An alternative form of carbon dioxide (C0
2) fixation found in *
C4 plants (see diagram). In such plants the first product of C0
2 fixation is not the three-carbon phosphoglyceric acid (see
Calvin cycle) but the four-carbon oxaloacetate. This is formed by the carboxylation of phosphoenolpyruvate (PEP) by the enzyme PEP carboxylase. The oxaloacetate is then either reduced to malate or transaminated to form aspartate. These reactions occur in the cells of the mesophyll. The malate or aspartate is then transported to bundle-sheath cells situated around the leaf veins (see
Kranz structure) and decarboxylated to form C0
2 and pyruvate. The C0
2 so released reacts with ribulose 1,5-bisphosphate to form two molecules of phosphoglyceric acid. The normal Calvin sequence of reactions then commences. The pyruvate is returned to the mesophyll cells where it is converted to PEP with the concomitant formation of a molecule of AMP from ATP. This step, which uses up two high-energy phosphate bonds, is the reason why, overall, C4 plants require 30 molecules of ATP for each molecule of glucose synthesized whereas C
3 plants only require 18. PEP carboxylase has a far higher affinity for C02 than RUBP carboxylase and C
4 plants are consequently more efficient at fixing C0
2 than C
3 plants. This accounts for their lower *
compensation points. See also
photorespiration.
