch (a glucose polymer) inside the chloroplast. The other pathway involves export of the triose phosphate from the chloroplast in exchange for inorganic phosphate. Further enzymatic reactions occurring in the cytosol convert the triose phosphate into sucrose (formed by joining glucose and fructose). In contrast to the case in animals, sucrose rather than glucose is the major carbohydrate transported in most plants, and starch rather than glycogen is the major storage form. Sucrose is largely exported from the leaf to supply energy to the roots, developing leaves, reproductive organs and other heterotrophic tissues, but it can also be accumulated in the vacuole during the day. Starch accumulation in the chloroplast and sucrose storage in the vacuole allow export to continue from the leaves during the night when photosynthesis is not possible. However, in addition to conversion into starch and sucrose, a significant portion of the triose phosphate is readily converted within the chloroplasts to amino acids and fatty acids. Regulation Regulation of photosynthetic carbon metabolism is required so that key enzymes function only during the light, when energy is available. This allows other enzymes to participate in catabolic pathways during the night. Regulation also allows the rate of metabolite movement through the numerous steps of the Calvin cycle to be tightly coupled with the rate of ATP and NADPH production by photosynthetic electron transport in the thylakoid membranes. This reduces fluctuations in metabolites or oscillations within the cycle that might otherwise occur when environmental conditions change. The primary signal for the presence of ‘light’ is the availability of reducing equivalents from photosynthetic electron transport. A minor portion is diverted to a small protein, thioredoxin. Thioredoxins, present in all living organisms, function as protein disulfide oxidoreductases, reducing disulfide bridges in target proteins to the reduced (–SH) form or re-oxidizing them to the disulfide (S–S) form (Buchanan, 1991). The enzymes modulated by thioredoxin in C3 photosynthesis are glyceraldehyde phosphate dehydrogenase (E3), fructose bisphosphatase (E6), sedoheptulose bisphosphatase (E8), and phosphoribulose kinase (E12) (see Figure 1). An increase in stromal pH from 7 to 8 and an increase in the magnesium ion concentration due to the light- ENCYCLOPEDIA OF LIFE SCIENCES / & 2002 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net Photosynthetic Carbon Metabolism dependent accumulation of protons into and release of magnesium ions from the thylakoid membranes provide another means of regulation. The activities of the enzymes, Rubisco, fructose bisphosphatase and sedoheptulose bisphosphatase, are increased by these changes in the stromal environment. The activity of Rubisco is also modulated by light in a unique manner by another stromal protein, Rubisco activase (Portis, 1992). Although the details of light modulation are not fully characterized, Rubisco activase responds to the level of ATP in the stroma, which increases from dark to light conditions, and the redox poise. In some species, a specific sugar phosphate inhibitor of Rubisco, carboxyarabinitol 1-phosphate, is also implicated in light/ dark regulation and the removal of this inhibitor is facilitated by Rubisco activase. C4 Photosynthesis The oxygenase activity of Rubisco is very costly to the productivity of plants that perform C3 photosynthesis (Ogren, 1984). Calculations show that photosynthesis would increase by 30–40% in C3 plants if this reaction did not occur. Plants also face the problem of balancing water loss with CO2 uptake through their stomata. Raising the concentration of CO2 around Rubisco in a spatially separate area after the CO2 has entered the leaves would help to alleviate both problems. Plants that perform C4 photosynthesis utilize this strategy (Hatch, 1987) and initially capture CO2 to form four-carbon