Which of the following macromolecules provides the most energy per gram?

Fats are the macromolecules that provide the most energy per gram, primarily due to their chemical structure and composition. Fats, or lipids, are composed of long chains of carbon and hydrogen atoms. When these molecules are metabolized, they yield a high amount of adenosine triphosphate (ATP), which is the energy currency of the cell. Specifically, fats provide about 9 calories per gram, significantly more than carbohydrates and proteins, which offer approximately 4 calories per gram each.

The reason that fats are more energy-dense has to do with their higher ratio of carbon and hydrogen atoms compared to oxygen atoms, making them more reduced. When fats are oxidized during cellular respiration, they release a greater amount of energy compared to other macromolecules. Nucleic acids, although vital for the storage and transmission of genetic information, do not primarily function as energy sources and therefore do not contribute significantly to dietary energy. This distinctive energy content makes fats the most efficient macromolecule for energy storage in the body.