2.1 Modes of photosynthesis

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Millions of years of evolution have produced significant bio-chemical variants for fixation of atmospheric CO2 into carbo-hydrate, namely (1) C3 (three-carbon fixation product), (2) C4 (four-carbon fixation product), (3) CAM (crassulacean acid metabolism) and (4) SAM (submerged aquatic macrophytes).

Despite much diversity in life form and biochemical process, all of these pathways focus upon a single enzyme which is by far the most abundant protein on earth, namely ribulose-1,5-bisphosphate carboxylase/oxygenase, or Rubisco (Figure 2.1a). Localised in the stroma of chloroplasts, this enzyme enables the primary catalytic step in photosynthetic carbon reduction (or PCR cycle) in all green plants and algae (Bassham and Calvin 1957). Although Rubisco has been highly conserved throughout evolutionary history, this enzyme is surprisingly inefficient with a slow turnover of active sites and a rather feeble discrimination between alternative substrates (CO2 and O2), a combination that severely restricts photosynthetic performance of C3 plants under ambient conditions of 20% O2 and 0.035% CO2. Accordingly, and in response to CO2 limitation, C4, CAM and SAM variants have evolved with metabolic concentrating devices which enhance Rubisco performance.