Structures of Cell membrane, its components

THE CELL MEMBRANE:

                                               This membrane, also called plasma membrane or plasmalemma, which envelops the cell completely and acts as a selective structural barrier between the contents of the cell and contents of the extracellular space. Noted that the cell membrane is much more than just a boundary structure and participates in various functions of the cell.

Structure:

                     Due to very thin structure of the plasmalemma, cannot be seen in tissue sections under the light microscope. So we used a highly magnified electron micrographs to see a plasmalemma clearly. In  micrographs, the cell membrane generally seen to have an average thickness of 7.5 nm and a trilaminar (3 - layered) structure consisting of two electron dense lines separated by an electron - lucent central zone; each layer is roughly 2.5 nm in thickness. Remembered that in different cells, the thickness of the cell membrane is variable and ranges from 7.5-10 nm.

cell membrane

Components:

                        The cell membrane or plasmalemma mainly composed of the following components:

• ·     Phospholipids.
• ·       Proteins.
• ·       Cholesterol.
• ·       Carbohydrates.

Fluid Mosaic Model:           

                                  The cell membrane or plasmalemma consists of a bimolecular layer of phospholipids (called lipid bilayer) in which protein and cholesterol molecules are embedded. Because the lipid bilayer is fluid in nature and the large molecules of protein are suspended in it exhibit a mosaic pattern, this model of the cell membrane structure is known as the fluid mosaic model.

 It is to be noted that the intracellular membranes, which enclose the nucleus and organelles of the cell, are also have the same trilaminar structure as that of the cell membrane.

Fluid mosaic model

Ø       Phospholipids:

                               Each phospholipid molecule of the lipid bilayer of the cell membrane consists of a polar (hydrophilic) head and a nonpolar (hydrophobic) tail. The head of each phospholipid molecule is composed of glycerol conjugated to a nitrogenous compound by a phosphate bridge. The nitrogenous compound may be choline, serine or ethanolamine. The nonpolar tail of each phospholipid molecule is made up of two long chains of fatty acids which are covalently linked to the glycerol component of the polar head of the phospholipid molecule. In the bimolecular layer the hydrophilic heads of the phospholipid molecules lie at the outer and inner faces of the cell membrane, while the hydrophobic tails are directed toward the middle of the bilayer.

Phospholipid

Ø        Cholesterol:

                            The cholesterol molecules lie between the long fatty acid chains of the phospholipids. Cholesterol stiffens the cell membrane and, therefore, the membrane fluidity is diminished by a high cholesterol content.

Ø      Proteins:

             Protein molecules make up approximately 50 % of the total membrane mass. The membrane proteins mainly divided into two groups:

(1)     Integral proteins: The integral proteins are either embedded within the lipid bilayer or pass through the bilayer completely.

(2)    Peripheral proteins: The peripheral proteins are not embedded in the lipid bilayer but instead, stay loosely associated with the integral proteins at the internal and external surfaces of the cell membrane; weak electrostatic forces bind the peripheral proteins to the integral proteins.

           Most of the membrane proteins are integral proteins, Some of these proteins are embedded in the cell membrane in such a way that one of their ends is anchored in the lipid bilayer, while the other end projects on the internal (cytoplasmic) or external (extracellular) surface of the cell membrane. However, majority of the integral proteins pass through the entire thickness of the cell membrane and hence are called transmembrane proteins. As indicated by their name, the transmembrane proteins span the lipid bilayer from one surface of the cell membrane to the other. Some of the transmembrane proteins span the membrane only once and, a therefore, known as one proteins.

Proteins

However, many of the transmembrane proteins are quite long and folded so that they make several passes through the membrane and hence are called multipass proteins.

Ø       Carbohydrates:

                         Chains of carbohydrate residues (sugars), which are generally oligosaccharides, are attached on that surface of the plasma membrane which faces away from the cytosol. Therefore, they are located on the outer surface of the cell membrane and on the inner surface of the membranes enclosing the cytoplasmic organelles.

    These carbohydrate residues are bound either to the heads of the phospholipid molecules to constitute glycolipids, or they are attached to the membrane proteins to form glycoproteins.

Most of the component proteins of the plasmalemma as well as intracellular membranes are of basic nature Therefore, the cell membrane and all the intracellular membranous structures, e.g., the mitochondria, Golgi apparatus, and smooth endoplasmic reticulum, attract acid dyes and are stained eosinophilic in the ordinary H & E stained tissue sections.

    On some cells, especially on the epithelial cells of the body. the glycoproteins and glycolipids of the cell membrane constitute a layer called glycocalyx. Under the EM, the glycocalyx appears as a fuzzy coat over the outer surface of the cell.

Ø      Glycocalyx:

                    The glycocalyx, also called cell coat, performs several functions which may vary in different cells. Generally, the molecules of the glycocalyx enable the cell to recognize the other cells, help in cell association and adhesion, and serve as receptor sites for hormones.

·       The glycocalyx covering the luminal surface of the absorptive cells (enterocytes) of the small intestine plays an important role in the digestion of the food materials.

·       The glycocalyx covering the endothelial cells lining the blood vessels also contains many important enzymes like angiotensin - converting enzyme (ACE), antithrombin III, and lipoprotein lipase.