Bilayer Composition – Biological Membranes

by Kevin Ahern, PhD

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    00:00 The amphiphilic substances that make up the lipid bilayer are molecules like what I show on the screen here. This includes molecules like glycerophospholipids and I’ll move to these fairly quickly. Amphiphilic of course means that it has a portion of the molecule that is very polar, that’s the part that is associated with water and its on the outside layers of the lipid bilayer. The inside part is the nonpolar tail, and these are where the fatty acids are attached.

    00:26 Another type of amphiphilic molecule we find in a lipid bilayer, are those of the sphingolipids, specifically on the screen you can see a sphingolipid called sphingomyelin. Sphingomyelin also has an arrangement such that it has a polar head as you can see on the bottom part in green and a nonpolar tail that has long chains sticking off of them that you can see green at the top of that.

    00:47 Now lipid bilayers are not the only part of the cell membrane. Cell membranes also include other compounds, and one of the important compounds found inside the cell membrane is that of proteins. Now this figure illustrates the different types of proteins that can be found within a lipid bilayer. First of all we see the integral membrane proteins.

    01:08 Integral membrane proteins are proteins who have their structure that projects through both sides of the lipid bilayer as you can see here. Peripheral membrane proteins are proteins that don't project all the way through, and may interact to limited extent with a part of the lipid bilayer as you can see with the two peripheral proteins here. Membranes that are called associated proteins are not in fact interacting with the lipid bilayer, but typically interacting with some other component of the bilayer. And last we have anchored membrane proteins. These are proteins that have typically a fatty acid sticking off of them, and the fatty acid is interacting with the nonpolar fatty acids in the hydrophobic part of the lipid bilayer.

    01:52 Another component of the lipid bilayer is the molecule cholesterol. Now, cholesterol has a bad rap for causing problems associated with heart attacks and so forth. However cholesterol is in the body partly because of its need to help lipid bilayers to have the fluidity and the properties that they have. Cholesterol is a very important lipid that's found in lipid bilayers, particularly in brain tissue. If you take brain tissue and dry it, you'll discover that 14% of the dry weight of brain tissue is cholesterol. Now, one of the ways that the cholesterol can attach itself to a lipid bilayer is through a fatty acid attachment site shown here and free cholesterol as well can also attach to a lipid bilayer.

    02:35 This schematically shows the structure of a lipid bilayer and with this bilayer we can see of course the lipid components that are shown with the yellow tails and so forth, various proteins that are embedded in the membrane. Now these proteins have various things attached to them. So for example the yellow circles that you can see are typically oligosaccharides or short carbohydrate molecules that are attached to proteins in the lipid bilayer. The red molecule that's shown in cross-section there is a protein that typically is involved in the transport of materials across the lipid bilayer. Now transport proteins are very, very important for cells, because transport proteins can help cells to get the nutrients in that they need. A common nutrient needed by cells for example is glucose.

    03:20 Cells get glucose in the body typically from outside and have to be brought in. Glucose does not freely cross the lipid bilayer by itself and so cells will typically have a series of glucose transport proteins that facilitate the movement of glucose into the cell from the exterior.

    03:38 These glucose transport proteins are called GLUTs, G-L-U-T-S. And these GLUTS are varied and they're important in helping the cell to get the nutrients as I said. One of the things or one of the ways that GLUTs production is stimulated is by the hormone insulin.

    03:57 Insulin causes cells to move GLUTs to the cell surface where they move out and grab glucose that's floating around in the bloodstream. Now you might remember that insulin is involved in the response of the body to high blood sugar, and the way that the body reduces high blood sugar is by moving GLUTs to the surface of a cell, and taking that glucose out of the blood and bring it into a cell.

    04:23 Now when we look at the lipid bilayer, one of the things we begin to discover is that bilayers are sort of tailored for the environments in which the cell is living. A really good example is that of the archaeans. Now the archaeans are a very odd group of single cell organisms. They typically occupy habitats that are very inhospitable and that other cells would typically not survive in. So archaeans have had to adapt to that environment, and one of the ways in which they've adapted to that environment is that they have altered their lipids that are contained in their lipid bilayer so that they are more resistant to the harsh conditions. Now we can see here a typical archaean amphiphilic lipid that's in the lipid bilayer. It has first of all at the top, a polar head. At the bottom it has a nonpolar tail, and that's very much like the lipids that are found in a cell that's in a normal environment or a non-toxic environment. But the difference between what is present in the archaean lipids and the cells of for example of a human being are a couple things.

    05:28 One, we see that the side chains, that is the fatty acids hanging off of the glycerol molecule that you see here are branched, they have carbons sticking off of them. Now that's thought to provide a protective function so that these fatty acids as they are, are not damaged and it turns out that the oxidation of fatty acids that are branched is much more complicated than it is of straight chain fatty acids.

    05:57 Another thing that we see in the archaean membrane is that instead of having ester bonds between the glycerol and the fatty acid, that archaeans instead have ether linkages.

    06:07 Now this may be important because ether bonds are stabler in the environments, the harsh environments, where the archaeans live than an ester bond is. So we see adaptation of the lipid bilayer according to the needs of the organism.

    About the Lecture

    The lecture Bilayer Composition – Biological Membranes by Kevin Ahern, PhD is from the course Biochemistry: Basics.

    Included Quiz Questions

    1. They have a polar head and a pair of polar tails
    2. They contain fatty acids
    3. They contain at least one phosphate
    4. They contain glycerol
    1. It contains a fatty acid
    2. It contains glycerol
    3. It does not contain phosphate
    4. It is not found in membranes
    1. They mediate cellular signaling and transport of materials
    2. They are called integral if they have a fatty acid anchoring them
    3. They are called peripheral if they project through both layers of the lipid bilayer
    4. They have non-polar fatty acids preferentially in their interiors
    1. It involves more unsaturated fatty acids for cells in colder temperatures
    2. It does not include cholesterol
    3. It includes proteins, lipids, and phosphates, but not carbohydrates
    4. It does not involve charged substances

    Author of lecture Bilayer Composition – Biological Membranes

     Kevin Ahern, PhD

    Kevin Ahern, PhD

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