Significance of C4 Cycle

The C4 pathway's two-cell CO2-concentrating mechanism gives C4 plants several distinct physiological advantages over C3 plants, despite its higher direct ATP cost. First, because PEP carboxylase has such a high affinity for CO2, C4 plants growing in the tropical and sub-tropical regions they are typically native to can continue fixing CO2 efficiently even at very low atmospheric CO2 concentrations - down to below 10 ppm, versus roughly 50 ppm needed by C3 plants. Second, this same high-affinity CO2 capture lets C4 plants keep their stomata more closed for a given rate of carbon fixation, which reduces water loss and gives C4 plants a degree of drought tolerance that C3 plants lack. Third, because PEP carboxylase - unlike RuBisCO - is completely insensitive to oxygen, rising oxygen concentrations have no inhibitory effect at all on the C4 cycle's initial fixation step. Fourth, and following directly from the near-total absence of photorespiration in C4 plants, their CO2 compensation point (the CO2 concentration at which photosynthetic uptake exactly balances respi …