2.1.1  Photosynthetic carbon reduction

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Figure 2.1 Photosynthetic carbon reduction (PCR cycle) utilises ATP and NADPH produced by thylakoid electron transport to drive CO2 fixation by Rubisco (a). CO2 is incorporated into a live-carbon sugar phosphate to produce two three—carbon sugar phosphates which can either be exported from the chloroplast for sucrose synthesis, be recycled to make more live—carbon
acceptors, or be used to make starch. The appearance of radioactive carbon in three-carbon sugar phosphates and then in starch and sucrose following photosynthesis in 14CO2 was evidence for the pathway of photosynthesis. (b) (Original drawing courtesy Bob Furbank)

The biochemical pathway of CO2 fixation was discovered by feeding radioactively labelled CO2 in the light to algae and then extracting the cells and examining which compounds accumulated radioactivity. Figure 2.1(b) shows a typical labelling ‘pattern’ for a C3 plant. Here, a short burst of labelled CO2 was given to the plants, then the label was ‘chased’ through the photosynthetic pathway by flushing with unlabelled air. Atmospheric CO2 is initially incorporated into a five-carbon sugar phosphate (ribulose-1,5-bisphosphate or RuBP) to produce two molecules of the phosphorylated three-carbon compound 3-phosphoglycerate, often referred to as the acidic form 3-phosphoglyceric acid (3-PGA). Hence, plants which use Rubisco as their primary enzyme of CO2 fixation from the air are called C3 plants. Consequently, in C3 plants, 3-PGA is the first labelled sugar phosphate detected after a pulse of 14CO2 has been supplied (Figure 2.1b). In the PCR cycle, 3-PGA is phosphorylated by the ATP produced from thylakoid electron transport (Section 1.2.3) and then reduced by NADPH to produce triose phosphate. Triose phosphate is the first stable product from Rubisco activity.

Newly synthesised triose phosphate faces three options. It can be (1) exported to the cytosol for sucrose synthesis and subsequent translocation to the rest of the plant, (2) recycled within the chloroplast to produce more RuBP or (3) diverted to produce starch (see Figure 2.1a, and starch grain in Figure 1.7). This is shown by the time-course of the appearance of radioactivity in starch and sucrose after it has passed through 3-PGA (Figure 2.1b). The energy requirements of the PCR cycle are three ATP and two NADPH per CO2 fixed, in the absence of any other energy-consuming processes.

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