The dark reactions of photosynthesis

The sequences of reactions by which C02 is reduced to carbohydrate are sometimes referred to as the 'dark' reactions of photosynthesis because C02 can be fixed in the dark by a leaf or photosynthetic organism if the appropriate reagents are available. The dark reactions take place separately from the light-driven reactions in the stroma or cytoplasm, as indicated in Fig. 1.10. Electrons from the light-driven process in the thylakoid membrane reduce either nicotinamide adenine dinucleotide (NAD') or its phosphorylated form (NADP'), and the reduced forms NADH or NADPH provide the reducing power for CO2 fixation, with the help of some additional free energy in the form of adenosine triphosphate (ATP) generated by photosynthetic phosphorylation. There are several mechanisms of C02 reduction, characteristic of different photosynthetic species. The reductive pentose cycle or C3 cycle (so called because the

first 'stable' product of C02 reduction is a three-carbon compound) is the commonest mechanism, operating in algae and most plants. Some plants, especially those indigenous to hot climates, such as corn (maize) and sugar cane, operate the C4 cycle. Edwards and Walker discuss these and other carbon fixation cycles such as the CAM cycle in Chapter 4; Blankenship (2002) provides a full account.