The bulk of the pancreas is an exocrine gland secreting pancreatic fluid into the duodenum after a meal. However, scattered through the pancreas are several hundred thousand clusters of cells called islets of Langerhans . The islets are endocrine tissue containing four types of cells. The beta cells of the islets secrete insulin.
Beta cells have channels in their plasma membrane that serve as glucose detectors.
Insulin affects many organs. Insulin stimulates liver cells to take up glucose from the blood and convert it into glycogen . In each case, insulin triggers these effects by binding to the insulin receptor - a transmembrane protein embedded in the plasma membrane of the responding cells. Diabetes mellitus is a disorder quite distinct from the similarly-named diabetes insipidus . It results from destruction of the beta cells of the islets.