Final answer:
Despite the simultaneous activity of glycogen phosphorylase and glycogen synthase that would notionally cancel each other's effects on glycogen, there is an ATP cost involved in the phosphorylation of glucose to create G-6P for glycogen synthesis. However, the breakdown process does not directly consume ATP. The system is regulated to prevent such a futile cycle.
Step-by-step explanation:
The cost in ATP per glucose residue, if glycogen phosphorylase and glycogen synthase were to remove and add glucose residues simultaneously, would effectively cancel each other out, leading to a net zero gain in the number of glucose units in glycogen. However, it's important to note that the glucose used in glycogen synthesis comes from glucose-6-phosphate (G-6P), which is already phosphorylated.
The synthesis of G-6P from glucose requires one ATP molecule (during the phosphorylation step catalyzed by hexokinase in glycolysis), and the glycogenolysis process also involves the conversion of glucose-1-phosphate to G-6P without a direct ATP cost. The entire process is an interconversion where energy is expended during synthesis and conserved during breakdown.
In this cycle, if glucose is added by glycogen synthase at the same rate it is removed by glycogen phosphorylase, there would be a continuous expenditure of ATP without a net increase in glycogen stores. Therefore, the metabolic futility of this simultaneous activity suggests a highly regulated system where either glycogen synthesis or glycogenolysis predominates according to cellular needs.