Final answer:
C4 photosynthesis requires the expenditure of ATP to convert PEP and CO2 into oxaloacetate, and later into malate or aspartate, providing efficient carbon fixation in hot and arid conditions despite using more ATP than C3 photosynthesis.
Step-by-step explanation:
In the context of C4 photosynthesis, the expenditure of ATP is required to convert a molecule into a more energy-rich intermediate that can then undergo further reactions in the photosynthesis process. Specifically, during C4 photosynthesis, phosphoenolpyruvate (PEP) is combined with carbon dioxide (CO2) to form oxaloacetate in a reaction catalyzed by the enzyme PEP carboxylase. This step is ATP-intensive because ATP is later used to convert the oxaloacetate into malate or aspartate, which is then transported to bundle sheath cells where the CO2 is released to eventually enter the Calvin cycle of the C3 photosynthesis pathway. The release of CO2 within the bundle sheath cells is critical as it provides a high concentration of CO2 to the Calvin cycle, enabling it to operate efficiently even under conditions of low external CO2 concentration.
The reason C4 photosynthesis requires more energy in the form of ATP is because of the additional steps to initially fix CO2 into a four-carbon compound, as opposed to C3 photosynthesis where CO2 is directly fixed into a three-carbon compound. However, this extra energy expense is balanced by increased efficiency in water use and the ability to function effectively in higher temperatures and dry conditions.