acids with the enzyme phosphoenolpyruvate (PEP) carboxylase (F1, Figures 2, 3 and 4). PEP carboxylase has two advantages over Rubisco for CO2 capture. The substrate is bicarbonate, which is present at 80-fold greater concentrations than CO2, and O2 is not a substrate or competitive inhibitor. Pyruvate phosphate dikinase (F3, Figures 2 and 3) is another key enzyme in C4 photosynthesis (Edwards et al., 1985). It regenerates the PEP utilized for CO2 capture using pyruvate, ATP and phosphate, and also forms AMP and pyrophosphate in a unique double phosphorylation mechanism. Much of the current understanding of C4 photosynthesis was pioneered by the research of M. D. Hatch and co-workers beginning in the 1960s when studies with 14CO2 first indicated that the fourcarbon acids malate, aspartate and oxaloacetate were labelled before phosphoglyceric acid in some species. Hence the term C4 photosynthesis or the Hatch–Slack pathway is used to designate these adaptations to the Calvin cycle. For efficient use of PEP carboxylase to concentrate CO2 around Rubisco, these two enzymes are physically separated in two different photosynthetic cell types. Consequently, the leaves of C4 plants typically have a distinctive structure called Krantz anatomy. A sheath of Mesophyll cell Bundle sheath cell CHLOROPLAST CO2 Oxaloacetate HCO3– F1 Malate NADP+ NADPH + H+ Oxaloacetate CHLOROPLAST Malate NADP+ F2 CO2 Pi Phosphoenolpyruvate NADPH + H+ Phosphoenolpyruvate AMP PPi Calvin cycle F3 ATP Pi Pyruvate Pyruvate Triose phosphate Figure 2 Mechanism for concentrating CO2 in C4 photosynthesis of the NADP malic enzyme type. (Adapted from Heldt H-W (1997) Plant Biochemistry and Molecular Biology. Oxford: Oxford University Press.) Malate and pyruvate are the major metabolites moving between the mesophyll and bundle sheath cells. Specific metabolite membrane transporters are indicated by solid black circles. Key enzymes are identified and discussed in the text (F1, EC 4.1.1.31; F2, 1.1.1.40; F3, 2.7.9.1). Mesophyll cell Bundle sheath cell MITOCHONDRION Aspartate a-KG Aspartate a-KG Glu Oxaloacetate Glu Oxaloacetate NADH + H+ CO2 CHLOROPLAST CO2 NAD+ Malate HCO3– F1 Pi Phosphoenolpyruvate NAD+ CHLOROPLAST F2a Phosphoenolpyruvate PPi AMP CO2 NADH + H+ F3 ATP Pyruvate Glu a-KG Alanine Calvin cycle Pi Pyruvate Triose phosphate Pyruvate Glu a-KG Alanine Figure 3 Mechanism for concentrating CO2 in C4 photosynthesis of the NAD–malic enzyme type. (Adapted from Heldt H-W (1997) Plant Biochemistry and Molecular Biology. Oxford: Oxford University Press.) Aspartate and alanine are the major metabolites moving between the mesophyll and bundle sheath cells. Specific metabolite membrane transporters are indicated by solid black circles. Key enzymes are identified and discussed in the text (F1, EC 4.1.1.31; F2a, 1.1.1.38; F3, 2.7.9.1; a-KG, alpha-ketoglutarate; Glu, glutamate). ENCYCLOPEDIA OF LIFE SCIENCES / & 2002 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net 3 Photosynthetic Carbon Metabolism Mesophyll cell Bundle sheath cell Aspartate a-KG Aspartate a-KG Glu Oxaloacetate Glu Oxaloacetate Pi F1 HCO3– ATP F4 CO2 ADP Phosphoenolpyruvate Phosphoenolpyruvate CHLOROPLAST CO2 Malate NADP+ Oxaloacetate Pi CHLOROPLAST CO2 NADPH Oxaloacetate MITOCHONDRION Malate NAD+ CO2 Calvin cycle Triose phosphate NADH + H+ HCO3– Phosphoenolpyruvate Phospheonolpyruvate AMP PPi ATP Pyruvate Glu Pi Pyruvate a-KG Alanine Respiratory chain ADP ATP Pyruvate Glu a-KG Alanine Figure 4 Mechanism for concentrating CO2 in C4 photosynthesis of the phosphoenolpyruvate (PEP) carboxykinase type. (Adapted from Heldt H-W (1997) Plant Biochemistry and Molecular Biology. Oxford: Oxford University Press.) Aspartate and PEP are the major metabolites moving between the mesophyll and bundle sheath cells. However, some malate also is used in the bundle sheath cells for respiration by mitochondria to meet the high demand for ATP by PEP carboxykinase. cells. Specific metabolite