Photosynthetic Carbon Metabolism

Abstract

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.

Keywords: photosynthesis; photorespiration; chloroplast

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.

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).

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).

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).

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).

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References

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: 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.

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Further Reading

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.

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Portis, Archie R(May 2001) Photosynthetic Carbon Metabolism. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0001385]