CO₂ Reduction

Linear electron transport in oxygenic photosynthesis is the reduction of NADP⁺ to NADPH by water, which results in the formation of molecular oxygen:

⇒ Equation [8] : 2H₂O + 2NADP⁺ → O₂ + NADPH + 2H⁺ .

NADPH is incapable of reducing CO₂ by itself; ATP is also required. The CO₂ acceptor in photosynthesis is the fivecarbon, phosphorylated sugar ribulose 1, 5-bisphosphate. CO₂ cleaves this sugar into 2 mol of the three-carbon sugar acid 3-phosphoglycerate, a compound that is also an intermediate in glycolysis. The enzyme that catalyzes this reaction, ribulose 1, 5-bisphosphate carboxylase/oxygenase, or rubisco, is present in very high concentrations within chloroplasts, which makes it among the most abundant proteins in the biosphere.

Recall that in glycolysis one of the two steps in which ATP is formed is the conversion of 1,3- bisphosphoglycerate to 3-phosphoglycerate. The acyl phosphate at the 1-position of the bisphosphorylated sugar acid is transferred to ADP to form ATP. The conversion of 3-phosphoglycerate to carbohydrates occurs by a pathway that is essentially the reverse of glycolysis. It must be emphasized, however, that glycolysis and photosynthetic carbon metabolism take place in separate intracellular compartments. Glycolysis occurs in the cytoplasm and uses NAD⁺ as the electron acceptor. The photosynthetic reduction of 3-phosphoglycerate occurs inside chloroplasts in the aqueous space known as the stroma. The enzymes in the two compartments are not the same even though they catalyze similar reactions. For example, the triose phosphate dehydrogenase in the cytoplasm is very specific for NAD⁺, whereas that in the chloroplast stroma is equally specific for NADPH.

Therefore, ATP is required for the reduction by NADPH of 3-phosphoglycerate to the oxidation level of a carbohydrate:

⇒ Equation [9]

ATP + 3-phosphoglycerate → ADP +1,3-bisphosphoglycerate

and the bisphosphoglycerate is in turn reduced by NADPH:

⇒ Equation [10]

NADPH + H+ + 1,3-bisphosphoglycerate → NADP+ +Pi + glyceraldehyde 3-phosphate.

Since two 3-phosphoglycerates are generated for each CO₂ assimilated, two NADPH and two ATP are required for reduction. This reaction is the only one in photosynthetic carbohydrate metabolism that is an oxidation– reduction reaction.

Glyceraldehyde 3-phosphate is a sugar phosphate and may be readily converted within chloroplasts to many sugars and the glucose polymer starch. Some of the glyceraldehyde 3-phosphate is used in a complex series of reactions to regenerate the five-carbon acceptor of CO₂, ribulose 1,5-bisphosphate. In the process, one phosphate is cleaved from one of the sugar phosphate intermediates. Thus, ribulose 5-phosphate, the product of the cycle, must be phosphorylated by using ATP as the phosphoryl donor. As a consequence, three ATP and two NADPH are required for each CO₂ taken up.

Photosynthesis must satisfy the energy requirements of all living tissues in plants, including roots, stems, and developing fruit. Up to 75% of the triose phosphate formed is exported from the chloroplasts in leaf cells to the cytoplasm where it is converted to sucrose, a major product of photosynthesis. In most plants, sucrose is transported to the rest of the plant where it is either stored as starch or broken down by glycolysis and the citric acid cycle in exactly the same way as it is in animals to produce the ATP needed to sustain life.