Oxidative phosphorylation, anaerobic metabolism and RQ

What you’ll learn

• Explain chemiosmosis and oxygen’s role.
• Explain lactate formation and recovery.
• Interpret redox indicators, respirometers and respiratory quotient.

Understand the idea

Reduced NAD and FAD donate electrons to the mitochondrial electron transport chain in the inner membrane. Energy released during electron transfer drives proton pumping from the matrix to the intermembrane space. Protons flow back through ATP synthase, coupling their electrochemical gradient to ATP production. Oxygen accepts electrons at the end of the chain and combines with protons to form water. Cristae provide membrane area; an intact inner membrane maintains the gradient.

When electron transfer cannot oxidise reduced NAD fast enough, animal cells can reduce pyruvate to lactate and regenerate NAD for glycolysis. This permits a small ATP yield without oxygen directly entering that pathway. After activity, lactate can be converted back to pyruvate for oxidation or used in glucose synthesis, particularly in the liver. Continued elevated oxygen consumption supports recovery processes; lactate is not merely a permanently accumulating waste.

Respiratory quotient compares carbon dioxide production with oxygen consumption in matching units. Carbohydrate oxidation gives RQ near one; fats generally give lower values. In a respirometer with carbon dioxide absorbed, gas-volume change estimates oxygen uptake after temperature, pressure and apparatus effects are controlled. Paired appropriate measurements can establish RQ. A redox indicator in a supervised yeast investigation changes when reduced, providing an indirect measure that needs consistent volumes, temperature and starting conditions.

Key formulae

$$\mathrm{RQ}=\frac{\text{carbon dioxide produced}}{\text{oxygen consumed}}$$

$$\text{oxygen uptake rate}=\frac{\text{volume change}}{\text{time}\times\text{mass}}$$

Worked example

A sample consumes $1.20\,\mathrm{cm^3}$ oxygen and releases $0.84\,\mathrm{cm^3}$ carbon dioxide in the same interval. Calculate RQ and interpret it cautiously.

1. Both volumes refer to the same interval and conditions.
2. $\mathrm{RQ}=0.84/1.20=0.70$.
3. This is consistent with a substantial contribution from fat oxidation, but mixtures of substrates and measurement uncertainty prevent identification of a single molecule.

Answer: RQ is 0.70, consistent with predominantly fat-based respiration under the usual assumptions.