Glycogen is mainly stored in which tissues?

• A. Liver and skeletal muscle
• B. Brain
• C. Adipose tissue
• D. Heart

Answer: (A)

Glycogen serves as the rapid glucose reserve in mammals, stored mainly in the liver and skeletal muscle. In the liver, glycogen is mobilized to maintain blood glucose during fasting or between meals, releasing glucose into the bloodstream via glycogenolysis thanks to glucose-6-phosphatase, which completes the final step to free glucose. In skeletal muscle, glycogen provides a local energy source for contraction; however, muscle tissue lacks glucose-6-phosphatase, so glucose released from muscle glycogen cannot enter the bloodstream and is used within the muscle for glycolysis during activity. The brain relies on circulating glucose and has little stored glycogen, adipose tissue stores energy as triglycerides, and the heart contains some glycogen but not primarily as a storage reservoir. Therefore, the main glycogen storage tissues are the liver and skeletal muscle.