Final answer:
Glycogen is converted to glucose through glycogenolysis and proteins can be utilized to produce ATP during the postabsorptive (fasting) state of metabolism, where glucagon acts to maintain blood glucose levels, and ketogenesis may contribute to ATP production when glucose is scarce.
Step-by-step explanation:
Glycogen is converted to glucose through glycogenolysis and proteins can be used to make ATP during the postabsorptive state of metabolism. During this state, after the body's immediate glucose needs have been met and insulin levels decrease, stored glycogen is broken down to maintain blood glucose levels. If glycogen stores are depleted, the body can break down proteins into amino acids which can then be converted into glucose or used directly for ATP production through processes such as ketogenesis and ketone oxidation.
In the postabsorptive state, the pancreas releases glucagon, which signals the liver to convert stored glycogen back into glucose, to be used by the cells for energy. Additionally, in this state, muscle cells can also release glucose into the bloodstream and catabolized amino acids from muscle proteins can generate ATP after undergoing ketogenesis and ketone oxidation in the liver.