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. But it can basically vary anywhere from, let's say, 18% to 75% of protein by mass. It really depends on the actual function of that cell and the function of that membrane. Now, there are two categories of proteins that we can find in the membranes of our body.