In a previous module you looked at the role of the cell membrane. One of its functions is to control the movement of molecules between the intracellular and extracellular fluid, and its chemical structure helps it to perform that function. Because of its lipid core, molecules that are polar do not pass easily through the cell membrane. Insulin is an important molecule that affects cell function. The exterior of the molecule is mainly polar, whereas the interior is mainly non-polar. Will insulin be able to pass freely through the cell membrane and if not, how does it influence cell function?

Insulin is unable to pass through the cell membrane so it must communicate with the cell from the outside. It does this by combining with receptors on the surface of the cell. These receptors are structures that can specifically interact with the insulin molecule. When the hormone and the receptor combine the end result is a change in cellular function.