A gram of fat oxidized by respiration produces approximately twice as much ATP as a gram of carbohydrate. Which of the following best explains this observation?

• A. Fats are produced when cells take in more food than they need.
• B. Fats are less soluble in water than sugars.
• C. Fats do not form true macromolecules as sugars do.
• D. Fats are closely related to lipid molecules, which are the basic building blocks of cellular membranes.
• E. Fats are better electron donors to oxygen than are sugars.