280-2 Freeze Fracture Electron Microscopy of Cell Membranes


Let’s look at the structure and function now, of a typical plasma membrane. We’re really focusing on eukaryotic cells here. Cells are sugar coated and by that I mean there are covalently bound oligosaccharides, shown in this illustration as blue structures. So we have the proteins embedded in the membrane, and we have the sugars that are covalently attached on the external surface, on the outside of the cell. So we say the cell is ‘sugar-coated’. We refer to the sugar coated cell surface as the ‘glycocalyx’. We have several kinds of structures that are sugar coated , or that have sugars attached. There are short oligosaccharide chains attached to proteins and these are glycoproteins. And then we have structures called ‘proteoglycans’, which are still transmembrane proteins, that is, proteins that pass through the entire phospholipid bilayer, but have longer polysaccharide chains attached. And the fact that these are much longer polysaccharide chains, that is glycoside-linked sugar chains differentiates them from glycoproteins. Also shown are phospholipids to which sugars have been attached, and these are ‘glycolipids’, and we saw an example before. This is a cell, any cell. Think of it as a blood cell. It has multiple functions based on the proteins that are embedded in the membrane. Some proteins are receptors for receiving chemical signal information. Some of the proteins are required in the case of say, an amoeboid cell in order to get movement and enlargement of a cell in order to change its shape. So for example, a phagocyte will change its shape as it engulfs a bacterium in your bloodstream. And then there are of course all these proteins which are involved in either import or export of molecules. Here in this kind of cell, you might expect to find such proteins pretty well evenly diffused around the surface of the cell membrane. But as it says down below, different regions of cell membranes can actually have different functions. Well, we will take a look at that. Let’s talk about the fluid mosaic model. It used to be called the fluid mosaic “theory” of membrane structure. You can take purified membranes, or even whole cells, freeze them in ice, go get a hammer and chisel, and tap at the frozen ice block containing the membranes, and what will happen is, membranes will fracture along the line of least resistance, the interaction of the hydrophobic fatty acid tails of the phospholipids. So you will split the phospholipid bilayer into two halves. And there’s a cartoon that I drew for you… and the idea is that if you split the membrane in half, some of the proteins will travel, or stick with one half of the phospholipid bilayer, some with the other. I’ve drawn it so they’re all on one side, which is typically see when you actually look in the electron microscope. And what you can see is, if there’s a protein stuck to one side of the phospholipid bilayer, because it was frozen, nothing had a chance to shift, and you can actually see a groove or a hole even, where that same protein was embedded in the other side of the phospholipid bilayer. This kind of electron microscopy tells us that there are indeed proteins embedded in the phospholipid bilayer, that they are not for example, sitting on either side of the membrane, that is interacting say, just with the phosphate heads of the phospholipids. But they are actually embedded, and they stick all the way through the membranes. All the ones shown here with one exception, called the ‘peripheral’ membrane protein, are called ‘transmembrane’, or ‘integral’ membrane proteins, because they span the entire phospholipid bilayer. The illustration shows the oligosaccharides, and so without my telling you, you should be able to identify what ‘s outside and what’s inside relative to this bit of plasma membrane. The glycoside-linked sugars in the glycopeptides and in one case, glycolipid, must be on the side of the cell.

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