Why do some plants run a different kind of photosynthesis?

Corn, sugar cane and sorghum run a different version of photosynthesis from most plants. It is called C4, and it exists because the enzyme that captures carbon dioxide, RuBisCO, also reacts with oxygen. That reaction makes a toxic product that costs energy to recycle through photorespiration. C4 photosynthesis reduces photorespiration by concentrating CO2 around RuBisCO.

The way it does this is to split the work between two rooms. C4 leaves generally contain two partially isolated compartments, mesophyll cells and bundle-sheath cells. In the mesophyll, a different enzyme, PEP carboxylase, fixes CO2 into a four-carbon acid, hence the name C4. That acid diffuses to the bundle sheath, where it is decarboxylated, which releases CO2 and creates a CO2-rich environment around RuBisCO.

The extra steps need more ATP to regenerate the starting molecule, but concentrating CO2 allows high rates of photosynthesis at higher temperatures. C4 plants also lose less water: at 30 degrees C, C3 grasses lose approximately 833 water molecules per CO2 fixed, against only 277 for C4 grasses.

The pathway was elucidated by Marshall Hatch and Charles Slack, in Australia, in 1966. It is so useful that it has evolved on at least 62 independent occasions in 19 plant families.