Pathway Breakdown

Glycogen breakdown in muscle begins with glycogen phosphorylase cleaving one glucose unit as glucose-1-phosphate (G1P), which converts to glucose-6-phosphate (G6P) via phosphoglucomutase, bypassing the hexokinase step that consumes ATP for free glucose. G6P enters glycolysis, investing 1 ATP (phosphofructokinase) to form fructose-1,6-bisphosphate, then splits into two triose phosphates. The payoff phase produces 4 ATP (2 per glyceraldehyde-3-phosphate via phosphoglycerate kinase and pyruvate kinase) and 2 NADH, yielding a net of 3 ATP per glucose unit before lactate formation.​

To Lactate Conversion

Under anaerobic conditions in muscle, the 2 NADH reduce 2 pyruvate to 2 lactate via lactate dehydrogenase, regenerating NAD⁺ for continued glycolysis without ATP gain or loss. Thus, the net ATP remains 3, as no oxidative phosphorylation occurs and NADH is not shuttled. This contrasts with free glucose glycolysis, which nets 2 ATP due to an extra initial ATP investment.​

Common Options Explained

• 2 ATP: Correct for free glucose to lactate (2 invested, 4 produced), but glycogen saves 1 ATP.​

• 3 ATP: Accurate for glycogen glucose unit to lactate, as confirmed in muscle energy metabolism.​

• 36-38 ATP: Applies to aerobic glucose oxidation (full TCA and ETC), irrelevant anaerobically.​

• 0 or negative: Impossible, as substrate-level phosphorylation generates net positive ATP.​

Introduction to Energy Yield

In muscle during intense anaerobic exercise, glycogen serves as a rapid ATP source via glycogenolysis and glycolysis to lactate. One glucose unit from glycogen produces a net 3 ATP molecules, higher than free glucose’s 2 ATP due to bypassing one phosphorylation step. This process fuels short bursts without oxygen, vital for CSIR NET life sciences topics like bioenergetics.​

Step-by-Step Mechanism

Glycogen (n residues) + Pi → Glycogen (n-1) + Glucose-1-phosphate (no ATP used). G1P → G6P (phosphoglucomutase, no ATP). G6P → Fructose-6-P → Fructose-1,6-bisphosphate (1 ATP invested). Splits to 2 GAP; each GAP yields 2 ATP and 1 NADH (total 4 ATP, 2 NADH). 2 Pyruvate → 2 Lactate (NADH used, no extra ATP). Net: 3 ATP, 2 lactate.​

• Advantage in muscle: No glucose-6-phosphatase; G6P directly glycolyzes.

• Equation summary: Glycogenn + 3 ADP + 3 Pi → Glycogenn–1 + 2 lactate + 3 ATP.​

Why Net 3 ATP?

Standard glycolysis from glucose: -2 ATP (hexokinase, PFK) +4 ATP = net 2. Glycogen skips hexokinase: -1 ATP +4 ATP = net 3. Lactate step balances redox without ATP change. Sources confirm this for anaerobic muscle metabolism.​

Exam Relevance for CSIR NET

Questions like “net number of ATP molecules produced” test pathway differences. Remember: glycogen to lactate = 3 ATP; glucose to lactate = 2 ATP; aerobic glucose = ~30-32 ATP. Muscle prefers glycogen for efficiency during sprints.​