Insulin release by the β cells of the pancreas involves a number of interrelated metabolic reactions in both the cytosol and the mitochondria. The islet must sense that blood glucose levels are rising. This is a critical step in insulin release. The glucose-phosphorylating enzyme, glucokinase, serves this function. GLUT 2 transports glucose through the plasma membrane making this glucose available to glucokinase. The glucose is then processed through glycolysis. Once glucose is made available to glucokinase, this enzyme migrates to the mitochondrial membrane pore protein. The ATP produced by the mitochondrial OXPHOS system is needed by glucokinase as well as by the sodium-potassium pump. Insulin secretion also involves some ion shifts. The Na⁺K⁺ pump becomes active as does the K⁺ channel, allowing these ions to move in and out of the cell. The calcium ion moves in. This ion serves two functions: (1) it stimulates OXPHOS, and (2) in sulin exocytosis.