May provide energy to overcome environmental stresses. [(1->4)-alpha-D-glucosyl](n) + phosphate = [(1->4)-alpha-D-glucosyl](n-1) + alpha-D-glucose 1-phosphate During logarithmic growth and early developmental phases, the activity of GP1 is dependent on 5'AMP. It then seems to convert to an AMP independent type. This alteration in activity coincides with phosphorylation of GP1. Present throughout cellular differentiation. In D.discoideum glycogen phosphorylase exists as 2 developmentally regulated forms, GP1 and GP2, which are the products of separate genes. Belongs to the glycogen phosphorylase family.