When a poison such as cyanide blocks the electron transport chain, glycolysis and the citric acid cycle also eventually stop working. Which of the following is the best explanation for this?

• A. A high level of NADH is present in the cell.
• B. The uptake of oxygen stops because electron transport was inhibited.
• C. Electrons are no longer available from the electron transport chain to power glycolysis and the citric acid cycle.
• D. They run out of ADP.
• E. NAD⁺ and FAD are not available for glycolysis and the citric acid cycle to continue.

Answer: (E)

Verified step by step guidance

1. Understand the role of the electron transport chain (ETC) in cellular respiration: The ETC is the final stage of aerobic respiration, where electrons are transferred through a series of proteins to ultimately reduce oxygen to water. This process generates a proton gradient used to produce ATP.
2. Recognize the importance of NAD+ and FAD in cellular respiration: NAD+ and FAD are coenzymes that accept electrons during glycolysis and the citric acid cycle, becoming NADH and FADH2. These reduced forms carry electrons to the ETC.
3. Identify the impact of blocking the ETC: When the ETC is blocked, NADH and FADH2 cannot donate their electrons, leading to an accumulation of these reduced coenzymes and a depletion of their oxidized forms, NAD+ and FAD.
4. Connect the availability of NAD+ and FAD to glycolysis and the citric acid cycle: Both glycolysis and the citric acid cycle require NAD+ (and FAD in the citric acid cycle) to accept electrons. Without these oxidized coenzymes, these pathways cannot continue to function.
5. Conclude why glycolysis and the citric acid cycle stop: The lack of NAD+ and FAD due to the blockage of the ETC prevents the continuation of glycolysis and the citric acid cycle, as these processes rely on the recycling of these coenzymes to proceed.