The respiratory quotient varies due to

Options

A. Respiratory substrate
B. Light and `O_2`
C. Respiratory product
D. Temperature

Answer

To solve the question regarding the variation of the respiratory quotient (RQ), we will follow these steps:

### Step 1: Understand the Concept of Respiratory Quotient (RQ)
The respiratory quotient (RQ) is defined as the ratio of the volume of carbon dioxide (CO2) produced to the volume of oxygen (O2) consumed during respiration. It is expressed mathematically as:
\[ RQ = \frac{\text{Volume of CO2 evolved}}{\text{Volume of O2 consumed}} \]

### Step 2: Identify Factors Affecting RQ
The RQ can vary based on different factors, primarily the type of respiratory substrate used. The main substrates include carbohydrates, fats, and proteins.

### Step 3: Analyze Different Respiratory Substrates
1. **Carbohydrates**: When carbohydrates are used as the respiratory substrate, the RQ is typically equal to 1. This is because the complete oxidation of glucose (a common carbohydrate) produces an equal amount of CO2 and consumes O2.

2. **Fats**: When fats are used as the substrate, the RQ is usually less than 1. This is due to the fact that fats yield more energy per molecule but produce less CO2 relative to O2 consumed.

3. **Proteins**: When proteins are metabolized, the RQ is approximately 0.9, as the oxidation of proteins also results in a different ratio of CO2 produced to O2 consumed.

### Step 4: Conclusion
Based on the analysis, the respiratory quotient varies primarily due to the type of respiratory substrate being utilized. Therefore, the correct answer to the question is:

**Answer: A. Respiratory substrate**