Energy - Carbohydrates, Lipids and Proteins as Respiratory Substrates

This section explains the relative energy values of carbohydrates, lipids, and proteins when used as respiratory substrates in living organisms. We will explore why these differences exist and how they are measured.

Respiratory Substrates

Respiratory substrates are substances that are broken down through cellular respiration to release energy. The primary respiratory substrate is glucose, but other molecules like fatty acids and amino acids can also be used.

Energy Yields

The energy released during cellular respiration is quantified by the amount of ATP produced. Different respiratory substrates yield different amounts of ATP per gram. This is due to the chemical structure of each molecule and the number of carbon-hydrogen bonds they contain. More carbon-hydrogen bonds generally mean more potential energy.

Relative Energy Values

The relative energy values are typically expressed as kilojoules (kJ) per gram (g). These values are based on experimental measurements.

Table of Relative Energy Values

Respiratory Substrate	Energy Yield (kJ/g)
Carbohydrates	3.5
Lipids (Fats)	9.0
Proteins	4.0

Explanation of Differences

The differences in energy yields are primarily due to the chemical composition of each molecule:

• Carbohydrates: Carbohydrates are composed of carbon, hydrogen, and oxygen. They have a moderate number of carbon-hydrogen bonds. The breakdown of carbohydrates involves a series of oxidation reactions, releasing energy.
• Lipids (Fats): Lipids are primarily composed of carbon and hydrogen, with a significantly higher proportion of hydrogen compared to carbohydrates. This high hydrogen content results in a large number of carbon-hydrogen bonds. The oxidation of these bonds releases a substantial amount of energy.
• Proteins: Proteins contain carbon, hydrogen, oxygen, and nitrogen. While they contain carbon-hydrogen bonds, the presence of nitrogen and the more complex structure of proteins lead to a lower energy yield compared to carbohydrates and lipids. Proteins are primarily used for structural and functional roles, not as a primary energy source.

Mathematical Representation

The theoretical energy yield of each substrate can be calculated using the following equations:

$$ \text{Theoretical Energy (kJ/g)} = \text{n} \times \Delta G_{hydration} $$ Where:

• n = number of carbon-hydrogen bonds
• $\Delta G_{hydration}$ = energy released per bond broken during oxidation

The values presented in the table are experimental values, which are influenced by factors such as the specific type of carbohydrate, lipid, or protein, and the conditions under which the respiration is carried out.