The ATP Requirement in C4 Cycle compared to C3 Cycle

The C4 cycle and the C3 cycle are two different pathways involved in photosynthesis. While both pathways ultimately produce glucose, they differ in terms of how carbon dioxide is fixed and the number of ATP molecules required.

C3 Cycle
1. The C3 cycle, also known as the Calvin cycle or the light-independent reactions, occurs in the stroma of the chloroplasts.
2. In this cycle, carbon dioxide is directly fixed by the enzyme ribulose bisphosphate carboxylase/oxygenase (RuBisCO) to form a three-carbon compound, phosphoglycerate (PGA).
3. PGA is then converted to glyceraldehyde-3-phosphate (G3P) with the help of ATP and NADPH, which are generated in the light-dependent reactions.
4. For every three molecules of carbon dioxide fixed, one molecule of G3P is formed.

C4 Cycle
1. The C4 cycle, also known as the Hatch-Slack pathway, is an additional pathway that some plants have evolved to increase their photosynthetic efficiency in hot and dry conditions.
2. In this pathway, carbon dioxide is initially fixed by the enzyme phosphoenolpyruvate carboxylase (PEP carboxylase) in the mesophyll cells of the leaves to form a four-carbon compound, oxaloacetate (OAA).
3. OAA is then converted to malate or aspartate and transported to the bundle sheath cells, where it releases carbon dioxide.
4. In the bundle sheath cells, carbon dioxide is fixed by RuBisCO to form PGA, which then proceeds through the C3 cycle.
5. Since the C4 cycle requires additional steps and energy expenditure to pump malate or aspartate between cells, it requires more ATP molecules compared to the C3 cycle.

The ATP Requirement
1. In the C3 cycle, each fixation of carbon dioxide requires 3 ATP molecules to produce one molecule of G3P.
2. In the C4 cycle, the initial fixation of carbon dioxide by PEP carboxylase in the mesophyll cells requires 2 ATP molecules per carbon dioxide molecule.
3. The subsequent regeneration of PEP in the mesophyll cells also requires 1 ATP molecule.
4. In the bundle sheath cells, where the C3 cycle occurs, each fixation of carbon dioxide requires 3 ATP molecules, similar to the C3 cycle.
5. Therefore, in total, for each fixation of carbon dioxide in the C4 cycle, 2 ATP molecules are required in the mesophyll cells and 3 ATP molecules are required in the bundle sheath cells, resulting in a total of 5 ATP molecules.
6. Comparatively, in the C3 cycle, only 3 ATP molecules are required.
7. Thus, the C4 cycle requires an additional 2 ATP molecules compared to the C3 cycle.
8. Therefore, the correct answer is option 'A': 12 extra ATP molecules are required in the C4 cycle compared to the C3 cycle.