Recall that C4 plants tend to have higher rates of photosynthesis when compared with C3 plants (under normal 21 percent O2 atmospheric conditions).
If photosynthetic rates are higher, why are all plants not C4 plants?

a. Because of mesophyll cells, C4 photosynthesis requires more chlorophyll than C3 photosynthesis.
b. Because the regeneration of PEP consumes ATP, C4 photosynthesis consumes more energy than C3 photosynthesis.
c. Because of rubisco, C4 photosynthesis consumes more O2 than C3 photosynthesis.
d. Because of leaf veins, C4 photosynthesis requires more H2O than C3 photosynthesis.

In C3 plant fixation of carbon occurs rubisco the Calvin cycle enzyme that add CO2 to ribulose bisphosphate to produce a three carbon compound 3-phosphoglycerate.

C4 plants have an alternate mode of carbon fixation that forms a four-carbon compound as its first products. In C4 plants there are two distinct types of photosynthetic cells; bundle-sheath cells and mesophyll cells. The Calvin cycle is confined in the chloroplasts of the bundle-sheath cell.

In the first step of this mechanisms an enzyme present only in the mesophyll PEP carboxylase adds CO2 to PEP to form oxaloacetate a four-carbon products.

The four-carbon is exported to the bundle-shealth cells where it releases CO2, which is reassimilated into organic material by rubisco and the Calvin cycle. The same reaction regenerate pyruvate. ATP is used to convert pyruvate to PEP, allowing the reaction cycle to continue.

In C4 plants ATP is the price for concentrating CO2 in the bundle-shealth. C4 photosynthesis has higher ATP requirements than the C3 pathway.