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 have been converted to CO2; this occurs during the citric acid cycle (CAC).

1. Relevant knowledge points and concepts
- Cellular respiration has four main stages: glycolysis, pyruvate oxidation (link reaction), citric acid cycle (Krebs cycle), and the electron transport chain (ETC)/oxidative phosphorylation.
- “Complete oxidation” refers to conversion of the carbon skeleton of glucose (C6H12O6) into CO2.
- The CAC oxidizes the acetyl groups (from acetyl-CoA) completely to CO2; NADH and FADH2 produced feed the ETC.

2. Step-by-step reasoning
- Start with one glucose (6 carbons).
- Glycolysis: glucose → 2 pyruvate (each 3C). No CO2 released in glycolysis; glucose is only partially oxidized to pyruvate.
- Pyruvate oxidation: each pyruvate (3C) → acetyl-CoA (2C) + CO2. For one glucose (2 pyruvates) this produces 2 CO2 and 2 acetyl-CoA.
- Citric acid cycle: each acetyl-CoA (2C) enters the CAC and is fully oxidized to 2 CO2. For two acetyl-CoA (from one glucose) the CAC produces 4 CO2.
- Total CO2 from pyruvate oxidation + CAC = 2 (from pyruvate oxidation) + 4 (from CAC) = 6 CO2, accounting for all six carbons of glucose.

3. Calculations / intermediate steps
- Glycolysis: 6C glucose → 2 × (3C pyruvate) (CO2 = 0)
- Pyruvate oxidation: 2 × (3C → 2C + CO2) → CO2 = 2
- CAC: 2 × (acetyl 2C → 2 CO2) → CO2 = 4
- Total CO2 = 0 + 2 + 4 = 6 CO2 (complete oxidation)

4. Why the correct answer is correct
- After the citric acid cycle, every carbon from the original glucose molecule has been released as CO2, so glucose is considered completely oxidized at the end of the CAC.

5. Why other options are incorrect
- Glycolysis (d): only partially oxidizes glucose to pyruvate; no complete oxidation.
- Pyruvate oxidation (c): removes one CO2 per pyruvate but does not oxidize all carbons from glucose.
- Electron transport chain (b): consumes electrons from NADH/FADH2 and reduces O2 to water to generate ATP; it does not further oxidize the carbon skeleton (CO2 formation is completed earlier).

6. Conclusion
- Therefore, the citric acid cycle (a) is the stage at which glucose is considered completely oxidized, matching the provided answer.

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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