Advanced Placement (AP) Biology Practice Exam

Which pathway leads to the production of ATP, NADH, and FADH2?

1. Glycolysis

2. Krebs cycle

3. Electron transport chain

4. All of the above

The correct answer is: Krebs cycle

The Krebs cycle, also known as the citric acid cycle or tricarboxylic acid (TCA) cycle, is a central metabolic pathway that takes place in the mitochondrial matrix. During this cycle, acetyl-CoA, derived from carbohydrates, fats, or proteins, is oxidized through a series of enzymatic reactions. As the Krebs cycle progresses, it produces energy-rich molecules: ATP is generated directly through substrate-level phosphorylation, while NADH and FADH2 are produced as reduced coenzymes. NADH carries high-energy electrons to the electron transport chain, while FADH2 serves a similar role. These electron carriers are vital because they help drive the oxidative phosphorylation process, which ultimately leads to the majority of ATP production in cellular respiration. In contrast, glycolysis, which occurs in the cytoplasm, also produces ATP and NADH, but it does not produce FADH2. The electron transport chain itself does not produce ATP, NADH, or FADH2; rather, it utilizes the electron carriers produced in glycolysis and the Krebs cycle to generate a proton gradient, which in turn facilitates the production of ATP through ATP synthase. Hence, while glycolysis and the electron transport chain do play crucial