Final answer:
The C4 photosynthetic pathway, though more water-use efficient, is less energy efficient than the C3 pathway due to its higher ATP requirement.
Step-by-step explanation:
One of the main disadvantages of the C4 photosynthetic pathway is that it requires more adenosine triphosphate (ATP) compared to the C3 pathway. The initial steps of C4 photosynthesis involve the enzyme PEP carboxylase catalyzing the fixation of carbon dioxide into a four-carbon compound, which consumes extra ATP.
This compound is then transported to the bundle sheath cells where it releases CO2 to be used in the Calvin cycle.
Though this pathway is more water-use efficient and advantageous in hot, dry climates, the energy cost due to the increased ATP requirement makes it less efficient in terms of energy than the simpler C3 pathway.