membrane transporters are indicated by solid black circles. Key enzymes are identified and discussed in the text (F1, EC 4.1.1.31; F4, 4.1.1.49; a-KG, alpha-ketoglutarate; Glu, glutamate). cells surrounds the vascular bundles (hence termed bundle sheath cells) and they are encircled by another group of cells, the mesophyll cells. The cells are separated by walls containing cellulose, but remain in communication via connections called plasmodesmata. This allows the diffusive fluxes of metabolites that cycle between the cells, allowing release of the captured CO2 within the bundle sheath cells and resulting in the elevation of the CO2 concentration around Rubisco. C4 plants are subdivided into three types according to where and how the CO2 is released from the four-carbon acids for recapture by Rubisco (Hatch, 1987). In most C4 species, the oxaloacetate formed by PEP carboxylase in the cytosol of the mesophyll cells is transported into the chloroplast and is reduced to malate. The malate leaves the chloroplasts and diffuses through the plasmodesmata to the bundle sheath cells. There malate is decarboxylated in the bundle sheath chloroplasts via NADP–malic enzyme (F2, Figure 2), which must not be confused with NADP– malate dehydrogenase in the mesophyll chloroplasts. The pyruvate formed by this reaction is transported out of the chloroplast, diffuses back to the mesophyll cells and is 4 transported into the chloroplasts. There, PEP is regenerated with pyruvate phosphate dikinase (F3, Figure 2) using ATP, which serves to drive the cycle and thereby effectively transport and concentrate the CO2 in the bundle sheath cells. Familiar species using this type of C4 photosynthesis are maize, sugar cane, sorghum and crabgrass. In other C4 species, such as millet, pigweed and purslane, the decarboxylation of malate occurs in the mitochondria of the bundle sheath cells via NAD–malic enzyme (F2a, Figure 3). This is accompanied by several other differences in the metabolite pathways (which may be seen by comparing Figures 2 and 3). Most notably, aspartate, rather than malate, is the major metabolite diffusing from the mesophyll cells. It is readily interconverted with oxaloacetic acid in each cell type via transamination. Nitrogen balance is maintained by diffusion of alanine from bundle sheath to mesophyll cells. Transamination reactions also readily interconvert alanine and pyruvate for this purpose. The third variation has been found in some fast-growing tropical grasses. Aspartate is also the major metabolite moving from the mesophyll cells, but in these species decarboxylation occurs largely in the cytosol of the bundle ENCYCLOPEDIA OF LIFE SCIENCES / & 2002 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net Photosynthetic Carbon Metabolism sheath cells with yet another enzyme, phosphoenolpyruvate (PEP) carboxykinase (F4, Figure 4), which uses ATP. As a result, the ATP demand necessary for driving the cycle resides largely in the bundle sheath cells rather than with pyruvate phosphate dikinase in the mesophyll cells. To meet this demand, some malate also diffuses to the mitochondria in the bundle sheath cells. Oxidation of the malate to pyruvate produces NADH, which is used by the respiratory chain to produce ATP. The pyruvate is converted to alanine, which returns to the mesophyll to complete a second cycle. As shown in Figure 4, this secondary cycle results in some additional CO2 transport. In some species, a strict classification into only one of these three C4 types cannot be made because decarboxylation does not occur by a single mode. Furthermore, other species are classified as C3 –C4 intermediates. Spatial separation of the carboxylation enzymes and the distinct anatomy are not as marked in some of these species. In other C3 –C4 intermediates, the C4 pathway is not fully functional and the CO2 release occurring during photorespiration appears to be confined to the bundle sheath cells, which