State the respiratory quotient of a carbohydrate and explain why it has that value.

It is 1.0, because the balanced equation for the oxidation of glucose shows six molecules of carbon dioxide produced for every six molecules of oxygen used.

State the approximate respiratory quotients of a lipid and of a protein.

About 0.7 for a lipid and about 0.9 for a protein.

A set of germinating seeds consumed 15.0 cm³ of oxygen and released 10.5 cm³ of carbon dioxide in the same period. Calculate the respiratory quotient and identify the substrate being used.

The respiratory quotient is 10.5 ÷ 15.0 = 0.70, which indicates that lipid is the main respiratory substrate.

OCR A-Level Biology B H422

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Cellular Respiration

Respiratory substrates and respiratory quotients

3 things to know
Components 01, 02 and 03

Overview

Recall questions on respiratory substrates, calculating and interpreting respiratory quotients for carbohydrate, lipid and protein, quotients above 1.0 and in fermenting yeast, the energy released per gram by lipid and carbohydrate, and how amino acids enter respiration.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Explain why the respiratory quotient of a lipid is lower than that of a carbohydrate.

Key term

What is a respiratory substrate?

Key term

Define the respiratory quotient.

Questions from this subtopic

Each one with the model answer underneath.

1
What is a respiratory substrate?
Model answer
A respiratory substrate is an organic molecule that can be oxidised in respiration to release energy for the synthesis of ATP.
2
Define the respiratory quotient.
Model answer
The respiratory quotient is the volume of carbon dioxide produced divided by the volume of oxygen consumed in the same time.
3
State the respiratory quotient of a carbohydrate and explain why it has that value.
Model answer
It is 1.0, because the balanced equation for the oxidation of glucose shows six molecules of carbon dioxide produced for every six molecules of oxygen used.
4
State the approximate respiratory quotients of a lipid and of a protein.
Model answer
About 0.7 for a lipid and about 0.9 for a protein.
5
Explain why the respiratory quotient of a lipid is lower than that of a carbohydrate.
Model answer
A lipid contains proportionally far less oxygen and more hydrogen than a carbohydrate, so much more oxygen must be taken in from outside to oxidise it, while the carbon dioxide released per carbon atom is unchanged.
6
A set of germinating seeds consumed 15.0 cm³ of oxygen and released 10.5 cm³ of carbon dioxide in the same period. Calculate the respiratory quotient and identify the substrate being used.
Model answer
The respiratory quotient is 10.5 ÷ 15.0 = 0.70, which indicates that lipid is the main respiratory substrate.

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