Principles of Biochemistry Volume 1 - A. Lehninger 1985

Biomolecules
Lipids and Membranes
Membranes have a fluid-mosaic structure

Based on findings from chemical and electron-microscopic studies, as well as the striking similarities between The properties of synthetic phospholipid bilayers and natural membranes, S. Jonathan Singer and Garth Nicolson proposed the fluid-mosaic model of Membrane Structure in 1972 (Fig. 12-18). According to this model, the continuous structural foundation, or matrix, of the membrane is a polar lipid bilayer. At physiological temperatures, this matrix exists in a fluid state, a property maintained by the specific ratio of saturated to Unsaturated Fatty acids within the hydrophobic tails of the polar Lipids. The fluid-mosaic model also posits that integral Membrane Proteins feature hydrophobic amino acid R-groups on their surfaces, which allow them to effectively «dissolve» within the central hydrophobic region of the bilayer. Conversely, peripheral, or extrinsic, proteins predominantly display hydrophilic R-groups on their surfaces, binding to the charged hydrophilic polar HEAD groups of lipids via electrostatic forces. Integral proteins—which include Enzymes and transport proteins—exhibit activity only when embedded within the Hydrophobic core of the bilayer, where they acquire the spatial conformation necessary for functional activity. It is worth emphasizing once again that no covalent bonds are formed either between lipid molecules within the bilayer or between the membrane proteins and lipids.

Furthermore, the fluid-mosaic model implies that membrane proteins are free to diffuse laterally within the membrane plane. Peripheral proteins literally float On the surface of the bilayer «sea», whereas integral proteins, much like icebergs, are nearly fully submerged in the hydrocarbon interior (Fig. 12-18). However, the lateral mobility of membrane Proteins can be restricted by interactions between functionally related proteins, leading to The formation of clusters and, ultimately, a mosaic distribution of proteins within the fluid lipid bilayer. Such protein clusters are thought to be capable of lateral diffusion through the bilayer. This process likely underlies the phenomenon known as capping—the redistribution of specific membrane proteins into distinct patches or caps, which occurs in certain Cell types during their lifecycle. While the Singer-Nicolson model successfully accounts for many of the physical, chemical, and biological properties of membranes and is widely accepted as the most accurate representation of molecular Organization in membranes, we will see later that certain Structural Features of Introduction/36.html">Biological Membranes still fall outside The Scope of the fluid-mosaic model.

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Fig. 12-18. The fluid-mosaic model of membrane structure proposed by Singer and Nicolson