In warm, dry conditions the C-4 pathway permits fixation of CO2 with
reduced losses due to photorespiration. The initial CO2 fixing enzyme in the C-4 pathway is phosphoenolpyruvate carboxylase, or PEPcase, which has a higher affinity for CO2 than Rubisco. Fixation of carbon in the mesophyll of C-4 plants prevents wasteful photorespiration by Rubisco.

In the mesophyll, PEPcase fixes CO2 as 4-carbon compounds:

PEP carboxylase + PEP + CO2 → oxaloacetate (C4)
oxaloacetate → malate (C4)

CO2 is taken up by cells of the mesophyll, where PEPcase fixes CO2 as 4-carbon oxaloacetate and malate before transport to the bundle sheath, a specialized tissue in which photosynthetic carbon reduction (C-3) takes place in bundle sheath cell chloroplasts.

Conversion of 4-carbon malate to 3-carbon pyruvate releases CO2 to the bundle cell's Calvin cycle, where 3-phosphoglycerate is furmed under the action of Rubisco. Pyruvate is phosphorylated in the mesophyll into PEP by the phosphorus group donated by a single molecule of ATP. PEP is again utilized to fix CO2 and form PEP carboxylase under the enzymatic action of PEPcase (phosphoenolpyruvate carboxylase)

The C-4 pathway consumes 30 ATP for the synthesis of one molecule of glucose, while the C-3 pathway consumes 18 ATP for the synthesis of one molecule of glucose. However, the reduction of wasteful photorespiration by Rubisco, in which tropical plants lose more than half photosynthecized carbon, more than compensates for the extra cost of ATP.

Table ~ comparison of C-3, C-4, CAM plants :

The C-4 pathway is also called the Hatch-Slack pathway for its Australian co-discoverers. The pathway is a more recent evolutionary development than the C-3 cycle, having arisen during the Cenozoic. Because C-4 carbon fixation has evolved on several occasions in different groups of plants, it is an example of convergent evolution. C4 plants, with their characteristic dimorphic chloroplasts, became more common during the Mesozoic, and today represent about 5% of plant biomass. Plants that employ C-4 metabolism include maize, sorghum, sugarcane, Eleusine, Amaranthus, and switchgrass (Panicum virgatum).

C4 photosynthesis: principles of CO2 concentration and prospects for its introduction into C3 plants.
C4 photosynthesis has a number of distinct properties that enable the capture of CO2 and its concentration in the vicinity of Rubisco, so as to reduce the oxygenase activity of Rubisco, and hence the rate of photorespiration.
Richard C. Leegood, C4 photosynthesis: principles of CO2 concentration and prospects for its introduction into C3 plants (Free Full Text Review Article), Journal of Experimental Botany, Vol. 53, No. 369, pp. 581-590, April 1, 2002.

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