Cinelecture 28 – Biological Membranes I
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Cinelecture 28 – Biological Membranes I

We’re now ready to tackle Membranes: Structure and Function. And shown here is a nice electron micro graphical mitochondria. Where you can see this smooth outer membrane and then the highly folded inner membrane in the cristi. So lets talk about membrane structure. We know that phospholipids are arranged in a bilayer, we’ve covered that already. And what we haven’t talked about is that Proteins can be inserted in the lipid bilayer of membranes. And the model for the membrane involves a mosaic of proteins, that is, that floats in the fluid lipid bilayer. So the lipid bilayer is rather fluid. And the proteins float in them rather like buoys. B-U-O-Y, bouys float in the ocean. Or like boats on a pond. And we’ve covered this already. Here is our phospholipid containing the fatty acid non-polar tails, and the polar head groups. And we’ve shown that in three different ways. Now here’s a model of a membrane shown schematically. In which we can identify certain components that are important to membrane biology. First, of course, the lipid bilayer itself. The phospholipid bilayer. And then we can talk about integral proteins that are embedded in the phospholipid bilayer. These include channel proteins that we’ll talk about later. And other proteins that link the extra cellular environment to the internal environment, By being bound to, or interacting with, cytoskeletal elements. For example we’ve talked about the integrants previously. Cholesterol can be inserted into the membrane as we will see. And that cholesterol is a lipid, as you know, it’s a steroid alcohol. And it’s hydrophobic portion can insert into the hydrophobic region of the phospholipid bilayer. It affects membrane fluidity as we will see. And there are peripheral proteins that are found near the inner bilayer in the cytoplasm. And they can be associated with proteins that are integral membrane proteins. So integral membrane proteins are ones that are embedded in the membrane. Often integral membrane proteins are glycosylated. Glucose or other sugars have been attached to those proteins. Sugars are also found attached to certain lipids. So we can have glycolipids as well as glycoproteins. In fact if you look at the surface of cells you’ll find that you could conceivably refer to them as sugar coated. Because there are so many sugars attached to molecules in the membrane. Here shown are extracellular matrix proteins and we know that those are, you know the plasma membrane lie above the plasma membrane in the extracellular environment. So this is a schematic that gives an overview of some of the components of the membrane. What we envision, in terms of the fluid mosaic model, Is that this lipid bilayer, these lipid molecules, are actually bouncing around and moving around, And proteins can float in them and change positions. Much like a boat on a lake. If we consider membranes and talk about specifically the components that they have. They have the phospholipid bilayer. Which serves as a barrier to permeability. Because of the hydrophobic interior of the lipid bilayer, polar molecules cannot pass easily, from the extracellular environment to the intracellular environment. or viceversa Transmembrane proteins, that we’ve talked about, they span the membrane. And they can have extracellular domains as well as intracellular domains. And we’ve mentioned peripheral membrane proteins. And then cell surface markers. The glycoproteins and glycolipids often mark a cell type as belonging to a particular cell class. And cells can recognize each other based on the nature of the glycoproteins and glycolipids that are on the cell surface. These mark the cell. Here’s a transmission electron micrograph, false-ly colored, showing two cells. Cell 1 and Cell 2 Here is the plasma membrane phospolipid bilayer of cell 2 Here is the phospolipid bilayer of cell 1 And here is the inter cellular space. We’ve talked a little about freeze fracturing a little before. Where you take a cell or tissue and freeze it and then crack it with a knife blade. And often the cracking separates a phospolipid bilayer. The crack occurs between the monolayers of the lipid bilayer. And when that happens you can peer at the hydrophobic surface of the inner membrane. And you can see proteins that are inserted in there. So here is such an example Here’s the external surface of the plasma membrane, and here’s where it cracked. And we are now looking at the lower half of the lipid bilayer. And you can see that there are proteins embedded in that lower half. And you can actually see proteins sticking out of the upper half as well into the extracellular environment. So we know about the structure of phospholipids, Glycerol is a 3-carbon polyalcohol And there are 2 fatty acids, remember, attached to that glycerol. And the phosphate group is attached to the glycerol as well. Endowing the phospholipid with it. A water-loving or hydrophilic head We talked about phospholipids will spontaneously form a bilayer in water. With fatty acids on the inside and the phosphate groups on both surfaces. As shown here Many experiments have shown the fluid mosaic model of the membrane. Mosaic being pieces that are embedded a matrix. Here what we have are the proteins are kind of like mosaic pieces and they float around in the membrane. And for example you can label proteins on a mouse cell, on a human cell. And using polyethylene glycol you can cause those cells to fuse. And then you can watch and see what happens to the proteins in the hybrid cell. And what you find is that they become intermixed Due to the fluid mosaic nature of the membrane, proteins are floating around the membrane surface and freely mixing. There is another experiment called flouresence recovery after photobleaching. FRAP And we’re gonna have a little video on that right now In the technique calld FRAP. F-R-A-P. Florescence recovery after photobleaching, proteins are labeled with a florescence tag. In this case a protein that is embedded in the membrane of the endoplasmic reticulum has been tagged with the green florescence protein. Photobleaching with a laser will eliminate and oblate the florescence coming from the area that is illuminated with the laser. And what you will see is that Other labeled proteins, that haven’t been photobleached and are outside of the area of photobleaching quickly move into the photobleached area. Showing that the membrane is a fluid mosaic In which proteins float rather like rafts in a sea of phospholipids. So lets look at that right now Here is the area that will be illuminated by a laser and you can see the photobleaching. Recovery period shows rather quickly that florescent proteins move into the photobleached area. Now in this case Proteins that are anchored to the inside of the nuclear membrane, in the nuclear lamina, and are firmly anchored there, have been photobleached. After equivalent recovery time periods, on the order of minutes, No movement of those proteins occur because they are anchored to lamin. Now lets move onto the malleability of the membrane in terms of it’s fluidity being influenced by environmental factors. Remember when we talked about fats we pointed out that unsaturated fatty acids in triglycerides create a lower melting point because the fats don’t pack as tightly. Well that’s also true for phospholipids. Saturated fatty acids make the membrane less fluid than unsaturated fatty acids. So you can have, just like in triglycerides, you can have differences in the fatty acid tails of phospholipids. that differ in their degree of saturation that is their degree of hydrogenation. Kinks, that are caused by double bonds, keep the phospholipids from packing tightly and therefore unsaturated fatty acids create membranes that are more fluid because they are not packing as tightly Cholesterol, which is a steroid alcohol, can increase or decrease membrane fluidity depending on the temperature At high temperatures, cholesterol embedded in the regular phospholipid bilayer tends to make the membrane less fluid At high temperatures it interacts with the hydrophobic fatty acid tails on the phospholipids But at low temperatures cholesterol can actually protect the membrane against freezing Because membranes tend to congeal just as fats will congeal And cholesterol will keep the phospholipids from packing too tightly at low temperatures. And thereby keep the membrane more fluid And, as you might expect, warm temperatures do make the membrane more fluid than cold temperatures In some organisms there are some bacteria for example that will actually have enzymes that desaturate the fatty acid tails of the phospholipids in the membrane so that they can tolerate cold, and thats because as you desaturate, You move to a case of unsaturation with double bonds which keeps the phospholipids from packing too tightly and becoming frozen So cold tolerance can be achieved enzymatically in this case Now lets move onto membrane proteins We will pick up with membrane proteins in the next section

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