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Insulin receptor, molecular model. The insulin receptor is a transmembrane protein, that is it spans the cellular membrane. The part of the receptor that sits on the outside of the cell, the ectodomain, is seen here, complexed with the antigen-binding fragment (Fab) of an antibody. Binding of insulin to the extracellular part of the protein activates a number of changes inside the cell that lead to the uptake of glucose, the synthesis of glycogen and fatty acids and glycolysis, the production of energy from glucose.