Why does glucagon cause glycogenolysis in the liver?

Glucagon causes glycogenolysis in the liver by binding to glucagon receptors on liver cells, activating a signaling cascade that increases cyclic AMP (cAMP) levels. This activates protein kinase A (PKA), which in turn activates glycogen phosphorylase, the enzyme that breaks down glycogen into glucose-1-phosphate. This glucose-1-phosphate is then converted into glucose-6-phosphate, which can be dephosphorylated to free glucose and released into the bloodstream. The release of glucose into the bloodstream helps to raise blood glucose levels during fasting or in response to low blood sugar.