Also referred to as the cell membrane, plasma membrane is the membrane found in all cells, which separate the inner part of the cell from the exterior. A cell wall is found to be attached to the plasma membrane to its exterior in plant and bacterial cells. Plasma membrane is composed of a lipid layer which is semipermeable. It is responsible to regulate the transportation of materials and the movement of substances in and out of the cell.
In addition to containing a lipid layer sitting between the phospholipids maintaining fluidity at a range of temperatures, the plasma membrane also has membrane proteins. This also includes integral proteins passing through the membrane which act as membrane transporters and peripheral proteins attaching to the sides of the cell membrane. It loosely serves as enzymes which shape the cell. Plasma membrane is selectively permeable to organic molecules and ions, it regulates the movement of particles in and out of organelles and cells.
Plasma Membrane Function
This membrane is composed of a phospholipid bilayer implanted with proteins. It forms a stable barrier between two aqueous compartments, which are towards the outside and inside of a cell in plasma membrane. The embedded proteins perform specialized functions which include cell-cell recognition and selective transport of molecules. Plasma membrane renders protection to the cell along with providing a fixed environment within the cell. It is responsible for performing different functions. In order for it allow movement of substances such as white and red blood cells, it must be flexible such that they could alter shape and pass through blood capillaries.
In addition, it also anchors the cytoskeleton to render shape to a cell and in associating with extracellular matrix and other cells to assist the cells in forming a tissue. It also maintains the cell potential. Plasma membrane is responsible for interacting with other, adjacent cells which can be glycoprotein or lipid proteins. The membrane also assists the proteins to monitor and maintain the chemical climate of the cell, along with the assistance in the shifting of molecules across the membrane.
Plasma Membrane – Components
It is composed of the following constituents:
- Phospholipids – forms the ultimate fabric of the membrane
- Peripheral proteins – present on the outer or inner surface of phospholipid bilayer but are not implanted in the hydrophobic core
- Cholesterol – folded between the hydrophobic tails of phospholipid membrane
- Carbohydrates – found to be attached to the lipids or proteins on the extracellular side of the membrane, leading to the formation of glycolipids and glycoproteins
- Integral proteins – found to be implanted in the phospholipid bilayer
Structure Of Plasma Membrane
Plasma membrane is a fluid mosaic of proteins, lipids and carbohydrates. It is impermeable to ions and water soluble molecules crossing membranes only through carriers, transmembrane channels and pumps. The transmembrane proteins nourish the cell with nutrients, regulate the internal ion concentration and set up a transmembrane electrical potential. Change in a single amino acid in one Cl− channel and plasma membrane pump can lead to human disease cystic fibrosis. On the basis of location of the membrane in the body, lipids can make up anywhere from 20-80% of the membrane, the rest being proteins.
It is composed of a phospholipid bilayer, which is two layers of phospholipids back-to-back. Phospholipids are lipids with a phosphate group associated with them. The phospholipids have one head and two tails where the head is polar and water-loving or hydrophilic. Tails on the other hand are nonpolar and water fearing or hydrophobic.
Fluid Mosaic Model
The description of the structure of plasma membrane can be carried out through the fluid mosaic model as a mosaic cholesterol, carbohydrates, proteins and phospholipids.
First proposed in 1972 by Garth L. Nicolson and S.J. Singer, the model explained the structure of plasma membranes. The model evolved with time however, it still accounts for the functions and structure of plasma membranes the best way. The model describes plasma membrane structure as a mosaic of components which includes proteins, cholesterol, phospholipids, and carbohydrates; it imparts a fluid character on the membrane.
Thickness of the membrane is in the range 5-10nm. The proportion of constituency of plasma membrane i.e., the carbohydrates, lipids and proteins vary from cell to cell. For instance, the inner membrane of the mitochondria comprises 24% lipid and 76% protein, in myelin, 76% lipid is found and 18% protein.
Chief fabric of this membrane comprises phospholipid molecules that are amphiphilic. The hydrophilic regions of such molecules are in touch with the aqueous fluid outside and inside the cell. The hydrophobic or the water hating molecules on the other hand are non-polar in nature. One phospholipid molecule comprises a three-carbon glycerol backbone along with 2 fatty acid molecules associated to carbons 1 and 2, and one phosphate-containing group connected to the third carbon.
This organisation provides a region known as head to the molecule on the whole. The head, which is a phosphate-containing group possesses a polar character or a negative charge while the tail, another region containing fatty acids, does not have any charge. They tend to interact with the non-polar molecules in a chemical reaction however, do not typically interact with the polar molecules.
The hydrophobic molecules when introduced to water, have the tendency to form a cluster. On the other hand, hydrophilic areas of the phospholipids have the tendency to form hydrogen bonds with water along with other polar molecules within and outside the cell. Therefore, the membrane surface interacting with the exterior and interior of cells are said to be hydrophilic. On the contrary, the middle of the cell membrane is hydrophobic and does not have any interaction with water. Hence, phospholipids go on to form a great lipid bilayer cell membrane separating fluid inside the cell from the fluid to the exterior of the cell.
The second major component is formed by the proteins of the plasma membrane. Integrins or integral proteins integrate fully into the structure of the membrane, along with their hydrophobic membrane, ranging from regions interacting with hydrophobic regions of phospholipid bilayer. Typically, single pass integral membrane proteins possess a hydrophobic transmembrane segment consisting of 20-25 amino acids. Few of these traverse only a portion of the membrane linking with one layer whereas others span from one to another side of the membrane, thereby exposing to the flip side.
Few complex proteins consist of 12 segments of a one protein, highly convoluted to be implanted in the membrane. Such a type of protein has a hydrophilic region/s along with one or more mildly hydrophobic areas. This organisation of areas of the proteins has the tendency to align the protein along with phospholipids where the hydrophobic area of the protein next to the tails of the phospholipids and hydrophilic areas of protein protrudes through the membrane is in touch with the extracellular fluid or cytosol.
The third most important component of the plasma membrane are carbohydrates. They are generally found on the outside of the cells and linked either to lipids to form glycolipids or proteins to form glycoproteins. The chain of this carbohydrate can comprise two to sixty monosaccharide units which could be branched or straight.
Carbohydrates alongside peripheral proteins lead to the formation of concentrated sites on the surface of the cell which identify each other. This identification is crucial to cells as they permit the immune system to distinguish between the cells of the body and the foreign cells/tissues. Such glycoproteins and glycoproteins are also observed on the surface of viruses, which can vary thereby preventing the immune cells to identify them and attract them.
On the exterior surface of cells, these carbohydrates, their components of both glycolipids and glycoproteins are together known as glycocalyx, which is extremely hydrophilic in nature attracting huge quantities of water on the cell surface. This helps the cell to interact with its fluid like environment and also in the ability of the cell to acquire substance dissolved in water.
You just learnt about the structure of plasma membranes. Learn some other interesting concepts important for NEET, at BYJU’S.