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  Membranes and membrane structure, Biology tutorial

Introduction:

Living cells, whether prokaryotic or eukaryotic do have biological membranes that could serves as the barrier between cellular components and whole extracellular environment. Biological membranes have trilaminar appearance when viewed under microscope, with two dark bonds on each side of light band. Overall width of different mammalian membranes is 7-10 nm, few membranes have smaller widths particularly intracellular ones. Although, electron microscopy has given us with very static picture of membranes, membranes are very dynamic with the movement which allows cellular and subcellular structures in eukaryotic cells to adjust their shape and move.

Chemical composition of biological membranes:

Lipids and proteins are the two major components of all membranes but amount differs really between different membranes. Intracellular membranes are known to have some proportion of protein due to greater enzymatic activity of the membranes. Membranes also have high amount of different polysaccharide (sugars) in form of glycoprotein and glycolipid. Free carbohydrates don't exist in membranes.

Lipids of membrane:

Three major lipid components of membranes are phosphoglycerides, sphingolipids and cholesterol. Individual cellular membranes also have small quantities of other lipids like triacylglycerol and diol derivatives. Percentage of each of major lipids differs significantly in different membranes and is most probably associated to specific roles of individual membranes.

Membrane proteins:

Membrane proteins are categorized into two. Peripheral membrane proteins (or extrinsic) that simply isolated from membranes by treatment of membrane with salt solution of low or high ionic strength, or extremes of pH or name is used to imply the physical location on surface of membrane. Peripheral protein many with enzymatic activity are frequently soluble in water and free of lipids.

Integral (or intrinsic) proteins need rather drastic treatment like use of detergent or organic solvents to be extracted from membrane. They generally have tightly bound lipid, which if removed leads to denaturation of protein and loss of its biological function. Removal of integral proteins leads to disruption of membrane while peripheral proteins can be removed with little or no change in integrity of membrane.

Carbohydrate of membranes:

Carbohydrates present in membranes are completely in form of oligosaccharides covalently joined to proteins to form glycoproteins and to lesser amount of lipids to create glycolipids. Sugars found in glycoproteins and glycolipids include Mannose, Glucose, Fucose, Galactose, N-acethylglucosamine, N-acetylgalactosamine and Sialic acid.

Molecular structure of membranes:

Basic structural characteristic of all membranes is derived from physicochemical properties of major lipids components, phosphoglycerides and sphingolipids. These are ampiphatic compounds with the hydrophilic head and hydrophobic tail .

These ampiphatic compounds because of their low solubility in water react in the unique fashion in aqueous systems. Under correct state these lipids molecules will come together to create spheres termed micelles with hydrophobic tails interacting to exclude water and charged polar groups on outside. Specific concentration of lipid needes for micelle formation is known as critical micelle concentration.

Also, depending on state, ampiphatic lipids will interest to form the bimolecular leaf structure with two layers of lipid in which polar group are at interface between aqueous medium and lipid whereas the hydrophobic tails react to form the environment which excludes water. This bilayer conformation is basic lipid structure of all biological membranes. Lipids bilayers are very stable structures. The lipid bilayer can close in on itself, forming the spherical vesicle separating external space from the internal compartment. These vesicles are known as liposomes. Based on physicochemical properties of lipids, their biochemical and electron microcopy investigations, knowledge of structure of biological membrane evolved. Basic structure is bimolecular leaf arrangement of lipids in which phosphoglycerides, sphingolipids and cholesterol are oriented so that hydrophobic portions of molecules interact to minimize the interaction with water or other polar lipids. Polar heads of ampiphatic compounds are at interface with aqueous environment. This arrangement of lipids is same as that if synthetic phospholipids, liposomes. The major problem to resolve, though has been to describe interaction of integral and peripheral proteins with lipid bilayer.

The number of models for biological membrane structure has been suggested dating back to 1935 by H Davson and Danielle that was refined by J.D. Robertson later. In early 1970s, G.L. Nichoson and S.J. Singer suggested Mosaic model for membranes in which it was recommended that proteins are on surface and on lipid bilayer. Some proteins could span lipid bilayer with their polar groups in contact with aqueous environment on both sides and hydrophobic portions interacting with lipids in interior of membrane. This model has been extensively refined and is referred to as fluid mosaic model to point to movement of both lipids and proteins in membrane.

Properties of biological membranes:

  • Biological membranes have fluidity (both lipids and protein more) and degree of fluidity is dependent on temperature and composition of membrane. At low temperatures, lipids are in gel crystalline state and as temperature increases, there is stage transition in liquid - crystalline state.
  • They have specific recognition sites like receptors.
  • There is asymmetric distribution of lipid components across biological membranes. Every layer of bilayer of lipids has different composition with respect to individual phosphoglycerides and phospholipids.
  • There is asymmetric distribution of lipid components across biological membranes. Every layer of bilayer of lipid has different composition with respect to individual phosphoglycerides and phospholipids.
  • They have electrically charged surface groups that support different electrical potential across membrane structure.
  • Biological membranes permit diffusion of solute molecules through them.

Function of membranes:

  • Recognition of certain molecular signals.
  • They act as components of nerve cells.
  • They manage movement or translocation of molecules in and out of cell.
  • They act as receptors for hormones.

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