Which organ stores glycogen to provide glucose during fasting?

• A. Liver
• B. Muscle
• C. Brain
• D. Adipose

Answer: (A)

During fasting the liver provides circulating glucose by breaking down glycogen and releasing free glucose into the bloodstream. This works because liver cells have glucose-6-phosphatase, which converts glucose-6-phosphate to free glucose that can enter the blood. Muscle stores glycogen for its own energy needs and lacks glucose-6-phosphatase, so it cannot supply glucose to other tissues. The brain uses glucose but does not store glycogen, and adipose tissue stores energy as triglycerides rather than glycogen. Thus, the organ that stores glycogen to provide glucose during fasting is the liver.