Does the net gain of ATP in glycolysis differ when glycogen, rather than glucose, is the starting material? If so, what is the change?
The correct answer and explanation is :
Yes, the net gain of ATP in glycolysis differs when glycogen is the starting material instead of glucose.
Change in ATP Yield
- When glucose is the starting material, 2 ATP are consumed in the preparatory phase, but 4 ATP are produced in the payoff phase, leading to a net gain of 2 ATP.
- When glycogen is the starting material, it is broken down into glucose-1-phosphate (G1P) by glycogen phosphorylase, which is then converted into glucose-6-phosphate (G6P) without consuming ATP.
- Normally, in glycolysis, glucose is phosphorylated to G6P by hexokinase, which consumes 1 ATP. However, since G6P is already formed from glycogen breakdown, this ATP cost is bypassed.
- This results in a net gain of 3 ATP instead of 2 ATP when starting from glycogen.
Explanation
Glycogen is a stored polymer of glucose found mainly in the liver and muscles. During glycogenolysis, glycogen phosphorylase cleaves α-1,4-glycosidic bonds to release glucose-1-phosphate. Unlike free glucose, which requires ATP for phosphorylation by hexokinase, G1P is readily converted to G6P by phosphoglucomutase without ATP consumption.
Once G6P enters glycolysis, it follows the standard pathway, producing 4 ATP in the payoff phase. Since no ATP was required for its initial phosphorylation, only one ATP (used in the phosphofructokinase-1 step) is subtracted from the total ATP yield. This leads to a net gain of 3 ATP per glucose unit from glycogen, compared to 2 ATP per free glucose molecule.
This ATP advantage is particularly important in muscle cells during intense exercise, where rapid ATP production is crucial. The efficiency of ATP generation from glycogen makes it a preferred energy source in high-energy-demand situations.