Advanced Placement (AP) Biology Practice Exam

What is produced alongside NADH and FADH2 in the Krebs cycle?

1. Oxygen

2. Water

3. ATP

4. Carbon dioxide

The correct answer is: ATP

In the Krebs cycle, also known as the citric acid cycle, ATP is indeed produced alongside NADH and FADH2. This cycle involves a series of enzymatic reactions that process acetyl-CoA into carbon dioxide while transferring energy to electron carriers. During this process, one molecule of ATP (or GTP, which is readily converted to ATP in some organisms) is produced directly through substrate-level phosphorylation for each turn of the cycle. While the other compounds mentioned in the choices may be associated with cellular respiration and metabolic processes, they are not produced directly in the Krebs cycle in the same sense as ATP. Carbon dioxide is a product of the decarboxylation reactions during the Krebs cycle, where carbon is lost from the citric acid molecules, contributing to cellular respiration as a waste product. Oxygen and water, while vital in the overall process of cellular respiration, are not products of the Krebs cycle itself; rather, oxygen is used during the electron transport chain, and water is formed at that stage as a byproduct of the reduction of oxygen. Thus, the production of ATP in the Krebs cycle highlights its role as a key metabolic pathway facilitating energy transfer.