Question

What is meant by the respiration quotient (RQ)? When will the value of RQ be 1 and when will it be less than 1?

Answer 1

Depending upon the respiratory substrate that is used in respiration the amount of oxygen is utilized and the amount of carbon dioxide is evolved. their ratio is calculated.

Complete answer:
Oxygen is consumed and carbon dioxide is released during aerobic respiration. The ratio of the volume of carbon dioxide evolved to the volume of oxygen utilized or consumed in aerobic respiration is called respiratory quotient (RQ). It is also known as the respiratory ratio.
RQ= Volume of carbon dioxide evolved/volume of oxygen consumed
The respiratory substrate in aerobic respiration alters the respiratory quotient.
If the respiratory substrate is carbon dioxide and it is completely utilized or consumed then the respiratory ratio is equal to 1 because an equal amount of oxygen and carbon dioxide are consumed and evolved.
${ C }_{ 6 }{ H }_{ 12 }{ O }_{ 6 }+{ 6O }_{ 2 } \rightarrow { 6CO }_{ 2 }+{ 6H }_{ 2 }O+Energy \\\ RQ= \frac{6CO_2}{6O_2}=1.0$
When fats are used as a respiratory substrate in the respiration. then the respiratory ratio will be less than one because the amount of oxygen consumed is more than the amount of carbon dioxide evolved.
$2({ C }_{ 51 }{ H }_{ 98 }{ O }_{ 6 })+{ 145O }_{ 2 } \rightarrow { 102CO }_{ 2 }+{ 98H }_{ 2 }O+energy\\\ RQ=\frac{102CO_2}{145O_2}=0.7$

Additional information:
- In living organisms, respiratory substrates are more than one
- Pure proteins and pure fats are not used as respiratory substrates
- Carbohydrates such as glucose is one of the important respiratory substrates in the body because most of the energy in the body is produced from glucose and is also one of the important essential fuel for brain cells, red blood cells, etc.
liver cells can oxidize lipids and excess amino acids.

Note: When proteins are used as respiratory substrates. the respiratory ratio is approximately 0.9 which is almost equal to one. This gives us evidence that proteins are important respiratory substrates next to carbohydrates.