The use of lipids and proteins as respiratory substrates

Alternative respiratory substrates are substances other than glucose, such as lipids and proteins, that can be oxidised by cells to release energy.

Lipids as respiratory substrates

How lipids can be used as a respiratory substrate:

1. Lipids are hydrolysed into glycerol and fatty acids.
2. Glycerol is converted into triose phosphate and enters the glycolysis pathway.
3. Fatty acids are broken down into two-carbon fragments and converted into acetyl coenzyme A, which enters the Krebs cycle.

Proteins as respiratory substrates

How proteins can be used as a respiratory substrate:

1. Proteins are hydrolysed into amino acids.
2. The amino group is removed from the amino acids (deamination).
3. Three-carbon compounds are converted into pyruvate, while four- and five-carbon compounds are converted into intermediates in the Krebs cycle.

The amino acids produced from hydrolysis of proteins are generally used only as a last resort for releasing energy because they have many other important, specialised functions.

Comparing the energy in respiratory substrates

Different respiratory substrates release different quantities of ATP.

This table compares the energy yields from different respiratory substrates, highlighting their unique roles in cellular respiration.

Respiratory quotients

The respiratory quotient (RQ) is the ratio of the volume of carbon dioxide produced to the volume of oxygen used during respiration.

Respiratory quotients can be calculated using the following formula:

$\text{RQ }=\frac{{\text{volume of CO}}_2\text{ produced }}{{\text{volume of O}}_2\text{ consumed }}$

These values can be measured using a respirometer.

Interpreting respiratory quotients

RQ values can tell us about various metabolic conditions. For example, we can work out the specific substrates being respired, as lipids and proteins require more oxygen for their oxidation so their RQ values are lower.

An RQ of more than 1 indicates that anaerobic respiration is occurring.

In plants, low RQ values might suggest that photosynthesis is occurring, as the CO₂ released during respiration is being used.