What is a fatty acid and how is it stored as energy?

• A. A long hydrocarbon chain; stored as triglycerides in adipose tissue.
• B. A short sugar molecule stored as glycogen in liver.
• C. A protein chain used for muscle contraction.
• D. A lipid that serves as an energy source but stored as glycogen.

Answer: (A)

Fatty acids are long hydrocarbon chains that serve as the building blocks of fats and a major energy source. In animals, energy is stored by packaging these fatty acids into triglycerides, which are molecules where three fatty acid chains are attached to a glycerol backbone. These triglycerides are stored in adipose tissue as an energy reserve. When energy is needed, enzymes break down the triglycerides to release fatty acids, which are then transported to tissues and oxidized in mitochondria to produce a large amount of ATP.

The other options mix up what is stored and what fatty acids are. A short sugar molecule stored as glycogen is a carbohydrate storage form, not lipid. A protein chain used for muscle contraction refers to actin and myosin, not energy storage. Saying a lipid serves as an energy source but is stored as glycogen conflicts with the fact that glycogen is a carbohydrate storage molecule, while fats are stored as triglycerides.