Which statement best describes lipids?

• A. They are soluble in water.
• B. They store energy.
• C. They are polymers of amino acids.
• D. They are inorganic minerals.

Answer: (B)

Lipids are energy-dense organic molecules used for long-term energy storage. Their long hydrocarbon chains are nonpolar, so they do not dissolve in water. This allows organisms to pack a large amount of energy into a small mass and store it efficiently, typically as triglycerides in adipose tissue. Lipids provide about 9 kcal per gram, higher than carbohydrates or proteins, which makes them ideal for sustained energy. They are not soluble in water, they are not polymers of amino acids (those are proteins), and they are organic molecules—not inorganic minerals.