Citric Acid Cycle Practice Test 2025 - Free Biochemistry Practice Questions and Study Guide

The correct answer is based on the biochemical process of converting succinyl CoA to succinate within the Citric Acid Cycle. In this reaction, the high-energy thioester bond present in succinyl CoA is broken. This bond is energetically rich, and the energy released during its cleavage is utilized to drive the phosphorylation of GDP to form GTP, a process known as substrate-level phosphorylation.

When the thioester bond is broken, it creates a more stable product, succinate, while providing enough energy to facilitate the addition of an inorganic phosphate to GDP, thereby synthesizing GTP. This is a classic example of how the energy contained in certain molecular bonds, specifically thioester bonds in this case, is harnessed to produce a high-energy molecule during metabolic processes.

The other options do not apply directly to this specific mechanism. Oxidative phosphorylation, for example, is associated with the electron transport chain and does not take place during this step of the Citric Acid Cycle. ATP hydrolysis can provide energy in other contexts, but it is not the primary driver for the phosphorylation of GDP in this reaction. The release of CO2 does occur during the conversion of succinyl CoA to succinate, but it