If we had a way of visualizing the plasma membrane of a cell, we would discover that it’s formed from lipids. We see such lipids there, phospholipids, many of them. We’ll talk about them shortly. That the outside of the cell, there are many proteins, you see them here, which thread their way through the plasma membrane, have an extracellular and intracellular part. And these transmembrane proteins, which reach from outside to inside, represent a very important way by which the cell senses its environment.