Cell membrane or plasma membrane or plasma lemma is the outer limiting membrane in most animal cells and it lies inner to cell wall in plant cells.
Many cell organelles of eukaryotic cells also have membrane covering. Vacuoles also are separated from cytoplasm by a membrane called tonoplast.
The cell membrane or plasma membrane and the sub cellular membranes together constitute the biological membranes, or biomembranes. Biomembranes are dynamic, quasifluid, and selectively permeable and film like structure of about 7.5 nm (75A0,) in thickness. Chemical composition
In 1895, C. Overton, applying the idea, “like dissolves the like” concluded that biomembranes are made of lipids. He based his findings on the observations that lipid- soluble substances entered cells much more rapidly than water-soluble substances.
Twenty years later, membranes isolated from Red Blood cells were chemically analyzed and found to be composed of proteins as well as lipids and small amount of carbohydrates. The proportional compositions of these substances vary from membrane to membrane. For examples, Myelin membranes have 18% protein, 79% lipid and 3% Carbohydrates.
Human erythrocyte membranes have 49% protein, 43% lipid and 8% Carbohydrates where as Spinach lamella has 70% protein, 30% lipid and no carbohydrates. Mitochondrial inner membrane has 76% protein and 24% lipids.
The lipids found in biomembranes can be phospholipids, glycolipids, sterols, and sphingolipids. The most abundant membrane lipids are phospholipids. About one hundred different types of phospholipids are associated with membranes. The sterols can be cholesterols, phytosterols and ergo sterols.
The Carbohydrates are mostly associated either with lipids as glycolipids or proteins as glycoproteins. These carbohydrates are branched or unbranched oligosaccharides of hexoses, fucoses, hexosamines, sialic acid etc.
The proteins can be structural proteins, carrier proteins, receptor proteins or enzyme proteins. Many enzymes (about 30 or more) are associated with biomembranes.
The ability of phospholipids to form biomembranes is built into their structures. A phospholipids is an ainphipathic/amphiatic molecule, meaning it has both a hydrophilic (water loving) or polar region and hydrophobic (water hating) or no polar region. Other types of membrane lipids are also amphipathic.
In case of phospholipids except cardiologic, two long non polar hydrocarbon tails are attached to a hydrophilic phosphate head. The hydrophilic regions of the lipids stay at the surface of the membrane in contact with water and the hydrophobic regions remain inside the core of the membrane sealed away from water.
The proteins may have hydrophilic and hydrophobic regions. The hydrophilic or polar regions of protein remain towards outside and the hydrophobic regions are folded inside the core or establish hydrophobic interactions with core lipid portion.
Irwin Langmuir (1917) dissolved lipids in organic solvent, benzene, and added the solution to water. After the water evaporated the lipid remained as a film covering the surface of water.
Two Dutch scientists E. Gorter and F. Grcndcl in 1925 compared the amount of lipid extracted from erythrocyte membranes to the total surface area of the cell and concluded that phospholipids form a bilayer.
Cell membranes are actually a bilayer, two molecules thick. Such a bilayer could exist as a stable boundary between the two aqueous compartments (one is the exterior of cell and the other is the cytoplasm).
This bilayer arrangement of lipids shelters the hydrophobic regions in the core of the membrane away from water and exposes the hydrophilic regions on the surface of the membrane.
Several models have been put forward explaining the arrangement of lipids and proteins in a bio membrane. People actually started constructing molecular models of cell-membrane much before its structure was actually resolved under electron microscope. H. Davson and J. Danielli in 1935 advocated the protein-lipid-protein sandwich model.
As per this model the lipid layer is sandwiched between two layers of protein. It was later modified to pleated-sheet model. Here the proteins form continuous sheets on both sides of lipid layer. In 1950 the electron micrographs obtained by heavy metal (osmium) staining of cell membrane showed a triple-layered structure of membrane.
There were two electron-dense bands separated by an electron transparent (unstained) layer. Basing on this observation in 1959 J. David Robertson put forward the Unit membrane concept. According to this concept all biological membranes have the basic unit membrane structure of three layers-two outer electron dense layers and one middle electron transparent layer. Each dense layer is constituted by protein of 15A0 thickness and the transparent layer is of bilayer of lipid of 45 A0 thicknesses.
Objections to unit membrane concept
(1) Not all membranes look trilamellar under electron microscope. The inner mitochondrial membrane appears like row of beads.
(2) All membranes are not alike as plasma membrane is 7-8nm in thickness, mitochondrial membrane in only 6nm.
(3) The chemical compositions of all membranes are not same. Membranes with different functions have different chemical compositions.
(4) Membrane proteins also have hydrophobic regions. If all the proteins are placed on the surface of the membrane then the hydrophobic regions of the proteins will be exposed to water. This will make the membrane unstable.