Question

A respiratory quotient approaching 0.7 indicates metabolism primarily of which macromolecule? (A) Carbohydrates (B) Lipids (C) Nucleic acids (D) Amino acids

Short answer

(B) Lipids

Step-by-step solution

Understand the Respiratory Quotient (RQ)

The respiratory quotient (RQ) is a ratio used to gauge the metabolic substrates being utilized. It is defined as the ratio of the volume of carbon dioxide produced to the volume of oxygen consumed during metabolism.

Know the RQ Values for Different Macromolecules

Different macromolecules have characteristic RQ values. Carbohydrates have an RQ of approximately 1.0, proteins have an RQ around 0.8, and lipids have an RQ close to 0.7.

Identify the Macromolecule With RQ Near 0.7

An RQ approaching 0.7 indicates that lipid metabolism is predominant. This is because the RQ value for lipids is about 0.7.

Match the Correct Option

Given that an RQ approaching 0.7 indicates lipid metabolism, the correct choice among the options provided is (B) Lipids.

Key concepts

metabolic substrates

Metabolic substrates are the molecules our bodies use for energy. They basically act as the body's fuel source. The main types of metabolic substrates are carbohydrates, proteins, and lipids. Each of these metabolizes differently in the body, releasing energy that we need for various activities, such as exercising, thinking, and even sleeping. Carbohydrates are usually the body's first choice for energy. Proteins are essential but are not the primary energy source. Lipids, or fats, provide a concentrated form of energy, often used when carbohydrate levels are low. Metabolic substrates are essential for maintaining the body's energy balance and overall health.

RQ values

RQ values, or Respiratory Quotient values, help us understand which macromolecules our body is using for fuel. The RQ value is calculated by measuring the ratio of carbon dioxide produced to oxygen consumed during metabolism. Different macromolecules have distinct RQ values:

• Carbohydrates have an RQ of around 1.0, meaning for every molecule of oxygen consumed, one molecule of carbon dioxide is produced
• Proteins have an RQ of approximately 0.8
• Lipids stand out with an RQ close to 0.7

These values help to identify which type of nutrient is being metabolized under various physiological conditions.

lipid metabolism

Lipid metabolism involves the breakdown of fats for energy. This is a complex process since lipids provide more energy compared to carbohydrates and proteins. During lipid metabolism, lipids are broken down into glycerol and fatty acids. These components then undergo further processes, such as beta-oxidation, to produce ATP, the energy currency of the cell. Fatty acids are particularly efficient in energy production because they can yield large amounts of ATP. Lipid metabolism is usually dominant when fasting, during prolonged exercise, or when carbohydrate stores are low.

carbon dioxide to oxygen ratio

The carbon dioxide to oxygen ratio, measured through the Respiratory Quotient (RQ), provides insight into the type of fuel the body is using. A higher ratio suggests carbohydrate metabolism, while a lower ratio indicates lipids. For example:

• RQ of 1.0 typically signals carbohydrate metabolism
• RQ around 0.8 often indicates protein metabolism
• RQ close to 0.7 suggests lipid metabolism

Understanding this ratio is important for conditions like exercise, where the type of substrate used can vary drastically. Monitoring these ratios can help optimize nutrition and performance, adjusting diets to ensure efficient energy use.