During intense exercise, a sprinter mobilizes muscle glycogen and channels glucosyl units directly into glycolysis at the glucose-6-phosphate level. Compared with glycolysis starting from free blood glucose, how many additional ATP molecules are generated per glucosyl unit entering glycolysis from glycogen under anaerobic conditions?

• A None, because glycogen breakdown consumes the equivalent of one ATP per unit released
• B Two additional ATPs, because both priming phosphorylations are avoided
• C Three additional ATPs, because glycogen phosphorylase produces glucose-6-phosphate directly
• D One additional ATP, because the hexokinase step is bypassed ✓

Explanation

Free glucose requires investment of one ATP at the hexokinase step before glycolysis yields a net of 2 ATP anaerobically. D glucosyl unit released from glycogen by phosphorylase emerges as glucose-1-phosphate, converted to glucose-6-phosphate without ATP expenditure, so it enters glycolysis past the priming step and yields a net of 3 ATP. Only the phosphorolytic cleavage itself is energetically free, making option A incorrect.

Reference: Lippincott Illustrated Reviews: Biochemistry, 8th ed.

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