What is the primary energy storage molecule for glucose in humans?

• A. Cellulose
• B. Glycogen
• C. Fructose
• D. Lactose

Answer: (B)

Glycogen serves as the body's main storage form of glucose, stored primarily in the liver and skeletal muscles. Its branched polymer structure allows rapid construction and, importantly, quick mobilization when glucose is needed. In the liver, glycogenolysis helps maintain blood glucose between meals by releasing glucose into the bloodstream, whereas in muscle, glycogen provides a local glucose source for anaerobic and aerobic energy during contraction. The branching (alpha-1,6) enables multiple ends for fast enzymatic attack by glycogen phosphorylase, speeding glucose release when energy demand rises. After breakdown, glucose-1-phosphate is formed and, in the liver, can be converted to free glucose to enter circulation via glucose-6-phosphatase; in muscle, the product mainly fuels glycolysis inside the muscle itself. Cellulose is a plant polymer indigestible by humans, fructose is a monosaccharide, and lactose is a disaccharide; none serve as the primary long-term storage form for glucose in the body.