Respiratory Quotient RQ: Value and Significance Explained

Respiratory Quotient, or RQ, is the ratio of carbon dioxide released to oxygen consumed during respiration. It tells us what kind of food substance is being oxidised — carbohydrate, fat, protein, or organic acid — without needing to analyse the tissues directly. Doctors, plant physiologists, and exercise scientists all use RQ (called RER in humans) as a quick window into metabolism.

This matters because RQ changes with diet, seed type, and health conditions. Germinating fatty seeds, ripening fruits, and patients on ventilators all show distinctive RQ values that guide decisions. It also helps verify the overall respiration equation and detect anaerobic respiration when oxygen uptake drops to zero.

Key Concepts

Definition: RQ = volume of CO2 evolved ÷ volume of O2 consumed, measured over the same time under the same conditions. For pure carbohydrates like glucose, RQ = 1.0. For fats, more oxygen is needed to oxidise carbon-hydrogen rich molecules, so RQ is about 0.7. For proteins, RQ is about 0.9. For organic acids like malic acid, the substrate already contains much oxygen, so RQ exceeds 1, about 1.33. During purely anaerobic respiration, CO2 may be released with no O2 uptake, so RQ is infinite. A Ganong respirometer is the classic school instrument for measuring RQ.

Formulas

RQ = CO2 released / O2 absorbed. Both volumes in the same units (mL or mol) at same temperature and pressure. Glucose: C6H12O6 + 6O2 → 6CO2 + 6H2O, so RQ = 6 ÷ 6 = 1.0. Fat (tripalmitin): 2(C51H98O6) + 145O2 → 102CO2 + 98H2O, so RQ = 102 ÷ 145 ≈ 0.70. Assumptions: complete aerobic oxidation, no CO2 retained or extra released by other processes. If fermentation also occurs, measured RQ rises above the pure aerobic value and must be interpreted carefully.

Worked Example

A respirometer shows germinating seeds consumed 45 mL O2 and released 31.5 mL CO2 in one hour. Calculate RQ and identify the substrate. Step 1: RQ = 31.5 ÷ 45 = 0.70. Step 2: Compare with known values: carbohydrate 1.0, protein 0.9, fat 0.7. Step 3: RQ 0.70 matches fat. Conclusion: the seeds are oily seeds like castor or groundnut, oxidising stored fat. If instead CO2 released were 45 mL, RQ would be 1.0, indicating starchy seeds like wheat respiring carbohydrate.

Common Misconceptions

• RQ is always 1. Correction: Only carbohydrates give 1; fats give about 0.7 and organic acids give more than 1.
• RQ above 1 means something is wrong. Correction: It is normal for organic acid respiration and for tissues converting carbohydrate to fat, where extra CO2 is released.
• RQ measures photosynthesis. Correction: RQ describes respiration only; photosynthesis has a separate photosynthetic quotient.

Real-World Applications

Seed technologists use RQ to judge what reserves a germinating seed is mobilising. Clinicians use respiratory exchange ratio to set nutrition for ICU patients and to detect overfeeding, which pushes RQ above 1. Sports scientists track RQ during exercise tests: rising toward 1.0 signals a shift to carbohydrate burning at high intensity. Food storage engineers monitor RQ of stored fruits to detect anaerobic spoilage early. Plant ecologists also compare RQ across species to understand metabolic strategies.

RQ is simply CO2 out divided by O2 in: 1.0 for carbs, about 0.7 for fats, about 0.9 for proteins, and above 1 for organic acids. One easy gas measurement thus reveals the fuel a living tissue is burning.