Final answer:
The citric acid cycle conversion coupled with ATP synthesis is the transformation of Succinyl-CoA to succinate as it directly generates GTP, which can be converted into ATP.
Step-by-step explanation:
The correct answer is b) Succinyl-CoA to succinate. This conversion is coupled with the synthesis of guanosine triphosphate (GTP), which is then used to generate adenosine triphosphate (ATP). In the citric acid cycle, the enzyme succinyl CoA synthetase catalyzes the reaction in which succinyl-CoA is converted to succinate. This step is unique as it is an example of substrate-level phosphorylation where a high-energy thioester bond in succinyl-CoA is used to generate ATP (or GTP) directly.
As such, it is coupled to an energy-converting step, distinguishing it from other steps in the cycle that are primarily involved in redox reactions or structural rearrangements of the molecules. During each turn of the citric acid cycle, there is a generation of three NADH, one FADH2, and one molecule of GTP (which can be converted to ATP), highlighting the importance of this cycle in cellular energy production.