Membranes are asymmetric, for example, because the sugars are only on one side of the membrane. Let's take a look at how one demonstrates this membrane asymmetry. If we attach ferritin to an iron binding protein to a lectin, a plant protein that binds sugars, there's a lectin called con-8. Now, the other side of the con-canavillon-8 is a sugar recognition site. So, what lectins like con-canavillon-8 can do is recognize sugars and bind to them wherever they find them. So, we take a tube full of this stuff and we bathe cross sections of cells to this ferritin tagged con-8 in the electron microscope. Here's a cross section of a cell in high power. The electrons that are beaming in an electron microscope cannot pass through iron. So, we say that iron is electron dense. In an electron micrograph, bits of real iron, I'm going to appear as little black dots. And the little black dots then are iron that's attached to ferritin, which is in turn attached to con-8, which, as you can imagine, is now attached to the glyco-calix, to the sugars on the cell surface of this cell. You can see that all of the black dots are on the outside of the cell, not on the inside. Remember, you don't actually see phospholipids with their fatty acid tails and their phosphate hydrophilic heads. You can't see that in an electron microscope. But again, the iron particles are bound only to the outside of the cell. So that means that sugars are only found on the outside or extracellular surface of plasma membranes.