Around the House Items That Accurately Represent a Cell Membrane
Chapter 3: Introduction to Cell Structure and Office
three.4 The Cell Membrane
By the end of this section, you volition be able to:
- Empathize the fluid mosaic model of membranes
- Describe the functions of phospholipids, proteins, and carbohydrates in membranes
A cell's plasma membrane defines the boundary of the cell and determines the nature of its contact with the environs. Cells exclude some substances, take in others, and excrete still others, all in controlled quantities. Plasma membranes enclose the borders of cells, but rather than being a static bag, they are dynamic and constantly in flux. The plasma membrane must be sufficiently flexible to allow certain cells, such every bit crimson blood cells and white blood cells, to modify shape equally they laissez passer through narrow capillaries. These are the more than obvious functions of a plasma membrane. In addition, the surface of the plasma membrane carries markers that allow cells to recognize one another, which is vital as tissues and organs form during early development, and which subsequently plays a role in the "cocky" versus "non-self" distinction of the immune response.
The plasma membrane also carries receptors, which are attachment sites for specific substances that collaborate with the cell. Each receptor is structured to bind with a specific substance. For case, 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 matter or toxins for disposal. Receptors on the plasma membrane's exterior surface interact with hormones or neurotransmitters, and permit their messages to be transmitted into the jail cell. Some recognition sites are used by viruses equally attachment points. Although they are highly specific, pathogens similar viruses may evolve to exploit receptors to gain entry to a prison cell by mimicking the specific substance that the receptor is meant to demark. This specificity helps to explicate why human immunodeficiency virus (HIV) or whatsoever of the v types of hepatitis viruses invade only specific cells.
Fluid Mosaic Model
In 1972, South. J. Singer and Garth L. Nicolson proposed a new model of the plasma membrane that, compared to before agreement, better explained both microscopic observations and the function of the plasma membrane. This was called the fluid mosaic model. The model has evolved somewhat over fourth dimension, just yet all-time accounts for the structure 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 change position, while maintaining the bones integrity of the membrane. Both phospholipid molecules and embedded proteins are able to diffuse quickly and laterally in the membrane. The fluidity of the plasma membrane is necessary for the activities of certain enzymes and ship molecules within the membrane. Plasma membranes range from v–ten nm thick. Every bit a comparison, man red blood cells, visible via light microscopy, are approximately viii µm thick, or approximately one,000 times thicker than a plasma membrane.
The plasma membrane is made upwards primarily of a bilayer of phospholipids with embedded proteins, carbohydrates, glycolipids, and glycoproteins, and, in animal cells, cholesterol. The amount of cholesterol in animal plasma membranes regulates the fluidity of the membrane and changes based on the temperature of the prison cell's surroundings. In other words, cholesterol acts every bit antifreeze in the jail cell membrane and is more abundant in animals that live in common cold climates.
The main material of the membrane is equanimous of two layers of phospholipid molecules, and the polar ends of these molecules (which look like a collection of balls in an creative person's rendition of the model) (Figure 3.22) are in contact with aqueous fluid both inside and outside the cell. Thus, both surfaces of the plasma membrane are hydrophilic. In contrast, the interior of the membrane, betwixt its two surfaces, is a hydrophobic or nonpolar region because of the fat acid tails. This region has no allure for water 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 span all or part of the membrane. Integral proteins may serve as channels or pumps to move materials into or out of the cell. Peripheral proteins are found on the exterior or interior surfaces of membranes, attached either to integral proteins or to phospholipid molecules. Both integral and peripheral proteins may serve every bit enzymes, every bit structural attachments for the fibers of the cytoskeleton, or every bit function of the cell'southward recognition sites.
Carbohydrates are the third major component of plasma membranes. They are always institute 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 may exist either straight or branched. Along with peripheral proteins, carbohydrates form specialized sites on the prison 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 past 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, as well as some cells of the key nervous organization. The hepatitis virus attacks simply liver cells.
These viruses are able to invade these cells, because the cells have bounden sites on their surfaces that the viruses take exploited with as specific glycoproteins in their coats. (Figure three.23). The jail cell is tricked by the mimicry of the virus coat molecules, and the virus is able to enter the cell. Other recognition sites on the virus'southward surface interact with the human allowed system, prompting the trunk to produce antibodies. Antibodies are fabricated in response to the antigens (or proteins associated with invasive pathogens). These same sites serve as places for antibodies to adhere, 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 private quickly 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's allowed organization in attacking the virus, because the antibodies will non recognize the new variations of the surface patterns.
Section Summary
The modern understanding of the plasma membrane is referred to equally the fluid mosaic model. The plasma membrane is equanimous of a bilayer of phospholipids, with their hydrophobic, fatty acrid tails in contact with each other. The landscape of the membrane is studded with proteins, some of which bridge the membrane. Some of these proteins serve to transport materials into or out of the cell. Carbohydrates are fastened to some of the proteins and lipids on the outward-facing surface of the membrane. These form complexes that function to place the cell to other cells. The fluid nature of the membrane owes itself to the configuration of the fat acid tails, the presence of cholesterol embedded in the membrane (in animal cells), and the mosaic nature of the proteins and protein-carbohydrate complexes, which are not firmly fixed in place. Plasma membranes enclose the borders of cells, merely rather than being a static bag, they are dynamic and constantly in flux.
fluid mosaic model: a model of the construction of the plasma membrane as a mosaic of components, including phospholipids, cholesterol, proteins, and glycolipids, resulting in a fluid rather than static grapheme
Media Attribution
- Figure 3.23: modification of work by U.s. National Institutes of Wellness/National Institute of Allergy and Infectious Diseases
Source: https://opentextbc.ca/biology/chapter/3-4-the-cell-membrane/
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