Membranes are the most important structural components of the cell; it is the protecting layer of the cell bounding the protoplasm and provides the interface for interaction between the outer and inner components. All cells in this living systems have such membranes around the protoplasm. Various cell organelles too are bounded by membrane. Chemically, structurally cell protoplasm is covered by a membrane called plasma membrane. In fact the active cytoplasmic fluid is pervaded with membranes. Some of the organelle membranes are highly specialized to perform specific functions. Most of the intracellular organelles are bounded by membranes, which actually make such structures compartmentalization of the protoplasm.
Analysis of chemical components found in membranes show variation from membrane to membrane, from organelle to organelle. Generally membranes are made up of proteins and lipids in various proportions as such as 1:1 to 1:3. The analysis of proteins and lipids of various types of membranes exhibit wide variety of structural components.
Proteins are polymers of amino acid residues. They can be isolated and separated in the individual components by the methods of SDS
polyacrylamide gel electrophoresis, column chromatography and ammonium sulfate precipitation methods. Some of the membrane proteins are structural ones and others are found to be enzymes, receptors or carriers. Many of them are located at the outer face of the membranes, extrinsic proteins or they be found in the core as intrinsic or integral proteins. Most of peripheral proteins are globular and they are either hydrophilic or partially hydrophobic. Integral proteins are however hydrophobic because they contain greater amount of nonpolar amino acids like lucine, valine, isoleucine, etc. Even such proteins contain some hydrophilic amino acid residues. The presence of variety of proteins and protein complexes provide structural and functional heterogeneity to membranes.
The only semi viscous to viscous part of the membranes contains a wide variety of lipids like phospholipids, sphingolipids, sulpholipids, phytosterols etc. Phospholipids are dipolar molecules with hydrophilic phosphate group at one and non polar hydrophobic fatty acid chains as tails at the other. Among them phosphotidyl serine, phosphotidyl ethanolamine phosphotidyl choline, phosphotidyl glycerol and cardiolipins are important.
Many of the lipids are associated with carbohydrates, such lipids are called glycolipids. Some of the membrane proteins are associated with carbohyrates and such protaries are called glycoproteins. They play important roles. Among sterols, cholesterols and phytosterols are common in plant membranes. Having polar heads and non polar tails, phospholipids play an important role in structural organization of the membrane.
Starting from Sand witch model proposed by Danieli Davson, the concept of membrane structure has undergone many modifications over the years. The unit membranes model of Robertson has been further improved by Singer and Nicholson as Fluid Mosaic model. This model is the most accepted one today, for it explains most of the observed membrane structures and functions. Singer and Nicholson model is based on studies like Freeze fracture electron microscopy, Nuclear Magnetic Resonance, X-ray diffraction, Fluorescence spectroscopy and biochemical analytical techniques. This model has also taken into account of energy relations like translational movements, vibrational movements and hydrophobic, elctrostatic and hydrogen bond interactions. Moreover the dynamic feature of the membranes has been explained mostly on the basis of energy translations.
According to Fluid Mosaic Model, various phospholipids and other lipid components from a bilayered structure at the interface of water, because the hydrophobic tails of lipids get oriented towards each other in such a way the hydrophilic heads are exposed towards water. As wide variety of proteins of different dimension are integrated into lipid bilayers so as to form mosaic of lipids and proteins.
Many proteins are located at the interphase between water and hydrophilic phospholipid layers, some are held and buried in the hydrophobic core and other are integrated in the core of lipid bilayers so as to occupy the entire core section of the membrane. There is a dynamic interaction between lipids and proteins; they exhibit lateral movement including rotational flip flop turnovers. The position of proteins with respect to lipids in the membrane is never constant and always there is constant flux thus exhibit in quasi fluidity as well as quasi crystalline semi solid state. The association of microfilaments and microtubules at inner face of the membranes further adds up to its dynamicity to a greater extent. The above described structure holds good for all the membranes. However plasma membrane and other cytoplasmic membranes differ in their chemical composition particularly with respect to proteins and specific lipids. Even the thickness
of the membrane varies from 60 – 100 Å. The plasma lemma in most of the cells being the surface membrane contains a wide variety of receptor and carrier proteins. It produces invagination to produce cytosolic endoplasmic reticulum.
Membranes being sheet like structures, posses a large surface area for many biochemical reactions. However, that function depends upon the protein and lipid contents. Plasma membrane as present at the outer surface, it has manifold function. Though they allow water to diffuse through in both directions, it prevents the free diffusion of both inorganic and organic solutes. Water movement is greatly facilitated by the presence of aquaporins. Thus it exhibits semi permeable property. However plasma lemma performs selective uptake of ions because specific solute carrier proteins found within the membrane. The plasma membrane is the first cellular structure that receives a wide variety of external stimuli like light, heat, chemicals, hormones, etc. Such stimuli are then passed on to cytoplasm or to the genetic material through specific receptor proteins. The surface membranes are also involved in bringing about changes in permeability and electro potential. They also take part in Pinocytosis and phagocytosis thus they facilitate the transportation of various substances in bulk. On the country, they are also responsible for the secretion of undigested materials and enzymes to the exterior surface. The presence of desmotubules of 200 A thicknesses, which traverse across the pit channels from one cell to another, is one of the unique features of the plasma membranes. Similar to plasma membrane other membranes also show specific functions. Thus membranes exhibit dynamisity in its structure and function.
Biogenesis of Membranes:
Plasma membrane can be considered as an organ by itself; it exhibits a characteristic structure and functions. Plasma membranes of different cell types exhibit different functions either in receiving the stimulus or transportation of materials to and fro. In spite of its diversity and uniqueness the synthesis of plasma membrane mainly depends upon the activity of endoplasmic reticulum and Golgi complex. A large number of endoplasmic reticular membranes which are engaged in protein translocation and modifications contribute to the plasma membrane synthesis. Many of the fatty acids required are synthesized at the cytosolic side of the ER membranes for this region contains anchored enzymes. There is continuous flow of membrane materials from trans golgi to plasma membranes and from plasma membrane inwards, there is dynamic equilibrium between these two components.
Below there are a host of diagrams that show the composition and structure of membranes in their vividness.