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The C4 Pathway

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The C4 Pathway - Lesson Summary

Carbon fixation in plants that have the first product of the Calvin Cycle as a 4 carbon compound.  These plants are called C 4 plants, and the process of carbon fixation is called the C 4 Pathway. The C 4 Pathway was discovered by M D Hatch and C R Slack in 1966. Carbon fixation in C 4 differs from C 3 plants as it has one extra step preceding the C 3 Calvin Cycle. C 4 plants are found in hot and dry areas and are adapted to tolerate high temperatures and can respond to high light intensities. Leaves whose anatomy shows vascular bundles encircled by bundle sheath cells are called leaves with Kranz anatomy. Kranz is derived from a German word which means ‘halo’ or ‘wreath’. Plants exhibiting Kranz anatomy are maize and sugarcane. In the C 4 Pathway, carbon dioxide is fixed in the mesophyll cells. The first step is the conversion of pyruvate to phosphoenolpyruvate by utilising 1 molecule of ATP in the presence of the enzyme pyruvate phosphate dikinase. Phosphoenol pyruvate, accepts carbon dioxide to form oxaloacetate, a 4 carbon compound. This reaction is catalysed by phospho enol pyruvate carboxylase. This enzyme ensures carboxylation even at low concentrations of carbon dioxide because of its greater affinity towards carbon dioxide compared to RuBisCo. Oxaloacetate is quickly converted to either malate and is transported to the bundle sheath cells through plasmodesmata, where it is decarboxylated back to pyruvate. Decarboxylation results in the release of a carbon dioxide molecule. Pruvate is transported back to the mesophyll cells through plasmodesmata and is phosphorylated to phospho enol pyruvate. Then the cycle of carboxylation is re-initiated. The released carbon dioxide is fixed in bundle sheath cells, which are rich in RuBisCo through the Calvin or C 3 Cycle. Thus, we find that C 4 plants undergo carboxylation twice, ie in mesophyll, through the C 4 Cycle and in bundle sheath cells via the Calvin Cycle. In C 4 plants, the C 4 and C 3 Pathways are separated in space, as they occur in different cells i.e. mesophyll cells and bundle sheath cells. As per energy requirements, the C 4 Pathway is a little expensive compared to the C 3 Pathway. This is because the C 4 Pathway consumes 2 ATP molecules for every carbon dioxide molecule fixed. Therefore, fixation of six molecules of CO 2 requires 12 ATP molecules.  When we include the ATPs used in the Calvin Cycle to the total ATPs used, the count goes up to 30, which are 12 more than in the C 3 Pathway.  Though the C 4 Pathway is quite expensive, it is an efficient method of carbon fixation for plants living in hot and dry conditions.    

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