Well, the function of a neuron is to basically carry an action potential to initiate an electrical signal and carry that signal the action potential along the axon. And so, in many of our neurons, on many of our neurons, the plasma membrane is actually myelinated by using these special lipids because it's the lipids that create this insulation layer that basically increases the rate at which that propagation actually takes place, because that signal can basically jump from one node to the next node. And so, because the function of the neuron and the plasma membrane on the neuron is to actually propagate, that signal will find a much higher amount of lipids compared to proteins. In fact, only about 18% of the plasma membrane of myelin neurons contains proteins by mass. On the other hand, if we study the membrane of mitochondria, we'll see that the function of this membrane is to actually produce high energy ATP molecules. And that involves moving many different types of molecules across the membrane, as well as creating proton gradients.