They actually move the molecules and particles across the cell membrane, and they also create an energy storage system, for instance, when they create proton gradients, that creates a separation of charge between the two sides of the membrane. And within that separation of charge, we store energy, electric potential energy. Now, depending on the type of membrane that we're examining, and depending on the functionality of that membrane, the mass ratio of lipids to proteins can basically differ. So it varies anywhere from four to one to one to four. And to see what we mean by that, let's compare the plasma membrane found on myelinated nerve cells to the membrane that encloses mitochondria. So what's the function of a neuron?