also raises the CO2 concentration in these cells and results in more efficient recycling. C4 photosynthesis requires additional enzymes to form the CO2 concentrating cycle and, as might be expected, several of these enzymes are also regulated by light. Malate is present in the cytosol and the activity of PEP carboxylase (F1, Figures 2, 3 and 4) is increased in the light by phosphorylation of the enzyme with a specific protein kinase, which results in reduced malate inhibition (Chollet et al., 1996). However, the nature of the light signal that controls the activity of the kinase is not clear. NADP malate dehydrogenase is modulated by thioredoxin as described previously. Pyruvate phosphate dikinase (F3, Figures 2 and 3) is also regulated by phosphorylation, but with a rather unusual regulatory protein (Edwards et al., 1985). In this case, the regulatory protein uses ADP for the phosphorylation, resulting in an inactive enzyme. Reactivation is catalysed by the same protein and dephosphorylation occurs via phosphorolysis, converting phosphate to pyrophosphate. Factors that control the activity of the regulatory protein in the light and dark have not been ascertained. The absence of Rubisco in the mesophyll cells and the need for additional ATP (about two molecules per CO2 incorporated) for the CO2 concentrating cycle results in quite different energy (i.e. ATP and NADPH) requirements for reactions occurring in the mesophyll and bundle sheath chloroplasts. This is reflected in their appearance. Mesophyll chloroplasts contain abundant thylakoid grana (enriched in photosystem II) and little starch, while the bundle sheath chloroplasts contain mostly non-appressed stroma thylakoids (enriched in photosystem I) and numerous starch granules. C4 photosynthesis is very advantageous over C3 photosynthesis in high-temperature environments or when water stress is frequent. The ability to concentrate CO2 around Rubisco means that the stomata can be more closed and water is conserved. C4 plants typically lose only 100–300 water molecules per CO2 incorporated, whereas C3 plants lose up to 1000. While only about 1% of the known species perform C4 photosynthesis, plants utilizing this pathway are predominant in those considered the most productive, but many are also considered to be the world’s worst weeds. CAM Plants CAM plants utilize another strategy to avoid photorespiration and most importantly to conserve water (Osmond, 1978). The acronym CAM is derived from Crassulacean Acid Metabolism, which in turn is derived from the family in which this pathway was first discovered and the prominent storage of the acid malate. However, plants in many other families use this form of metabolism: some familiar ones are cacti, orchids, pineapple, agave and epiphytes. CAM plants are typically found in mostly dry and even hot habitats such as deserts but are also found in other water-stressed environments such as salt marshes. In these plants there is a temporal separation in the activities of PEP carboxylase and Rubisco (Figure 5). PEP carboxylase (F1, Figure 5) is used to capture atmospheric CO2 during the night when temperatures are lower and the humidity is higher. The oxaloacetate formed by this reaction is reduced in the cytosol by NAD malate dehydrogenase (F5, Figure 5) to form malate, which is stored in large vacuoles in its acidic form. The presence of these large vacuoles is the reason many of these plants are succulent. Most of the starch made the previous day is metabolized during the night to supply the PEP needed for CO2 capture. Rubisco and the Calvin cycle are active during the day, but the stomata can be completely closed because decarboxylation of the stored malate via NADP–malic enzyme (F2, Figure 5) is used to provide the CO2 for Rubisco. Consequently, CO2 can accumulate to high levels and suppress photorespiration. The other product, pyruvate, is converted back into starch via pyruvate phosphate dikinase (F3, Figure 5) and several other enzymatic steps. Thus a large diurnal cycle of carbon between malic acid accumulation during the night and starch accumulation during the day occurs in CAM plants. Similarly to the variations observed in C4 photosynthesis, in a few CAM plants PEP carboxykinase or NAD–malic enzyme is used to release the CO2. Without some form of regulation there would be competition between Rubisco and PEP carboxylase during the day for the released CO2. Malate normally inhibits PEP carboxylase during the day while decarboxylation is occurring. During the night, PEP carboxylase is modified by phosphorylation, which reduces the inhibition by malate. ENCYCLOPEDIA OF LIFE SCIENCES / & 2002 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net 5 Photosynthetic Carbon Metabolism Night Day VACUOLE Malic acid ATP ADP + Pi Malic acid H+ H+ Malate Malate H+ CO2 Malate Oxaloacetate HCO3– F1 F5 Malate CHLOROPLAST Malate NADP+ F2 NADPH + H+ Pyruvate ATP Pi Pi Phosphoenolpyruvate NADH + H+ 3-Phosphoglycerate NAD+ AMP PPi Phosphoenolpyruvate ATP ADP 1,3-Bisphosphoglycerate CO2 F3 Triose phosphate Calvin cycle Triose phosphate Triose phosphate Starch Starch Figure 5 Mechanism for concentrating CO2 in CAM plants. Specific metabolite membrane transporters are indicated by solid black circles. Key enzymes are identified and discussed in the text (F1, EC 4.1.1.31; F2, 1.1.1.40; F3, 2.7.9.1; F5, 1.1.1.37). As in C4 plants, the additional metabolic reactions needed for the temporal separation of the initial CO2 capture and the subsequent recapture by the Calvin cycle use more energy, but in hot dry habitats the limiting resource is usually water. CAM plants are well adapted to these environments and typically lose only 50 water molecules for each CO2 gained. However, the capacity to store malic acid in vacuoles appears to be limited, resulting in slow growth even under favourable conditions. Some CAM plants (described as ‘facultative’ versus ‘obligate’) have developed the ability to switch between the C3 and CAM forms of photosynthesis in response to their environment. properties and mechanism of light/dark regulation. Annual Review of Plant Physiology 36: 255–286. Hatch MD (1987) C4 photosynthesis: a unique blend of modified biochemistry, anatomy and ultrastructure. Biochemica et Biophysica Acta 895: 81–106. Ogren WL (1984) Photorespiration: pathways, regulation, and modification. Annual Review of Plant Physiology 35: 415–442. Osmond CB (1978) Crassulacean acid metabolism: a curiosity in context. Annual Review of Plant Physiology 29: 379–414. Portis AR (1992) Regulation of ribulose 1,5-bisphosphate carboxylase/ oxygenase activity. Annual Review of Plant Physiology and Plant Molecular Biology 43: 415–437. References Further Reading Bassham JA (1962) The path of carbon in photosynthesis. Scientific American 206: 88–100. Buchanan BB (1991) Regulation of CO2 assimilation in oxygenic photosynthesis: the ferredoxin/thioredoxin system. Archives of Biochemistry and Biophysics 288: 1–8. Chollet R, Vidal J and O’Leary MH (1996) Phosphoenolpyruvate carboxylase: a ubiquitous, highly regulated enzyme in plants. Annual Review of Plant Physiology and Plant Molecular Biology 47: 273–298. Edwards GE, Nakamoto H, Burnell JN and Slack MD (1985) Pyruvate, Pi dikinase and NADP-malate dehydrogenase in C4 photosynthesis: Heldt H-W (1997) Plant Biochemistry and Molecular Biology. Oxford: Oxford University Press. Raghavendra AS (ed.) (1998) Photosynthesis: A Comprehensive Treatise. Cambridge: Cambridge University Press. von Caemmerer S and Furbank RT (eds) (1997) C4 photosynthesis: 30 (or 40) years on. Australian Journal of Plant Physiology 24 (4). [Special issue] Winter K and Smith JAC (eds) (1996) Crassulacean acid metabolism. Biochemistry, ecophysiology and evolution. Ecological Studies, vol. 114. Berlin: Springer-Verlag. 6 ENCYCLOPEDIA OF LIFE SCIENCES / & 2002 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net