And these types of pathways are known as secondary messenger systems. And secondary messenger systems are most often controlled by special protein complexes that involve g protein. So what exactly is the pathway of such a signal transduction process? So let's take a look at the following paragraph and then let's look at a specific example of a secondary messenger system. So our complex of proteins usually contains a transmembrane protein that is found on the membrane of that cell, the receptor cell that contains a binding side for the ligand and the ligand is the first messenger molecule. Now, this binding side is usually found on the extracellular side of the membrane as in this case.