The C4 Pathway and Kranz Anatomy

While most plants rely exclusively on the Calvin cycle described in the previous section, a significant number of plant species -- including many economically important tropical and sub-tropical species such as maize, sugarcane and sorghum, along with numerous tropical grasses -- have evolved an additional biochemical pathway that operates alongside, and before, the Calvin cycle. This pathway is called the C4 pathway (also known as the Hatch-Slack pathway, after the two biochemists who first worked it out), because its own first stable product is a four-carbon compound, distinguishing it clearly from the three-carbon first product of the ordinary C3 pathway.

The C4 pathway is inseparably linked to a distinctive leaf anatomy called Kranz anatomy (from the German word for "wreath"), visible in cross-section under the microscope. In a C4 leaf, each vascular bundle is surrounded by a conspicuous ring of large, thick-walled bundle sheath cells, packed with unusually numerous and unusually large chloroplasts; radiating outward from this ring is a looser layer of smaller mesophyll cells, containing fewer and smaller chloroplasts. This distinctive wreath-like arrangement of two concentric, chloroplast-bearing cell layers around each vein is the anatomical hallmark by which C4 leaves can be recognised, and it directly reflects the biochemical division of labour the pathway relies on.

The pathway begins in the cytoplasm of the outer mesophyll cells, where atmospheric CO2 is fixed by the enzyme PEP carboxylase (PEPcase) onto a three-carbon acceptor molecule, phosphoenolpyruvate (PEP), forming the four-carbon compound oxaloacetic acid (OAA) -- the first stable product that gives the pathway its name. PEP carboxylase has a considerably higher affinity for CO2 than RuBisCO does, and, critically, it has no competing oxygenase activity at all, which allows it to fix CO2 efficiently even under the relatively low internal CO2 concentrations that prevail when a leaf partly closes its stomata to conserve water in hot conditions. The OAA formed is rapidly converted to malic acid (or, in some C4 species, aspartic acid), and this four-carbon organic acid is then transported, through the numerous plasmodesmata connecting mesophyll and bundle sheath cells, into the neighbouring bundle sheath cells.

Once inside the bundle sheath cell, the four-carbon acid is enzymatically decarboxylated, releasing CO2 once again -- but this time, released directly inside the very cell that houses the Calvin cycle. This regenerated, locally concentrated CO2 is then fixed a second time, by RuBisCO, exactly as described in the Calvin cycle section, but with the crucial difference that this entire Calvin cycle takes place exclusively within the bundle sheath chloroplasts, never in the mesophyll. The three-carbon compound (pyruvate) left behind after decarboxylation is transported back into the mesophyll cell, where it is converted back into PEP -- a step that consumes additional ATP -- so that the mesophyll's CO2-fixing machinery can be reused for the next cycle. …