Really Good Article from Scientific American: LINK
The behavior of a cell depends on its immediate surrounding environment, known as the microenvironment. The assortment of growth factors in this microenvironment is the most important aspect regulating the behavior of that cell. All growth factors exert their effects by binding to a receptor. Receptors are proteins found on the surface of a cell that receive such chemical signals from the outside of the cell. Each growth factor has it’s own receptor; think of it as a key (the growth factor) fitting into a lock (the receptor). Growth factor receptors tend to be ‘transmembrane molecules‘; this means that one end of the receptor ‘sticks out’ through the cell membrane into the microenvironment while the other end projects inside the cell. By spanning across the cell membrane, growth factor receptors are able to communicate signals from outside the cell (e.g. presence of growth factors in the microenvironment) to the inside of the cell. Revisiting the lock and key analogy, think of it as a key that fits into a lock that protrudes through the door-frame, instead of being flush against the door.