Respiratory Quotient (RQ)
The respiratory quotient (RQ), also known as the respiratory ratio, is a crucial concept in understanding the efficiency and type of metabolism occurring within an organism. It is defined mathematically as:
$$ RQ = \frac{\text{Volume of } CO_2 \text{ evolved}}{\text{Volume of } O_2 \text{ consumed}} $$
The value of RQ varies depending on the substrate used in respiration:
- For Carbohydrates: When carbohydrates are oxidized completely, the RQ equals 1.
Example Reaction:
$$ C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy} $$
This results in:
$$ RQ = \frac{6CO_2}{6O_2} = 1.0 $$
- For Fats: The RQ is less than 1.
For instance, when tripalmitin (a fatty acid) is oxidized:
$$ 2(C_{51}H_{98}O_6) + 145 O_2 \rightarrow 102 CO_2 + 98 H_2O + \text{Energy} $$
The calculation yields:
$$ RQ = \frac{102 CO_2}{145 O_2} \approx 0.7 $$
- For Proteins: The RQ for protein metabolism typically hovers around 0.9.
Recognizing that living organisms do not exclusively utilize one substrate is essential; they metabolize a combination of carbohydrates, proteins, and fats, which results in variable RQ values during respiration. Understanding RQ allows for insights into the metabolic state and efficiency of organisms in different physiological conditions.