The respiratory quotient

A clue in the gases

A student puts germinating seeds in a closed tube. The seeds take in oxygen and give out carbon dioxide. Are the two volumes equal?

They are not always equal. The ratio depends on what is being respired. This ratio has a name.

It is the respiratory quotient (RQ), also called the respiratory ratio. A ratio has no unit, but the two volumes must be in the same unit, for example mL.

Gas volumes are in the ratio of their moles, so the RQ of a substrate follows from the balanced equation.

Carbohydrates: RQ = 1

When a carbohydrate such as glucose is oxidised completely, 6 carbon dioxide are released for 6 oxygen taken in.

Fats: RQ = 0.7

A fat has much less oxygen in its molecule than a carbohydrate, so it needs more oxygen from the air to burn. The ratio falls below 1. For the fat tripalmitin:

Proteins: RQ about 0.9

When a protein is the substrate, the ratio is about 0.9.

Organic acids: RQ above 1

An organic acid has more oxygen in its molecule than a carbohydrate does. So it needs less oxygen from the air, and the RQ is above 1. For oxalic acid:

This detail is a little beyond the textbook. What you need to remember is the direction: below 1 for fats and proteins, above 1 for acids. In a cell without oxygen, carbon dioxide is released and no oxygen is taken in, so the RQ is very large.

In real tissues

A living tissue never respires one pure substance. Pure proteins or fats are never the only substrates, so the RQ of a real tissue lies between the values above. A reading of 0.85, for example, suggests a mixture of fat and carbohydrate.

How it is measured

In a closed tube with germinating seeds, a small cup of KOH takes in the carbon dioxide. So the gas volume falls by the oxygen that is taken in. In a second tube with water in place of KOH, the carbon dioxide stays in the gas, and the volume falls by the oxygen taken in minus the carbon dioxide released. From the two readings, you can find both volumes and the RQ.

You should find that equal volumes give an RQ of 1. A smaller volume of carbon dioxide gives a value below 1, which points to fat. A larger volume gives a value above 1.

Try it in Python

The program works out the RQ from the equations of three substrates.

Practise with new numbers

Common mistakes

• Dividing oxygen by carbon dioxide. The carbon dioxide goes on top.
• Giving fats an RQ above 1. Fats need more oxygen, so the RQ is 0.7.
• Thinking that the RQ of a plant is always 1. It is 1 only for carbohydrate.
• Using different units for the two volumes. Both must be in the same unit.
• Saying that a real tissue has a single substrate. The RQ of a tissue is a mixture.

Remember these

• RQ is the volume of carbon dioxide released divided by the volume of oxygen taken in. It has no unit.
• Carbohydrate: 1. Fat: 0.7. Protein: about 0.9. Organic acid: more than 1.
• In real tissues, the substrates are mixed, so the RQ lies between the pure values.
• KOH takes in the carbon dioxide, so the fall of volume in its tube is the oxygen taken in.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Write the RQ as a ratio.

2. Which substrates have an RQ of 1, 0.7 and about 0.9?

3. Why is the RQ of a fat below 1?

4. A tissue takes in 30 mL of oxygen and releases 30 mL of carbon dioxide. What is the RQ, and what is the likely substrate?

5. What does KOH do in an experiment to find the RQ?