Glycogen breakdown — Source of glucose: In which tissues is glucose released from glycogen by phosphorolysis?

Introduction / Context:
Glycogenolysis mobilizes stored carbohydrate. The key enzymatic step is phosphorolysis, which cleaves alpha-1,4 bonds to release glucose-1-phosphate. Understanding where phosphorolysis occurs helps clarify differences between liver and muscle carbohydrate metabolism.

Given Data / Assumptions:

• Glycogen phosphorylase catalyzes phosphorolysis.
• Both liver and skeletal muscle store glycogen.
• Downstream fates of glucose differ between tissues.

Concept / Approach:
In both liver and muscle, the first step of glycogen breakdown is the same: glycogen phosphorylase uses inorganic phosphate to release glucose-1-phosphate. In liver, glucose-6-phosphatase converts glucose-6-phosphate to free glucose for export. Muscle lacks glucose-6-phosphatase and keeps glucose-6-phosphate for glycolysis to fuel contraction. Hydrolytic cleavage of glycogen to free glucose is not the physiological route in muscle; debranching enzymes handle branch points with transferase and limited hydrolysis of alpha-1,6 linkages, but the bulk release is via phosphorolysis.

Step-by-Step Solution:

Verification / Alternative check:
Enzyme assays show high glycogen phosphorylase activity in both tissues; only liver expresses significant glucose-6-phosphatase.

Why Other Options Are Wrong:

• “Muscles by hydrolysis” misstates the primary cleavage mechanism; hydrolysis is minor and at branch points only.
• Picking one tissue ignores the shared first step of glycogenolysis.

Common Pitfalls:
Confusing the tissue-specific presence of glucose-6-phosphatase (export vs. local use) with the initial cleavage mechanism (which is phosphorolysis in both).

Final Answer:
Both (a) and (b)