How does the amount of energy produced by respiratory substrates vary?

The correct option highlights that the energy produced by respiratory substrates—specifically lipids, proteins, and carbohydrates—varies significantly due to their distinct chemical structures and the metabolic pathways they undergo during cellular respiration.

Each type of substrate provides a different amount of energy because they are broken down differently in the body. Carbohydrates, for instance, are relatively easy to convert into energy, yielding a moderate amount of energy per gram. Lipids, on the other hand, contain more carbon-hydrogen bonds and therefore release more energy when metabolized, leading to a higher energy yield compared to carbohydrates. Proteins can be used for energy, but they are typically utilized for their building block amino acids instead, and their energy yield is less than that of carbohydrates due to the more complex processes needed for their breakdown.

This variability indicates that the body has a preference for different substrates based on energy needs and availability, allowing for efficient energy production according to the type of nutrient being metabolized.