 # Respiratory Quotient And Aerobic Respiration

Respiration by definition describes it as the act of breathing; the inhalation and exhalation of gases are well familiar with each one of us. This time, we venture across a different aspect of it.

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Let’s see what is respiratory quotient and how it relates to aerobic respiration. ## Respiratory Quotient

Aerobic respiration is the process most living things undergo to use food energy efficiently. In this aerobic respiration, oxygen is consumed and carbon dioxide is released. The actual ratio of the volume of carbon dioxide evolved to the volume of oxygen consumed during the act of respiration is called the respiratory quotient (RQ) or the respiratory ratio.

$RQ$ = $\frac{Volume~ of ~Carbon~ dioxide~ evolved}{ Volume~ of~ Oxygen~ consumed}$

The respiratory quotient depends on the type of respiratory substrate used during the act of respiration. Additionally, the respiratory quotient is a dimensionless number used in the calculations of the basal metabolic rate or BMR when estimated from the carbon dioxide production. These measurements are forms of indirect calorimetry. It happens to be measured using Ganong’s respirometer.

When the carbohydrates are completely oxidized which are used as a substrate, the respiratory quotient becomes one. Here equal amounts of carbon dioxide and oxygen are evolved and consumed, respectively.

$C_6H_{12}O_6 + 6 O_2 → 6 CO_2+ 6 H_2O + Energy$

$RQ$ = $\frac{6 ~CO_2}{6 ~O_2}$

But at the moment fats tend to involve in the act of respiration, the respiratory quotient drops to lower than one.

For example,

$C_{16}H_{32}O_2 + 23 O_2 → 16 CO_2 + 16 H_2O$

$RQ$ = $\frac{16~ CO_2}{23~ O2}$ = $0.696$

The RQ is approximately 0.7.

When proteins are respiratory substrates the ratio would be about 0.9. For example,

$C_{72}H_{112}N_{18}O_{22}S + 77 ~O_2 → 63~ CO_2 + 38~ H_2O + SO_3 + 9 ~CO (NH_2)_2$

$RQ$ = $\frac{63 ~CO_2}{77 ~O_2}$ = $0.8$

The RQ for protein is approximately 0.8.

One must recognize that in living organisms the respiratory quotient is rarely one, in fact, most often, it happens to be more than one. Pure proteins and fats are quite honestly never used as respiratory substrates.

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