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. And so what that means is this membrane is very, very active and it contains a high proportion of proteins. In fact, about 75% of proteins by mass is found within the membrane of mitochondria. But on average, if we examine the plasma membranes of the cells inside our body, we'll find about 50% of protein by mass.