This tutorial demonstrates how the structure of a signal molecule determines its function. Signal molecules can interact with either intracellular or extracellular receptors. For a signal molecule to bind with an intracellular receptor it must be able to pass through the cellular membrane. Generally signal molecules that enter the cell are nonpolar and fat soluble. These signal molecules can pass through the lipid bilayer of the cell membrane. Signals that bind with extracellular receptors are proteins or other types of molecules that cannot readily pass through the membrane.
This challenging tutorial addresses the concept at a high level of complexity.