Photosynthesis In Higher Plants

The C4 pathway, also called the Hatch and Slack pathway, is detailed in NCERT Class 11, Chapter 13 (Photosynthesis in Higher Plants). In C4 plants such as maize, sugarcane and sorghum, the first step of carbon fixation occurs in the mesophyll cells, where the three-carbon compound phosphoenolpyruvate (PEP) acts as the primary carbon dioxide acceptor. The enzyme PEP carboxylase combines PEP with CO2 to form the four-carbon acid oxaloacetate, which gives the pathway its name. Ribulose-1,5-bisphosphate is the CO2 acceptor in the Calvin cycle catalysed by RuBisCO, which in C4 plants operates only later in the bundle sheath cells, so it is not the primary mesophyll acceptor. Oxaloacetate is the first stable product of fixation, not the acceptor itself. 3-phosphoglyceric acid is the first product of the C3 cycle, not a C4 acceptor. A biological consistency check supports PEP: since the earliest detectable fixation product in C4 plants is a four-carbon acid, the acceptor must be a three-carbon molecule that gains one carbon, which is exactly phosphoenolpyruvate, and the high affinity of PEP carboxylase for carbon dioxide further explains why C4 plants fix carbon efficiently even at low concentrations.