Final answer:
The Citric Acid Cycle requires oxygen indirectly because it relies on the electron transport chain, which uses oxygen. Without oxygen, the electron carriers NADH and FADH2 cannot be recycled, leading to a halt in the cycle.
Step-by-step explanation:
The Citric Acid Cycle (CAC), also known as the Krebs Cycle, takes place in the mitochondria and involves a series of reactions that produce energy carriers such as NADH and FADH2. Although the cycle itself does not use oxygen directly, it is an aerobic process because the NADH and FADH2 generated must transfer their electrons to the electron transport chain (ETC), which uses oxygen as the final electron acceptor. If there is insufficient oxygen, the ETC can't operate. As a result, NADH and FADH2 cannot offload their electrons and return to their oxidized form, which is necessary for the continuation of the cycle. Thus, under anaerobic conditions, the CAC shuts down because the required regeneration of NAD+ and FAD is halted, preventing the further oxidation of metabolites and stalling the cycle.