The C₄ pathway

C₄ plants are adapted to dry tropical regions. Their first CO₂ fixation product is the 4-carbon OAA, but they still use the Calvin cycle as their main biosynthetic route.

Compared with C₃ plants they have a special leaf anatomy, tolerate higher temperatures, respond to high light intensity, lack photorespiration and have greater biomass productivity. Maize and sorghum are examples.

Kranz anatomy: around the vascular bundles lies a wreath-like layer of large bundle sheath cells (Kranz means wreath). These cells can be in several layers; they have many chloroplasts, no intercellular spaces, and thick walls that gases cannot pass through.

In mesophyll cells, the primary CO₂ acceptor is phosphoenol pyruvate (PEP), a 3-carbon molecule, and the enzyme is PEP carboxylase (PEPcase). Mesophyll cells lack RuBisCO.

The OAA formed in mesophyll is converted to other 4-carbon acids such as malic acid or aspartic acid, which move to the bundle sheath cells.

Inside bundle sheath cells the C₄ acids break down, releasing CO₂ plus a 3-carbon molecule, which goes back to the mesophyll to be converted into PEP again, so the cycle is completed.

The released CO₂ enters the Calvin cycle in the bundle sheath cells, which are rich in RuBisCO but lack PEPcase.

This pathway is also known as the Hatch and Slack pathway, and in effect it pumps CO₂ to where RuBisCO is, keeping CO₂ concentration high there.