Cellular respiration stoichiometry: For complete aerobic oxidation of carbohydrate, how many molecules of CO2 are exhaled for each molecule of O2 consumed (i.e., the respiratory quotient for carbohydrates)?

Introduction / Context:
The respiratory quotient (RQ) links gas exchange to metabolism. For substrates like glucose oxidized aerobically, RQ helps quantify how many carbon dioxide molecules are produced per oxygen molecule consumed, a classic physiology and biochemistry concept.

Given Data / Assumptions:

• Aerobic, steady-state oxidation of a carbohydrate such as glucose.
• Complete oxidation: glucose + O2 → CO2 + H2O with ATP production.
• No contribution from fats or proteins (which have different RQ values).

Concept / Approach:
For glucose, the balanced reaction is commonly written as: C6H12O6 + 6 O2 → 6 CO2 + 6 H2O. RQ is defined as CO2 produced / O2 consumed. For pure carbohydrate, this ratio equals 1. This reflects the matching of carbon oxidation to oxygen reduction when the substrate has an average oxidation state compatible with complete combustion to CO2 and H2O.

Step-by-Step Solution:
Write the overall equation: C6H12O6 + 6 O2 → 6 CO2 + 6 H2O.Compute the ratio: CO2 produced / O2 consumed = 6 / 6.Simplify the ratio to 1 CO2 per 1 O2.Therefore, for each O2, one CO2 is exhaled on average during carbohydrate oxidation.

Verification / Alternative check:
Indirect calorimetry in exercise physiology routinely measures RQ ≈ 1.0 for pure carbohydrate metabolism, confirming the stoichiometric calculation.

Why Other Options Are Wrong:

• 3, 6, 12: These do not reflect the standard molar ratio and would imply non-physiologic stoichiometry for carbohydrate.

Common Pitfalls:
Confusing mixed-fuel oxidation (fats have RQ ≈ 0.7; proteins ≈ 0.8–0.9) with pure carbohydrate; always specify the substrate when interpreting RQ.

Final Answer:
1