The Calvin cycle and C4 plants

The dark reactions (they need no light directly) use the ATP and NADPH from the light reactions to fix CO₂ into sugar. In plants they happen in the chloroplast stroma (in cyanobacteria, in the cytosol). Melvin Calvin and his co-workers worked out the Calvin cycle, in three stages:

• Stage I, fixation: the enzyme RuBisCO joins CO₂ to ribulose-1,5-bisphosphate (RuBP), giving two 3-phosphoglycerate (PGA). Since the first stable product has three carbons, this is the C3 cycle. RuBisCO can also grab oxygen: it is both a carboxylase and an oxygenase.
• Stage II, reduction: 6 ATP and 6 NADPH reduce 6 PGA to 6 glyceraldehyde-3-phosphate (G3P).
• Stage III, regeneration: one G3P leaves to make sugar or starch; the other five are rebuilt, using 3 ATP, into three RuBP.

C4 plants, such as maize, sorghum and sugarcane, have an extra step. Their leaves have Kranz anatomy: a ring of mesophyll cells around bundle sheath cells.

• In the mesophyll cells, the enzyme PEP carboxylase fixes CO₂ onto PEP, making oxaloacetate (4C), which becomes malate.
• Malate moves into the bundle sheath cells and releases CO₂ there, for RuBisCO and the Calvin cycle. The leftover pyruvate returns to the mesophyll to remake PEP.

So the Calvin cycle is common to all plants; C4 plants simply add a CO₂-concentrating step before it, which keeps CO₂ high around RuBisCO.