Answer: Acetyl CoA combining with oxaloacetate to form citrate.
- A Citrate combining with NADH to form isocitrate
- B Acetyl CoA combining with oxaloacetate to form citrate
- C Pyruvate combining with CO<sub>2</sub> to form oxaloacetate
- D Succinyl CoA combining with fumarate to form malate
Correct answer: B. Acetyl CoA combining with oxaloacetate to form citrate
Explanation: The Krebs cycle begins when the 2-carbon acetyl CoA combines with the 4-carbon oxaloacetate to form the 6-carbon citrate, catalyzed by citrate synthase.
Aerobic respiration unfolds in three locations: glycolysis in the cytoplasm yields a small amount of ATP directly, the Krebs cycle in the mitochondrial matrix harvests electron carriers (NADH, FADH2), and the electron transport chain in the inner mitochondrial membrane uses those carriers to generate the bulk of the ATP, with O2 needed only at this final stage.
Concept context
Glycolysis, the Krebs cycle, the electron transport chain, fermentation, and the respiratory quotient. The cellular energy-release counterpart to photosynthesis.