⛽ Alternative Respiratory Substrates

In the absence of sufficient glucose or depleted glycogen stores, the body resorts to other chemical molecules for respiration, namely lipids and proteins.

🧬 Lipids (Triglycerides)

Triglycerides, composed of a glycerol molecule and three fatty acids, can be broken down and utilized in respiration.

• Glycerol: Converted into pyruvate, which then enters the link reaction and Krebs cycle to produce ATP.

• Free Fatty Acids: Undergo beta oxidation to produce around 50 acetyl coenzyme A molecules, each of which can generate approximately 10 ATP molecules. Thus, from three fatty acids, at least 500 ATP molecules can be produced.

Fats have a high energy content due to the numerous carbon-hydrogen bonds in fatty acids, which release significant energy when broken.

💪 Proteins

When lipid reserves are also depleted, the body can utilize proteins as an energy source.

• Amino Acids: The building blocks of proteins, undergo deamination (removal of the amine group) to form pyruvate, which then enters the link reaction and Krebs cycle to generate ATP.

Reasons to avoid using protein for respiration:

• Proteins make up essential enzymes and structural components. Using proteins for respiration leads to muscle loss and enzyme deficiencies.

• The deamination process consumes ATP, reducing the net ATP production, making it inefficient.

🧮 Respiratory Quotient (RQ)

The respiratory quotient (RQ) is the ratio of carbon dioxide produced to oxygen consumed. It provides insights into the respiratory substrate being used.

RQ = CO2 produced/O2 consumed

For carbohydrates (glucose): $RQ = \frac{6 \text{ } CO_2}{6 \text{ } O_2} = 1$

For lipids: The RQ is approximately 0.7 due to the high oxygen consumption during beta oxidation.

For proteins: The RQ is about 0.9.

In experimental settings, RQ values can be calculated from given data and used to infer which respiratory substrate is being utilized.