Question at position 12 What is the primary substrate entering the Citric Acid Cycle?PhosphocreatineGlucosePyruvateLactateAcetyl CoANADHCO2

Question

BlackTom AI · Accepted Answer

Acetyl CoA is the primary substrate entering the Citric Acid Cycle because it supplies the two‑carbon acetyl unit that condenses with oxaloacetate (4C) to form citrate (6C), the first step of the cycle catalyzed by citrate synthase.

1) Relevant knowledge points and concepts
- The Citric Acid Cycle (TCA/Krebs) accepts a 2‑carbon acetyl group as its input; that acetyl group is carried by coenzyme A as acetyl‑CoA.
- Pyruvate (3C) produced by glycolysis must be converted to acetyl‑CoA by the pyruvate dehydrogenase complex (PDH) before entering the cycle.
- Fatty acid β‑oxidation and certain amino acid catabolic pathways also produce acetyl‑CoA.
- NADH and CO2 are products/cofactors in related reactions, not primary cycle substrates. Phosphocreatine stores phosphate for rapid ATP regeneration; lactate is a reversible product of pyruvate metabolism.

2) Step‑by‑step reasoning
- The direct substrate that enters the cycle is the acetyl group attached to CoA. The key entry reaction:
  acetyl‑CoA (2C) + oxaloacetate (4C) → citrate (6C) (enzyme: citrate synthase).
- How acetyl‑CoA is formed from glucose:
  pyruvate + CoA + NAD+ → acetyl‑CoA + CO2 + NADH + H+ (enzyme: pyruvate dehydrogenase).
  This shows pyruvate itself does not directly enter the TCA; it must be converted to acetyl‑CoA.

3) Intermediate chemical/transformation steps
- Glycolysis: Glucose (6C) → 2 pyruvate (3C).
- PDH: pyruvate (3C) → acetyl (2C) + CO2 + NADH; acetyl + CoA → acetyl‑CoA.
- TCA entry: acetyl‑CoA + oxaloacetate → citrate.

4) Why the correct answer is correct
- The cycle requires a 2‑carbon acetyl donor; acetyl‑CoA is exactly that and is the molecular species that combines with oxaloacetate to start the TCA. Other listed molecules are either upstream precursors, energy carriers, or products.

5) Why other options are incorrect (brief)
- Phosphocreatine: energy reserve for rapid ATP regeneration, not a TCA substrate.
- Glucose: a precursor; it is metabolized to pyruvate and then to acetyl‑CoA before TCA entry.
- Pyruvate: must be converted to acetyl‑CoA first; it does not directly condense with oxaloacetate.
- Lactate: converted to pyruvate by lactate dehydrogenase, then to acetyl‑CoA; not direct TCA substrate.
- NADH: a reduced electron carrier produced by PDH and the TCA, not the cycle’s input substrate.
- CO2: a product of pyruvate oxidation and the TCA, not the substrate entering the cycle.

6) Conclusion
Therefore, Acetyl CoA is the correct primary substrate entering the Citric Acid Cycle, matching the provided answer.

Question at position 12 What is the primary substrate entering the Citric Acid Cycle?PhosphocreatineGlucosePyruvateLactateAcetyl CoANADHCO2单项选择题

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