Energy: Respiration and Respiratory Substrates

This section focuses on the processes of respiration and how we can calculate the Respiratory Quotient (RQ) for different respiratory substrates. The RQ is a useful indicator of the type of fuel being used for energy production.

Respiration: The Process

Respiration is the process by which living organisms release energy from organic molecules. It can be broadly divided into two main types: aerobic respiration and anaerobic respiration.

Aerobic Respiration: This process occurs in the presence of oxygen and is the most efficient way to produce energy. The overall equation for aerobic respiration is:

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

Anaerobic Respiration: This process occurs in the absence of oxygen and is less efficient than aerobic respiration. There are different types of anaerobic respiration, including:

• Alcoholic Fermentation: Occurs in yeast and some bacteria, producing ethanol and carbon dioxide.
• Lactic Acid Fermentation: Occurs in muscle cells during intense exercise, producing lactic acid.

Respiratory Quotient (RQ)

The Respiratory Quotient (RQ) is the ratio of the volume of carbon dioxide produced to the volume of oxygen consumed during respiration.

$$RQ = \frac{Volume \, of \, CO_2 \, produced}{Volume \, of \, O_2 \, consumed}$$

The RQ value is characteristic of the respiratory substrate being used. It provides information about the proportion of carbon in the molecule that comes from carbohydrate, fat, or protein.

Calculating RQ Values

We can calculate the RQ for different respiratory substrates using the following equations:

• Carbohydrate: The balanced equation for the complete aerobic respiration of glucose is:
$$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$$

From this equation, we can see that for every mole of carbon in the glucose molecule, there are 6 moles of carbon dioxide produced and 6 moles of oxygen consumed. Therefore, the RQ for carbohydrate is:

$$RQ_{carbohydrate} = \frac{6}{6} = 1$$
• Fat: The balanced equation for the complete aerobic respiration of a fatty acid is:
$$C_6H_{12}O_2 + 12.5O_2 \rightarrow 6CO_2 + 6H_2O$$

From this equation, for every mole of carbon in the fatty acid molecule, there are 6 moles of carbon dioxide produced and 12.5 moles of oxygen consumed. Therefore, the RQ for fat is:

$$RQ_{fat} = \frac{6}{12.5} = 0.48$$
• Protein: The balanced equation for the complete aerobic respiration of a protein is:
$$C_6H_{14}N_2O_2 + 4O_2 \rightarrow 6CO_2 + 2H_2O + 4N_2$$

From this equation, for every mole of carbon in the protein molecule, there are 6 moles of carbon dioxide produced and 4 moles of oxygen consumed. Therefore, the RQ for protein is:

$$RQ_{protein} = \frac{6}{4} = 1.5$$

Summary of RQ Values

Respiratory Substrate	Respiratory Quotient (RQ)
Carbohydrate	1.0
Fat	0.48
Protein	1.5

Applications of RQ

The RQ value can be used to determine the primary source of energy for an organism. For example, if an organism has a high RQ value (e.g., 1.5), it is likely to be using a significant amount of protein for energy.