Which pathway produces the most ATP molecules? Select one: a. lactic acid fermentation b. glycolysis c. the Krebs cycle (citric acid cycle) d. the electron transport chain
The correct answer and explanation is:
Correct Answer: d. the electron transport chain
Explanation (300 Words):
ATP (adenosine triphosphate) is the primary energy currency of cells. It is produced through several metabolic pathways: glycolysis, the Krebs cycle (citric acid cycle), lactic acid fermentation, and the electron transport chain (ETC). Among these, the electron transport chain produces the most ATP molecules.
Let’s break down why:
- Glycolysis:
- Occurs in the cytoplasm.
- Breaks one glucose molecule into two pyruvate molecules.
- Produces a net gain of 2 ATP molecules per glucose.
- Lactic Acid Fermentation:
- Anaerobic process (does not require oxygen).
- Converts pyruvate into lactic acid.
- Produces only 2 ATP per glucose (from glycolysis alone).
- Used by muscle cells in low oxygen conditions.
- Krebs Cycle (Citric Acid Cycle):
- Takes place in the mitochondria.
- Processes each pyruvate from glycolysis.
- Generates 2 ATP per glucose (1 ATP per cycle × 2 cycles per glucose).
- Also produces high-energy electron carriers (NADH, FADH₂).
- Electron Transport Chain (ETC):
- Occurs in the inner mitochondrial membrane.
- Utilizes NADH and FADH₂ (from glycolysis and Krebs cycle).
- Electrons move through protein complexes, creating a proton gradient.
- ATP synthase uses this gradient to make ATP.
- Produces about 32–34 ATP molecules per glucose.
Thus, while glycolysis and the Krebs cycle prepare the energy carriers (NADH, FADH₂), the ETC is the powerhouse that converts most of that stored energy into ATP using oxygen as the final electron acceptor. Without the ETC, aerobic organisms wouldn’t be able to meet their energy demands efficiently.
In summary, the electron transport chain is the most efficient ATP-producing step in cellular respiration.