Of The Following Functions, Which Is Most Important For The Glycoproteins Of Animal Cell Membranes?
Chapter 3: Introduction to Cell Construction and Function
iii.4 The Prison cell Membrane
Past the end of this section, y'all will exist able to:
- Understand the fluid mosaic model of membranes
- Describe the functions of phospholipids, proteins, and carbohydrates in membranes
A jail cell's plasma membrane defines the purlieus of the cell and determines the nature of its contact with the environment. Cells exclude some substances, take in others, and excrete nonetheless others, all in controlled quantities. Plasma membranes enclose the borders of cells, merely rather than beingness a static bag, they are dynamic and constantly in flux. The plasma membrane must exist sufficiently flexible to allow certain cells, such every bit ruddy claret cells and white blood cells, to change shape as they pass through narrow capillaries. These are the more obvious functions of a plasma membrane. In addition, the surface of the plasma membrane carries markers that allow cells to recognize 1 another, which is vital as tissues and organs form during early development, and which later plays a part in the "cocky" versus "non-cocky" distinction of the immune response.
The plasma membrane likewise carries receptors, which are attachment sites for specific substances that interact with the cell. Each receptor is structured to demark with a specific substance. For example, surface receptors of the membrane create changes in the interior, such as changes in enzymes of metabolic pathways. These metabolic pathways might be vital for providing the cell with energy, making specific substances for the cell, or breaking down cellular waste or toxins for disposal. Receptors on the plasma membrane's exterior surface interact with hormones or neurotransmitters, and allow their letters to exist transmitted into the cell. Some recognition sites are used past viruses as zipper points. Although they are highly specific, pathogens like viruses may evolve to exploit receptors to proceeds entry to a jail cell by mimicking the specific substance that the receptor is meant to demark. This specificity helps to explain why man immunodeficiency virus (HIV) or any of the five types of hepatitis viruses invade only specific cells.
Fluid Mosaic Model
In 1972, Due south. J. Vocalizer and Garth L. Nicolson proposed a new model of the plasma membrane that, compared to earlier understanding, better explained both microscopic observations and the part of the plasma membrane. This was called the fluid mosaic model. The model has evolved somewhat over fourth dimension, just withal best accounts for the construction and functions of the plasma membrane as we now understand them. The fluid mosaic model describes the structure of the plasma membrane as a mosaic of components—including phospholipids, cholesterol, proteins, and carbohydrates—in which the components are able to flow and alter position, while maintaining the basic integrity of the membrane. Both phospholipid molecules and embedded proteins are able to lengthened chop-chop and laterally in the membrane. The fluidity of the plasma membrane is necessary for the activities of certain enzymes and transport molecules inside the membrane. Plasma membranes range from five–x nm thick. As a comparison, man red blood cells, visible via light microscopy, are approximately 8 µm thick, or approximately 1,000 times thicker than a plasma membrane.
The plasma membrane is made upward primarily of a bilayer of phospholipids with embedded proteins, carbohydrates, glycolipids, and glycoproteins, and, in fauna cells, cholesterol. The corporeality of cholesterol in animal plasma membranes regulates the fluidity of the membrane and changes based on the temperature of the cell'south environment. In other words, cholesterol acts every bit antifreeze in the cell membrane and is more abundant in animals that live in common cold climates.
The primary cloth of the membrane is composed of two layers of phospholipid molecules, and the polar ends of these molecules (which expect like a collection of balls in an creative person's rendition of the model) (Figure three.22) are in contact with aqueous fluid both inside and exterior the jail cell. Thus, both surfaces of the plasma membrane are hydrophilic. In contrast, the interior of the membrane, between its two surfaces, is a hydrophobic or nonpolar region because of the fatty acid tails. This region has no attraction for h2o or other polar molecules.
Proteins brand up the second major chemic component of plasma membranes. Integral proteins are embedded in the plasma membrane and may bridge all or role of the membrane. Integral proteins may serve as channels or pumps to move materials into or out of the cell. Peripheral proteins are institute on the outside or interior surfaces of membranes, fastened either to integral proteins or to phospholipid molecules. Both integral and peripheral proteins may serve every bit enzymes, as structural attachments for the fibers of the cytoskeleton, or as part of the cell'southward recognition sites.
Carbohydrates are the third major component of plasma membranes. They are always found on the exterior surface of cells and are spring either to proteins (forming glycoproteins) or to lipids (forming glycolipids). These carbohydrate bondage may consist of two–60 monosaccharide units and may be either directly or branched. Along with peripheral proteins, carbohydrates form specialized sites on the cell surface that permit cells to recognize each other.
Evolution in Action
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 chosen T-helper cells and monocytes, also equally some cells of the central nervous system. The hepatitis virus attacks only liver cells.
These viruses are able to invade these cells, considering the cells have binding sites on their surfaces that the viruses accept exploited with every bit specific glycoproteins in their coats. (Figure 3.23). The cell is tricked past the mimicry of the virus coat molecules, and the virus is able to enter the cell. Other recognition sites on the virus's surface interact with the homo allowed system, prompting the body to produce antibodies. Antibodies are made in response to the antigens (or proteins associated with invasive pathogens). These aforementioned sites serve every bit 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 alter rapidly, making the production of an effective vaccine confronting the virus very difficult. The virus population inside an infected individual chop-chop evolves through mutation into different populations, or variants, distinguished by differences in these recognition sites. This rapid change of viral surface markers decreases the effectiveness of the person'due south immune arrangement in attacking the virus, because the antibodies will not recognize the new variations of the surface patterns.
Section Summary
The modern agreement 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. The mural of the membrane is studded with proteins, some of which span the membrane. Some of these proteins serve to ship 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. These course complexes that office to identify the cell to other cells. The fluid nature of the membrane owes itself to the configuration of the fatty acid tails, the presence of cholesterol embedded in the membrane (in animate being cells), and the mosaic nature of the proteins and protein-carbohydrate complexes, which are not firmly stock-still in place. Plasma membranes enclose the borders of cells, only rather than being a static purse, they are dynamic and constantly in flux.
fluid mosaic model: a model of the structure of the plasma membrane every bit a mosaic of components, including phospholipids, cholesterol, proteins, and glycolipids, resulting in a fluid rather than static character
Media Attribution
- Figure 3.23: modification of piece of work past United states National Institutes of Health/National Institute of Allergy and Infectious Diseases
Source: https://opentextbc.ca/biology/chapter/3-4-the-cell-membrane/
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