All membranes are composed of  lipids and proteins, but the nature of the core, proteins and the surface architecture are not he same. Membranes are asymmetric, 70A to 100A thick, containing a double layer of lipids to which proteins molecules are attached on either side. Such a picture gave rise to a unit membrane hypothesis. The unit membrane model was considered a rigid structure, which however , did not explain the intricate behavior with respect to a multitude of function the membrane has to perform . Subsequently, physicochemical investigation revealed that the membrane is a dynamic structure, consisting of proteins of globular nature which keep on shuttling in and out. Consequently a new model was proposed by singer and Nicholson (1972), called fluid mosaic model .

The central plane of the membrane consists of a bimolecular phospholipid film. The lipid molecules are oriented in a specific manner, the non –polar hydrocarbon tails oriented towards the centre and the polar heads towards the surface. The lipid layer is a viscous fluid that permits movement of embedded proteins imparting asymmetry to the membrane. The composition of lipids is variable. For example, mammalian membrane. Similarly cardiolipins are found to be in greater proportions in the membranes of cell organelles. Mitochondrial membranes are rich in phospholipids and phosphatidyl choline.

structure lipids found in biological membranes

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