Some span only part of the membrane—associating with a single layer—while others stretch from one side of the membrane to the other, and are exposed on either side.
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. Peripheral proteins are found on either the exterior or interior surfaces of membranes; and weakly or temporarily associated with the membranes.
They can interact with either integral membrane proteins or simply interact weakly with the phospholipids within the membrane. Figure 5. 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 one of the core functional requirements noted above in "cellular membranes". The mosaic characteristic of the membrane, described in the fluid mosaic model, helps to illustrate its nature.
The integral proteins and lipids exist in the membrane as separate molecules and they "float" in the membrane, moving somewhat with respect to one another.
The membrane is not like a balloon, however, in that can expand and contract dramatically; 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, and the membrane will flow and self-seal when the needle is extracted.
The mosaic characteristics of the membrane explain some but not all of its fluidity. There are two other factors that help maintain this fluid characteristic. One factor is the nature of the phospholipids themselves. In their saturated form, the fatty acids in phospholipid tails are saturated with hydrogen atoms. There are no double bonds between adjacent carbon atoms. This results in tails that are relatively straight.
By contrast, unsaturated fatty acids do not have a full complement of hydrogen atoms on their fatty acid tails, and therefore contain some double bonds between adjacent carbon atoms; a double bond results in a bend in the string of carbons of approximately 30 degrees. Figure 6. Any given cell membrane will be composed of a combination of saturated and unsaturated phospholipids. The ratio of the two will influence the permeability and fluidity of the membrane.
A membrane composed of completely saturated lipids will be dense and less fluid, and a membrane composed of completely unsaturated lipids will be very loose and very fluid. Organisms can be found living in extreme temperature conditions. Both in extreme cold or extreme heat. What types of differences would you expect to see in the lipid composition of organisms that live at these extremes?
Saturated fatty acids, with straight tails, are compressed by decreasing temperatures, and they will 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. 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.
Animals have an additional membrane constituent that assists in maintaining fluidity. Cholesterol, which lies alongside the phospholipids in the membrane, tends to dampen the effects of temperature on the membrane. Thus, this lipid functions as a "fluidity buffer", preventing lower temperatures from inhibiting fluidity and preventing increased temperatures from increasing fluidity too much.
Thus, cholesterol extends, in both directions, the range of temperature in which the membrane is appropriately fluid and consequently functional. Cholesterol also serves other functions, such as organizing clusters of transmembrane proteins into lipid rafts. They regulate the transport of molecules, control information flow between cells, generate signals to alter cell behavior, contain molecules responsible for cell adhesion in the formation of tissues, and can separate charged molecules for cell signaling and energy generation.
Cell membranes are dynamic, constantly being formed and degraded. Membrane vesicles move between cell organelles and the cell surface. Some of these proteins serve to transport materials into or out of the cell.
Carbohydrates are attached to some of the proteins and lipids on the outward-facing surface of the membrane, forming complexes that function to identify the cell to other cells. The fluid nature of the membrane is due to temperature, the configuration of the fatty acid tails some kinked by double bonds , the presence of cholesterol embedded in the membrane, and the mosaic nature of the proteins and protein-carbohydrate combinations, which are not firmly fixed in place.
Plasma membranes enclose and define the borders of cells, but rather than being a static bag, they are dynamic and constantly in flux. Which plasma membrane component can be either found on its surface or embedded in the membrane structure? Which characteristic of a phospholipid contributes to the fluidity of the membrane? The fluid characteristic of the cell membrane allows greater flexibility to the cell than it would if the membrane were rigid. It also allows the motion of membrane components, required for some types of membrane transport.
Why do phospholipids tend to spontaneously orient themselves into something resembling a membrane? The hydrophobic, nonpolar regions must align with each other in order for the structure to have minimal potential energy and, consequently, higher stability.
Thus, the head orients to water, and the tail to other lipids. Skip to content Structure and Function of Plasma Membranes. Learning Objectives By the end of this section, you will be able to: Understand the fluid mosaic model of cell membranes Describe the functions of phospholipids, proteins, and carbohydrates in membranes Discuss membrane fluidity. Fluid Mosaic Model The existence of the plasma membrane was identified in the s, and its chemical components were identified in The fluid mosaic model of the plasma membrane describes the plasma membrane as a fluid combination of phospholipids, cholesterol, and proteins.
Carbohydrates attached to lipids glycolipids and to proteins glycoproteins extend from the outward-facing surface of the membrane. Phospholipids The main fabric of the membrane is composed of amphiphilic, phospholipid molecules.
The hydrophilic head group consists of a phosphate-containing group attached to a glycerol molecule. The hydrophobic tails, each containing either a saturated or an unsaturated fatty acid, are long hydrocarbon chains. In an aqueous solution, phospholipids tend to arrange themselves with their polar heads facing outward and their hydrophobic tails facing inward.
Proteins Proteins make up the second major component of plasma membranes. Integral membranes 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 Carbohydrates are the third major component of plasma membranes. Evolution Connection. Membrane Fluidity The mosaic characteristic of the membrane, described in the fluid mosaic model, helps to illustrate its nature.
Link to Learning. Career Connection. Section Summary The modern understanding of the plasma membrane is referred to as the fluid mosaic model. Review Questions Which plasma membrane component can be either found on its surface or embedded in the membrane structure? What is the primary function of carbohydrates attached to the exterior of cell membranes? Free Response Why is it advantageous for the cell membrane to be fluid in nature? Previous: Introduction.
Next: Passive Transport. On the inner or outer surface of the phospholipid bilayer; not embedded within the phospholipids. Vertebrates, invertebrates and protozoa bear different set of carbohydrates in their cells.
Curiously, some pathogens are able to "dress" superficial carbohydrates similar to those of the host cells. In this way, they cannot be detected.
There are differences in the carbohydrate composition of cells of vertebrate, invertebrate and protozoa. The sweet and sour of cancer: glycans as novel therapeutic targets. Nature reviews cancer. Extracellular matrix Structural proteins Carbohydrates Glycoproteins Types 3.
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