Along with peripheral proteins, carbohydrates form specialized sites on the cell surface that allow cells to recognize each other.
How Viruses Infect Specific OrgansSpecific glycoprotein molecules exposed on the surface of the cell membranes of host cells are exploited by many viruses to infect specific organs.
For example, HIV is able to penetrate the plasma membranes of specific kinds of white blood cells called T-helper cells and monocytes, as well as some cells of the central nervous system. The hepatitis virus attacks only liver cells. These viruses are able to invade these cells, because the cells have binding sites on their surfaces that the viruses have exploited with equally specific glycoproteins in their coats.
Figure 3. The cell is tricked by the mimicry of the virus coat molecules, and the virus is able to enter the cell. Antibodies are made in response to the antigens or proteins associated with invasive pathogens. These same sites serve as places for antibodies to attach, and either destroy or inhibit the activity of the virus. Unfortunately, these sites on HIV are encoded by genes that change quickly, making the production of an effective vaccine against the virus very difficult.
The virus population within an infected individual quickly evolves through mutation into different populations, or variants, distinguished by differences in these recognition sites. The modern understanding of the plasma membrane is referred to as the fluid mosaic model. The plasma membrane is composed of a bilayer of phospholipids, with their hydrophobic, fatty acid tails in contact with each other. Inheritance 5. Genetic Modification 4: Ecology 1.
Energy Flow 3. Carbon Cycling 4. Climate Change 5: Evolution 1. Evolution Evidence 2. Natural Selection 3. Classification 4. Cladistics 6: Human Physiology 1. Digestion 2. The Blood System 3. Disease Defences 4. Some complex proteins are composed of up to 12 segments of a single protein, which are extensively folded and embedded in the membrane. This type of protein has a hydrophilic region or regions, and one or several mildly hydrophobic regions.
This arrangement of regions of the protein tends to orient the protein alongside the phospholipids, with the hydrophobic region of the protein adjacent to the tails of the phospholipids and the hydrophilic region or regions of the protein protruding from the membrane and in contact with the cytosol or extracellular fluid.
Structure of integral membrane proteins : Integral membrane proteins may have one or more alpha-helices that span the membrane examples 1 and 2 , or they may have beta-sheets that span the membrane example 3. Carbohydrates are the third major component of plasma membranes. They are always found on the exterior surface of cells and are bound either to proteins forming glycoproteins or to lipids forming glycolipids.
These carbohydrate chains may consist of 2—60 monosaccharide units and can be either straight or branched. Along with peripheral proteins, carbohydrates form specialized sites on the cell surface that allow cells to recognize each other.
Similar types of glycoproteins and glycolipids are found on the surfaces of viruses and may change frequently, preventing immune cells from recognizing and attacking them. The glycocalyx is highly hydrophilic and attracts large amounts of water to the surface of the cell. The mosaic nature of the membrane, its phospholipid chemistry, and the presence of cholesterol contribute to membrane fluidity.
There are multiple factors that lead to membrane fluidity. First, the mosaic characteristic of the membrane helps the plasma membrane remain fluid. The integral proteins and lipids exist in the membrane as separate but loosely-attached molecules. The membrane is not like a balloon that can expand and contract; rather, it is fairly rigid and can burst if penetrated or if a cell takes in too much water.
However, because of its mosaic nature, a very fine needle can easily penetrate a plasma membrane without causing it to burst; the membrane will flow and self-seal when the needle is extracted. Membrane Fluidity : The plasma membrane is a fluid combination of phospholipids, cholesterol, and proteins. The second factor that leads to fluidity is the nature of the phospholipids themselves. In their saturated form, the fatty acids in phospholipid tails are saturated with bound hydrogen atoms; there are no double bonds between adjacent carbon atoms.
This results in tails that are relatively straight. In contrast, unsaturated fatty acids do not contain a maximal number of hydrogen atoms, although they do contain some double bonds between adjacent carbon atoms; a double bond results in a bend of approximately 30 degrees in the string of carbons. Thus, if saturated fatty acids, with their straight tails, are compressed by decreasing temperatures, they press in on each other, making a dense and fairly rigid membrane.
The relative fluidity of the membrane is particularly important in a cold environment. A cold environment tends to compress membranes composed largely of saturated fatty acids, making them less fluid and more susceptible to rupturing. Many organisms fish are one example are capable of adapting to cold environments by changing the proportion of unsaturated fatty acids in their membranes in response to the lowering of the temperature.
In animals, the third factor that keeps the membrane fluid is cholesterol. First, the scientists extracted the lipids with a variety of solvents, including acetone, from a known number of cells. Then they used the Langmuir trough to determine how large an area the lipids could cover.
Because they could measure the actual size surface area of a red blood cell and knew approximately how many of those cells they had in their sample , they could calculate the total surface area that would have to be covered by membrane. When the two numbers were compared, it was clear that the amount of lipid they had extracted could cover twice the area needed to enclose all the cells. Why would there be so much? Additional experiments showed that lipids could spontaneously form a bilayer when mixed with water Figure 1.
Together, these observations suggested that there may be a simple explanation for the results with the red blood cells. The plasma membrane of these cells likely consists of a double layer of lipid surrounding each cell. As it happens, Gortner and Grendel made some errors in their experiment. They failed to completely extract all the lipids from the cells, and they also underestimated the total surface area of the individual red blood cells. However, because these two errors canceled each other out, their final conclusions turned out to be correct, regardless of their miscalculations.
Thereafter, the idea of a lipid bilayer became the basis for future models of membrane structure. Sadava When the use of electron microscopy started to allow examination of the plasma membrane at high resolution, people noticed that the image clearly showed three layers, not two. In a key paper, Stoeckenius provided clear pictures of the three-layer structure. He then described in both words and diagrams how the lipid bilayer results in a three-layer image.
As it turns out, the inner and outer edges of the bilayer have a different composition than the interior. Under the view of the electron microscope, the outsides of the lipid bilayer show up as two darker layers, whereas the hydrophobic interior stains less densely, thus showing three apparent "layers" outside layers are represented as blue in Figure 1C.
The first clues to lipid bilayer structure came from results with red blood cell membranes. The ultimate discovery that the plasma membrane is a lipid bilayer with hydrophobic and hydrophilic properties changed the way this structure was viewed. Its semipermeable and liquid nature provided the groundwork for understanding both its physical and biological properties.
Edidin, M. Lipids on the frontier: a century of cell-membrane lipids Nature Reviews : Molecular Cell Biology 4 : — Gortner, E. On bimolecular layers of lipoids on the chromacytes of blood. Journal of Experimental Medicine 41 , — Langmuir, I. The constitution and fundamental properties of solids and liquids II: Liquids. Journal of the American Chemical Society 39 , — Overton, E. The probable origin and physiological significance of cellular osmotic properties.
Vierteljahrschrift der Naturforschende gesselschaft 44 , 88— In Biological Membrane Structure , trans. Park, R. Boston: Little Brown, Sadava, D. Cell Biology, Organelle Structure and Function.
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