Cell Membrane : The outer covering of the cell consisting of lipid bilayer with proteins embedded in it that seperates interior of all cells from the outside environment.It is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells.
Composition of cell membrane :
Lipids and proteins are the two major components of all cell membranes.Their concentrations vary greatly between membrane protein ranges from about 20% in the myelin sheath to over 70% in the inner membrane of mitochondria. Integral membranre proteins represent about 25% of sequenced genes. Membranes contain a small amount of various polysaccharides as glycoprotein and glycolipid but no free carbohydrate.
There are three major lipid components in eukaryotic membranes which are glycerophospholids, sphingophospholipids and cholesterol. Glycerophospholipids and sphingomyelin a sphingolipid that contains phosphate are classified as phospholipids. The free chains in phospholids and glycolipids usually contain an even number of carbon atoms ,typically between 16 to 20. The 16- and 18-carbon fatty acids are the most common phosphatidylethanolamine (Cephalin) and phosphatidylcholine (Lecithin) are the most common glycerophospholipids in membranes.
Bacteria and blue green algae contain glycerolipids in which a carbohydrate is attached directly to the glycerol.Some membranes contain small quantities of other lipids such as trigacylglyceroland diol derivatives as well as lipid covalently linked to protein.
Basic structure of cell wall :
All biological membranes have a common generall structure , each is very thin film of lipid and protein molecules held together mainly noncovalent interactions.Cell membranes are dynamic fluid sructures and most of their molecules are able to move about the plane of the membrane.Lipid and proeins are the two major components af all membranes. The lipid molecules are arranged as continuous double layer abot 5 nm thick. There are three major lipid components namely glycerophospholipids, sphingophospholipids and cholesterol which are amphiphatic in nature . Besides lipids and proteins membranes contain a small amounts of various polysaccharides as glycoprotein but no free carbohydrate.The amount of each depends upon the type of cell but in the majority of classes phospholipids are the most abundant.
Lipid bilayers from through the process of self-asembly in which the hydrophilic/polar heads of the phospholipids facing outward and the hydrophobic/non polar tails facing inward . The lipid bilayer provides the basic fluid structure of the membrane and serves as a relatively impermeable barrier to passage of most water soluble molecules.
The cell membranes contain many peripheral proteins those are losely associaed with the membrane through electroostatic interactions and hydrogen bonds or by covalently attached lipid anchors .
Integral proteins associated firmly with membranes by hydrophobic interactions between the lipid bilayer and their nonpolar amino acid side chains which are oriented toward the outside of the protein molecule. These proteins are the most abundant type of protein molecule to span the lipid bilayer.The peripheral proteins may serve as regulators of memrane bound enzymes or may limit the mobility of integral proteins by tethering them to intracellular structures.
Some membrane proteins are covalently linked to complex arrays of carbohydrate. The sugar moities of surface glycoproteins influence the folding of the proteins as well as their stabilities and intracellular destinations and they play a significant role in the specific binding of ligands to glycoprotein surface receptors.Some are covalently attached to one or more lipids which serve as hydrophobic anchors that hold the proteins to the membrane.
To describe the structural features of cell membranes S.J.Singer and G.L.Nicolson proposed a model namely Fluid Mosaic Model in 1972.
Biological Role of Cell Membranes :
1. The cell membrane surrounds the cytoplasm of a cell and in animal cells physically seperates the intracellular components from the extracellular environment.
2. It plays a role in anchoring the cytoskeleton to provide shape to the cell and in attaching to the extracellular matrix and other cells together to form tissues.
3. The membrane is differentially permeable and able to regulate what enters and exits in the cell thus facilitating the transport of materials needed for survival.
4. The membrane maintains cell potential. Thus the membrane work as a selective filter that allows only contain things to come inside or go outside the cell. To do so membrane employs a number of transport mechanism.
a. Diffusion : Some substances (small molecules, ions) such as CO2,O2 , and H2O can move across the plasma membrane by this passive process.
b.Osmosis : Membrane acts as a barier for certain molecules and ions , and they can occur in different concentrations on the two sides of membrane .By occuring difference membrane can set up a osmotic flow for the solvent, in this case water.
c. Mediated transport : By special membrane protein or permeases membrane allows to enter sugars and materials of growth such as amino acids and to leave waste of metabolism.
d. Endocytosis : It is a cellular proccess in which cells absorb molecules by engulfing them.
e. Exocytosis : It is a cellular process where cells eject waste products or chemical transmitters from the interior of the cell.
5. The protein molecules of cell membrane receive signals from the other cells or the outside environment and convert the signals to message that are passed to the organelles inside the cell.
