Why do lipids yield more energy per gram than carbohydrates when used as respiratory substrates?

Lipids contain a higher proportion of hydrogen atoms relative to oxygen atoms, meaning more hydrogen is available to donate electrons to the electron transport chain, resulting in more ATP production per unit mass.

Which type of respiratory substrate has a higher relative energy value per gram: lipids or carbohydrates?

Lipids have a higher relative energy value per gram than carbohydrates.

In terms of molecular structure, what feature of lipid molecules accounts for their greater energy yield compared to carbohydrates?

Lipid molecules are more reduced (contain more C–H bonds and fewer C–O bonds) than carbohydrates, so more oxidation can occur during respiration, releasing more energy.

If an organism needed to store the maximum amount of energy in the minimum mass, would it be more efficient to store lipids or carbohydrates? Explain why.

Lipids would be more efficient because they have a higher energy value per gram than carbohydrates, allowing more energy to be stored per unit mass.

Why do lipids have a higher relative energy value per gram than carbohydrates?

Lipids have more hydrogen atoms per molecule than carbohydrates, so more reduced NAD and FAD are produced, leading to more ATP synthesis.

How can proteins be used as respiratory substrates?

Proteins can be used after deamination, with the remaining carbon skeleton entering respiration pathways.

OCR A-Level Biology H420

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Respiration

Respiratory substrates and the respiratory quotient

1 thing to know
Components 01 and 03

Overview

Why lipids yield more energy per unit mass than carbohydrates, how proteins are used, and the respiratory quotient, the respirometer and the variables to control.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

Define the respiratory quotient (RQ).

Questions from this subtopic

Each one with the model answer underneath.

1
Why do lipids yield more energy per gram than carbohydrates when used as respiratory substrates?
Model answer
Lipids contain a higher proportion of hydrogen atoms relative to oxygen atoms, meaning more hydrogen is available to donate electrons to the electron transport chain, resulting in more ATP production per unit mass.
2
Which type of respiratory substrate has a higher relative energy value per gram: lipids or carbohydrates?
Model answer
Lipids have a higher relative energy value per gram than carbohydrates.
3
In terms of molecular structure, what feature of lipid molecules accounts for their greater energy yield compared to carbohydrates?
Model answer
Lipid molecules are more reduced (contain more C–H bonds and fewer C–O bonds) than carbohydrates, so more oxidation can occur during respiration, releasing more energy.
4
If an organism needed to store the maximum amount of energy in the minimum mass, would it be more efficient to store lipids or carbohydrates? Explain why.
Model answer
Lipids would be more efficient because they have a higher energy value per gram than carbohydrates, allowing more energy to be stored per unit mass.
5
Why do lipids have a higher relative energy value per gram than carbohydrates?
Model answer
Lipids have more hydrogen atoms per molecule than carbohydrates, so more reduced NAD and FAD are produced, leading to more ATP synthesis.
6
How can proteins be used as respiratory substrates?
Model answer
Proteins can be used after deamination, with the remaining carbon skeleton entering respiration pathways.

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AA phospholipid contains one glycerol molecule.

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