The existence of the plasma membrane was identified in the 1890s and its chemical components were identified in 1915. The main components identified at that time were lipids and proteins. The first widely accepted model of the structure of the plasma membrane was proposed by Hugh Davson and James Danielli in 1935; it was based on the "rail track" appearance of the plasma membrane in early electron micrographs. They theorized that the structure of the plasma membrane resembles a sandwich, with proteins corresponding to the bread and lipids to the filling. In the 1950s, advances in microscopy, particularly transmission electron microscopy (TEM), allowed researchers to see that the core of the plasma membrane consisted of a double layer rather than a single layer. A new model that better explains both microscopic observations and the function of this plasma membrane was developed by S.J. Cantor and Garth L. Nicolson in 1972.
The explanation proposed by Singer and Nicolson is calledliquid mosaic model. The model has evolved somewhat over time, but it still better explains the structure and functions of the plasma membrane as we understand it now. The fluid mosaic model describes the structure of the plasma membrane as a mosaic of components, including phospholipids, cholesterol, proteins, and carbohydrates, which give the membrane a fluid character. Plasma membranes are 5 to 10 nm thick. For comparison, human red blood cells, visible under a light microscope, are about 8 µm in diameter, or about 1,000 times the width of a plasma membrane. The membrane looks a bit like a sandwich (Cipher).
The main components of a plasma membrane are lipids (phospholipids and cholesterol), proteins, and carbohydrates attached to some of the lipids and some of the proteins. A phospholipid is a molecule composed of glycerol, two fatty acids, and a phosphate-bonded headgroup. Cholesterol, another lipid made up of four fused carbon rings, is found in the center of the membrane along with phospholipids. The proportions of proteins, lipids, and carbohydrates in the plasma membrane vary with cell type, but in a typical human cell, protein makes up about 50% of the mass composition, lipids (of all types) make up about 40% of the mass. composition, with the remaining 10 percent of the mass composition being carbohydrates. However, the concentration of proteins and lipids varies with different cell membranes. For example, myelin, a membrane growth of specialized cells that insulate peripheral nerve axons, is only 18% protein and 76% lipid. The mitochondrial inner membrane contains 76% protein and only 24% lipid. The plasma membrane of human red blood cells is composed of 30% lipids. Carbohydrates are present only on the outer surface of the plasma membrane and bind to proteins and formglycoprotein, binding to lipidsglycolipid.
phospholipid
The main membrane tissue consists of amphiphilic phospholipid molecules. Thehydrophilicor "love" portions of these molecules (which look like a collection of spheres in an artist's rendering of a model) (Cipher) are in contact with the aqueous fluid inside and outside the cell.hydrophobic, or water-hating molecules, tend to be nonpolar. They interact with other nonpolar molecules in chemical reactions, but generally do not interact with polar molecules. When placed in water, hydrophobic molecules tend to form a sphere or a cluster. The hydrophilic regions of phospholipids tend to form hydrogen bonds with water and other polar molecules both outside and inside the cell. Therefore, the inward and outward facing surfaces of the cell membrane are hydrophilic. Instead, the inside of the cell membrane is hydrophobic and does not react with water. Therefore, phospholipids form an excellent bilayer cell membrane that separates the fluid inside the cell from the fluid outside the cell.
A phospholipid molecule (Cipher) consists of a three-carbon glycerol backbone with two fatty acid molecules attached to carbons 1 and 2 and a phosphate-containing group attached to the third carbon. This arrangement gives the molecule as a whole a region called the head (the phosphate-containing group) that has a polar character, or negative charge, and a region called the tail (the fatty acids) that has no charge. The head can form hydrogen bonds, but the tail cannot. A molecule with this arrangement of a positively or negatively charged area and an uncharged or nonpolar area is calledamphiphilicor "doubly loving".
This property is fundamental to the structure of a plasma membrane, since phospholipids in water tend to arrange themselves with their hydrophobic ends facing each other and their hydrophilic heads facing outward. In this way, they form a lipid bilayer, a barrier formed by a phospholipid bilayer that separates water and other materials on one side of the barrier from water and other materials on the other side. In fact, when phospholipids are heated in aqueous solution, they tend to spontaneously form small globules or droplets (called micelles or liposomes), with their hydrophilic heads forming the outside and their hydrophobic tails forming the inside.Cipher).
