Rank carbohydrates, lipids, and proteins by energy potential based on C-C bonds.

• A. Most: Lipids; Middle: Carbohydrates; Least: Proteins.
• B. Most: Lipids; Middle: Proteins; Least: Carbohydrates.
• C. Most: Carbohydrates; Middle: Lipids; Least: Proteins.
• D. Most: Proteins; Middle: Carbohydrates; Least: Lipids.

Answer: (A)

The amount of energy you can get from a molecule largely depends on how reduced its carbon skeleton is—the more carbon–carbon and carbon–hydrogen bonds that can be oxidized, the more energy is released. Lipids have long hydrocarbon chains with many C–C and C–H bonds and relatively few highly oxidized groups, so their carbons are highly reduced and yield the most energy when broken down. Carbohydrates are more oxidized than lipids, containing more C–O bonds and functional groups, which means they don’t release as much energy per carbon. Proteins also contain carbon backbones, but they include nitrogen-containing groups and peptide bonds; handling and excreting nitrogen costs energy, and the carbon skeletons of amino acids don’t release as much energy upon complete oxidation as lipids do. So the ranking by energy potential from highest to lowest is: lipids, then carbohydrates, then proteins.