Question

(a) What is meant by the term fuel value? (b) Which is a greater source of energy as food, \(5 \mathrm{~g}\) of fat or \(9 \mathrm{~g}\) of carbohydrate? (c) The metabolism of glucose produces \(\mathrm{CO}_{2}(g)\) and \(\mathrm{H}_{2} \mathrm{O}(l) .\) How does the human body expel these reaction products?

Short answer

(a) Fuel value is the amount of energy released when a substance is consumed and metabolized by an organism, typically expressed in calories or joules per unit mass. (b) 5g of fat provides more energy as food than 9g of carbohydrate (45 cal > 36 cal). (c) The human body expels the reaction products of glucose metabolism (CO2 and H2O) through the respiratory system (CO2 is exhaled) and various processes such as urination, sweating, breath moisture, and fecal excretion (removal of H2O).

Step-by-step solution

a) Definition of fuel value

Fuel value is a measure of the amount of energy that is released when a substance (usually food) is consumed and metabolized by an organism, typically expressed in calories or joules per unit mass. In the context of nutrition, fuel value tells us how much energy a specific food provides for the body to perform various tasks.

b) Comparing energy content of fat and carbohydrate

To compare the energy content of fat and carbohydrate, we will use their fuel values, which are generally known. Fat has a fuel value of approximately 9 calories per gram (cal/g) while carbohydrate has a fuel value of approximately 4 cal/g. Given the amounts of fat and carbohydrate provided in the problem (5 g of fat and 9 g of carbohydrate), we can calculate the total energy content of each: Energy provided by fat = (Fuel value of fat) x (Amount of fat) = 9 cal/g * 5 g = 45 cal Energy provided by carbohydrate = (Fuel value of carbohydrate) x (Amount of carbohydrate) = 4 cal/g * 9 g = 36 cal Since 45 cal > 36 cal, 5 g of fat provides more energy as food than 9 g of carbohydrate.

c) Expelling reaction products of glucose metabolism

As mentioned in the question, the metabolism of glucose produces carbon dioxide (CO2) and water (H2O). The human body has evolved a mechanism to expel these reaction products: 1. Carbon dioxide (CO2): The primary method for CO2 removal is through the respiratory system. CO2 produced during cellular metabolism diffuses into the bloodstream, where it combines with water to form carbonic acid. This acid dissociates into bicarbonate ions and hydrogen ions, and the bicarbonate ions are carried along the bloodstream to the lungs. Here, they recombine with hydrogen ions to form carbonic acid, which then breaks down to release CO2 gas. This CO2 gas is expelled from the body through the process of exhalation. 2. Water (H2O): The water produced during glucose metabolism contributes to the body's overall water content. Excess water is removed from the body through various processes, the primary one being urination. Additionally, water is also lost through sweating, breath moisture, and fecal excretion.

Key concepts

Energy Content Comparison

The energy content of foods is an important concept for understanding how much energy different substances provide to our body. This energy is often measured in calories or joules. When comparing fats and carbohydrates, fats contain more calories per gram. This means fats can offer more energy than carbohydrates.

• Fats generally have a fuel value of about 9 calories per gram.
• Carbohydrates typically have a fuel value of about 4 calories per gram.

Given these fuel values, we calculate the energy provided by each food type. For example, 5 grams of fat offer 45 calories, whereas 9 grams of carbohydrates provide 36 calories. Thus, gram for gram, fats are denser in energy compared to carbohydrates.

Glucose Metabolism

Glucose plays a vital role in providing energy to our bodies. It undergoes a process known as metabolism, where it is broken down to release energy. This energy is crucial for maintaining bodily functions and supporting physical activities. When metabolized, glucose is converted into carbon dioxide (\( \mathrm{CO}_2 \)) and water (\( \mathrm{H}_2\mathrm{O} \)), along with a release of energy.

• During metabolism, glucose is oxidized in the presence of oxygen.
• This process involves several steps, including glycolysis, where glucose is first broken down in the cytoplasm.
• The reactions continue in the mitochondria during the Krebs cycle and oxidative phosphorylation.

Ultimately, the breakdown of glucose leads to the production of ATP (adenosine triphosphate), the energy currency of the cell, which powers various cellular processes.

Reaction Products Expulsion

After glucose is metabolized into carbon dioxide and water, the body needs to expel these products efficiently. Excreting these byproducts is crucial to maintain homeostasis and balance within the body's systems.
Carbon dioxide, a gaseous product, is primarily removed through respiration.

• It diffuses from body cells into the bloodstream and is transported to the lungs.
• In the lungs, it is expelled from the body when we breathe out.

Excess water formed during metabolism is managed through several routes.

• It is expelled mainly via urination, helping in regulating the body's fluid balance.
• Water is also lost through sweat, which aids in thermoregulation, and through breath moisture.
• Additionally, some amount of water is expelled through feces.

These mechanisms ensure that the byproducts of metabolism are not accumulated in the body, maintaining optimal function.