In which stage of cellular respiration, is the initial input (glucose), considered to have been completely oxidised?

Question

BlackTom AI · Accepted Answer

Complete oxidation of glucose means all six carbon atoms of C6H12O6 have been converted to CO2. That full conversion is achieved by the end of the citric acid (Krebs) cycle.

Relevant knowledge points:
- Glycolysis (cytoplasm) breaks glucose (6C) into two pyruvate molecules (2 × 3C). No CO2 is released during glycolysis, so glucose is only partially oxidized.
- Pyruvate oxidation (pyruvate → acetyl‑CoA) converts each 3C pyruvate to a 2C acetyl group, releasing one CO2 per pyruvate and producing NADH. This step removes 2 of the original 6 carbons as CO2 (total so far: 2 CO2), but carbons remain in acetyl‑CoA.
- Citric acid cycle (mitochondrial matrix) oxidizes each acetyl (2C) completely to 2 CO2. For two acetyl‑CoA (from one glucose) this yields 4 CO2. Combined with the 2 CO2 from pyruvate oxidation, all 6 carbons from glucose have been released as CO2 (total: 6 CO2).
- Electron transport chain (ETC) oxidizes NADH/FADH2 to regenerate NAD+ and FAD and reduces O2 to H2O; it does not convert carbon atoms to CO2.

Step-by-step carbon accounting:
1. Glucose (6C) → 2 pyruvate (2 × 3C) during glycolysis (0 CO2).
2. 2 pyruvate (2 × 3C) → 2 acetyl‑CoA (2 × 2C) + 2 CO2 during pyruvate oxidation (2 CO2 released).
3. Citric acid cycle: each acetyl‑CoA (2C) → 2 CO2; for two acetyl groups → 4 CO2 released.
4. Total CO2 produced = 2 (pyruvate oxidation) + 4 (citric acid cycle) = 6 CO2 → all glucose carbons oxidized.

Why answer b is correct:
The citric acid cycle is the stage that completes oxidation of the acetyl groups derived from glucose, resulting in conversion of all carbons to CO2.

Why other choices are incorrect:
- a. Pyruvate oxidation: only partial oxidation; removes 2 of 6 carbons as CO2.
- c. Glycolysis: no CO2 release; glucose still intact as carbons in pyruvate.
- d. Electron transport chain: handles electron transfer and ATP synthesis, not further oxidation of glucose carbons to CO2.

Therefore, the correct answer is b. Citric acid cycle.

In which stage of cellular respiration, is the initial input (glucose), considered to have been completely oxidised?单项选择题

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