Protein
Proteins form the second major component of plasma membranes.Integral Proteins(Some specialized types are called integrins), as the name suggests, they are fully integrated into the membrane structure, and their hydrophobic regions spanning the membrane interact with the hydrophobic region of the phospholipid bilayer (Cipher). Integral single-pass membrane proteins typically have a hydrophobic transmembrane segment composed of 20 to 25 amino acids. Some span only part of the membrane, connected to a single layer, while others extend from one side of the membrane to the other, leaving them exposed on both sides. Some complex proteins consist of up to 12 segments of a single highly folded protein embedded in the membrane.Cipher). This type of protein has a hydrophilic region or regions and one or more slightly hydrophobic regions. This arrangement of protein regions tends to orient the protein adjacent to the phospholipids, with the hydrophobic region of the protein adjacent to the phospholipid tails and the hydrophilic region(s) of the protein projecting from the membrane and in contact with the cytosol. . .
peripheral proteinsThey are found on the inner and outer surfaces of membranes attached to integral proteins or phospholipids. Peripheral proteins, along with integral proteins, can serve as enzymes, as structural junctions for cytoskeletal fibers, or as part of recognition sites in the cell. They are sometimes called "cell-specific" proteins. The body recognizes its own proteins and attacks foreign proteins associated with invading pathogens.
carbohydrates
Carbohydrates are the third major component of plasma membranes. They are always found on the outer surface of cells and bind to proteins (forming glycoproteins) or lipids (forming glycolipids) (Cipher). These carbohydrate chains can consist of 2 to 60 monosaccharide units and can be linear or branched. Along with peripheral proteins, carbohydrates form specialized sites on the cell surface that allow cells to recognize themselves. These dots have unique patterns that allow the cell to be recognized, just as each person's unique facial features allow it to be recognized. This recognition function is very important for cells because it allows the immune system to distinguish between body cells (called "self") and foreign cells or tissues (called "foreign"). Similar types of glycoproteins and glycolipids are found on the surface of viruses and can change frequently, preventing immune cells from recognizing and attacking them.
These carbohydrates on the outer surface of the cell, the carbohydrate components of both glycoproteins and glycolipids, are collectively called the glycocalyx (meaning "sugar shell"). The glycocalyx is highly hydrophilic and attracts large amounts of water to the cell surface. This supports the cell's interaction with its aqueous environment and the cell's ability to conserve substances dissolved in water. As discussed above, the glycocalyx is also important in cell identification, self-determination and non-self-determination, and embryonic development, and is used in cell-cell junctions to form tissues.
evolutionary connection
How viruses infect specific organsPatterns of glycoproteins and glycolipids on the surface of cells provide an opportunity for infection by many viruses. The HIV and hepatitis viruses only infect specific organs or cells in the human body. HIV can cross the plasma membranes of a subtype of lymphocyte called helper T cells, as well as some monocytes and cells of the central nervous system. The hepatitis virus attacks liver cells.
These viruses can enter these cells because the cells have binding sites on their surface that are specific and compatible for certain viruses (Cipher). Other recognition sites on the surface of the virus interact with the human immune system, prompting the body to produce antibodies. Antibodies are produced in response to antigens or proteins associated with invading pathogens, or in response to foreign cells, as can occur in organ transplantation. The same sites serve as sites for antibodies to bind and kill or inhibit virus activity. Unfortunately, these recognition sites in HIV change very quickly due to mutations, making it very difficult to produce an effective vaccine against the virus as the virus evolves and adapts. An HIV-infected person quickly develops different populations or variants of the virus, distinguished by differences in these recognition sites. This rapid change in surface markers reduces the effectiveness of the person's immune system in attacking the virus because the antibodies do not recognize the new variations in surface patterns. In the case of HIV, the problem is complicated by the fact that the virus specifically infects and destroys cells involved in the immune response, further disabling the host.
