C3, C4 and CAM Plants: A Comparative Overview

Bringing together the material of the preceding sections, C3, C4 and CAM photosynthesis can usefully be understood as three related but distinct evolutionary solutions to a single underlying biochemical tension: RuBisCO, the enzyme at the heart of the Calvin cycle, is both indispensable and imperfect, since its competing oxygenase activity gives rise to the wasteful photorespiratory pathway described earlier, particularly under hot, bright or water-limited conditions that reduce the internal CO2:O2 ratio available to it.

Ordinary C3 plants -- the great majority of plant species, including staple crops such as wheat and rice and most trees -- rely on the Calvin cycle alone, with RuBP as the sole CO2 acceptor, RuBisCO as the sole carboxylating enzyme, and PGA as the first stable product, all within the mesophyll chloroplasts of an ordinary leaf lacking any specialised Kranz anatomy. This is metabolically the most economical route, in terms of ATP expenditure per CO2 fixed, but it is also the route most vulnerable to significant photorespiratory carbon loss whenever conditions turn hot, bright and dry.

C4 plants, typified by maize, sugarcane and sorghum, address this vulnerability by adding an upstream CO2-concentrating step: PEP carboxylase fixes CO2 in mesophyll cells first, forming the four-carbon compound OAA, before the CO2 is released again and re-fixed by RuBisCO within a separate, enclosed compartment -- the chloroplast-rich bundle sheath cells of the leaf's distinctive Kranz anatomy. This spatial separation of the two carboxylation events, between two cooperating cell types, keeps the local CO2 concentration around RuBisCO consistently high, suppressing photorespiration almost entirely, at the cost of the extra ATP needed to regenerate PEP after each turn.

CAM plants, typified by cacti, Opuntia, Bryophyllum and pineapple, achieve a broadly similar CO2-concentrating effect through temporal rather than spatial separation, performing the very same two steps -- PEP carboxylase-mediated fixation and RuBisCO-mediated re-fixation -- within one single mesophyll cell, but at two different times of day: CO2 fixation and malic acid storage by night, when stomata are open, and Calvin cycle activity by day, when stomata remain closed to conserve water. Like C4 photosynthesis, CAM also suppresses photorespiration effectively, but its principal adaptive advantage is water conservation rather than light-use efficiency, which is reflected in its characteristically slower overall growth rate compared with both C3 and C4 plants. …