C4 carbon fixation

These intermediates diffuse to the bundle sheath cells, where they are decarboxylated, creating a CO2-rich environment around RuBisCO and thereby suppressing photorespiration.

The resulting pyruvate (PYR), together with about half of the phosphoglycerate (PGA) produced by RuBisCO, diffuses back to the mesophyll.

PGA is then chemically reduced and diffuses back to the bundle sheath to complete the reductive pentose phosphate cycle (RPP).

Additional biochemical steps require more energy in the form of ATP to regenerate PEP, but concentrating CO2 allows high rates of photosynthesis at higher temperatures.

[3] However, since the C3 pathway does not require extra energy for the regeneration of PEP, it is more efficient in conditions where photorespiration is limited, typically at low temperatures and in the shade.

The primary function of kranz anatomy is to provide a site in which CO2 can be concentrated around RuBisCO, thereby avoiding photorespiration.

A layer of suberin[7] is often deposed at the level of the middle lamella (tangential interface between mesophyll and bundle sheath) in order to reduce the apoplastic diffusion of CO2 (called leakage).

Although most C4 plants exhibit kranz anatomy, there are, however, a few species that operate a limited C4 cycle without any distinct bundle sheath tissue.

[8][9][10][11] Although the cytology of both genera differs slightly, the basic principle is that fluid-filled vacuoles are employed to divide the cell into two separate areas.

There is also evidence of inducible C4 photosynthesis by non-kranz aquatic macrophyte Hydrilla verticillata under warm conditions, although the mechanism by which CO2 leakage from around RuBisCO is minimised is currently uncertain.

Instead of direct fixation by RuBisCO, CO2 is initially incorporated into a four-carbon organic acid (either malate or aspartate) in the mesophyll.

This PGA is chemically reduced in the mesophyll and diffuses back to the bundle sheath where it enters the conversion phase of the Calvin cycle.

In the bundle sheath ASP is transaminated again to OAA and then undergoes a futile reduction and oxidative decarboxylation to release CO2.

This cycle bypasses the reaction of malate dehydrogenase in the mesophyll and therefore does not transfer reducing equivalents to the bundle sheath.

An increase in relative expression of PEPCK has been observed under low light, and it has been proposed to play a role in facilitating balancing energy requirements between mesophyll and bundle sheath.

The division of the photosynthetic work between two types of chloroplasts results inevitably in a prolific exchange of intermediates between them.

To reduce product inhibition of photosynthetic enzymes (for instance PECP) concentration gradients need to be as low as possible.

To meet the NADPH and ATP demands in the mesophyll and bundle sheath, light needs to be harvested and shared between two distinct electron transfer chains.

[13] The apportioning of excitation energy between the two cell types will influence the availability of ATP and NADPH in the mesophyll and bundle sheath.

For instance, average quantum efficiency is the ratio between gross assimilation and either absorbed or incident light intensity.

However, they will also have high rates of CO2 retro-diffusion from the bundle sheath (called leakage) which will increase photorespiration and decrease biochemical efficiency under dim light.

This increased water use efficiency of C4 grasses means that soil moisture is conserved, allowing them to grow for longer in arid environments.

[18] C4 carbon fixation has evolved in at least 62 independent occasions in 19 different families of plants, making it a prime example of convergent evolution.

[22] C4 metabolism in grasses originated when their habitat migrated from the shady forest undercanopy to more open environments,[23] where the high sunlight gave it an advantage over the C3 pathway.

[24] Drought was not necessary for its innovation; rather, the increased parsimony in water use was a byproduct of the pathway and allowed C4 plants to more readily colonize arid environments.

[26][27] Increasing the proportion of C4 plants on earth could assist biosequestration of CO2 and represent an important climate change avoidance strategy.