Both the liver and muscle contain glycogen, yet, unlike liver, muscle is not capable of contributing glucose to the circulation because muscle
**Core Concept**
The liver and muscle both store glycogen as a form of energy storage, but the liver has a unique ability to release glucose into the bloodstream through a process called glycogenolysis, whereas muscle does not. This is due to differences in the regulation and mechanisms of glycogen metabolism in these two tissues.
**Why the Correct Answer is Right**
Muscle glycogen is primarily used for local energy production within the muscle cells (myocytes) through a process called glycolysis, which does not involve the release of glucose into the bloodstream. This is because muscle cells lack the enzyme glucose-6-phosphatase, which is necessary for the conversion of glucose-6-phosphate to glucose, making it unavailable for release into the circulation. In contrast, liver cells have glucose-6-phosphatase, allowing them to convert glucose-6-phosphate to glucose, which can then be released into the bloodstream.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided, but we can assume the correct answer is not listed.
**Option B:** This is not a valid option, but if it were, it might imply that muscle lacks glycogen, which is incorrect.
**Option C:** This option might suggest that muscle lacks glucose-6-phosphatase, which is correct, but it does not directly explain why muscle cannot contribute glucose to the circulation.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that muscle glycogen is primarily used for local energy production, whereas liver glycogen serves as a critical source of glucose for the entire body.
**Correct Answer: C. Muscle lacks the enzyme glucose-6-phosphatase, which is necessary for the conversion of glucose-6-phosphate to glucose, making it unavailable for release into the circulation.**