Final answer:
Glycogenolysis effectively yields two additional ATP molecules compared to glycolysis starting from free glucose because it bypasses the energy-consuming step of glycolysis, leading to a potential net gain of four ATP molecules from a molecule of glucose derived from glycogen.
Step-by-step explanation:
When comparing the energy yield from glycogenolysis to glycolysis, it is essential to recognize the roles both processes play in energy metabolism. Glycolysis, which can occur with or without oxygen, produces a net gain of two ATP molecules from one glucose molecule after accounting for the two ATPs used during its energy-consuming phase. In contrast, glycogenolysis is the process of breaking down glycogen into glucose-1-phosphate (which is converted into glucose-6-phosphate and can then enter glycolysis), it does not directly produce ATP. However, since glycogenolysis bypasses the energy-consuming step of glycolysis that uses up two ATP, it effectively yields two additional ATP compared to glycolysis starting from free glucose. Therefore, when a molecule of glucose is derived from glycogenolysis, it has the potential to produce a net gain of four ATP molecules after the completion of glycolysis.