Stage II: Bundle Sheath Cells

The C4 pathway's second stage takes place inside the bundle sheath cells, once the mesophyll-derived malic acid has arrived there. Malic acid is decarboxylated - broken down with release of CO2 - to yield the 3-carbon compound pyruvic acid plus a molecule of CO2. Because this decarboxylation happens locally, right where the bundle sheath chloroplasts sit, it produces a high, concentrated pulse of CO2 exactly where RuBisCO is located; this freed CO2 immediately combines with RuBP and proceeds through the ordinary Calvin (C3) cycle, ultimately producing sugar that is exported out to the phloem for transport around the plant. Meanwhile, the pyruvic acid left behind after decarboxylation is transported back out of the bundle sheath cell to the mesophyll cell, where it is re-phosphorylated (at the cost of ATP) back into phosphoenolpyruvate, regenerating the CO2 acceptor used in stage I and closing the overall C4 cycle. It is precisely this local concentration of CO2 around RuBisCO in the bundle sheath - achieved by physically separating the initial CO2-c …