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Home arrow entries arrow H arrow Hatch-Slack pathway

Hatch-Slack pathway

An alternative form of carbon dioxide (C02) fixation found in *C4 plants (see diagram). In such plants the first product of C02 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 C02 and pyruvate. The C02 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 C3 plants only require 18. PEP carboxylase has a far higher affinity for C02 than RUBP carboxylase and C4 plants are consequently more efficient at fixing C02 than C3 plants. This accounts for their lower *compensation points. See also photorespiration.Image
 
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