Fluid Mosaic Model of Singer and Nicolson

Fluid Mosaic Model of Singer and Nicolson

	Fluid Mosaic Model of Singer and Nicolson The essential feature of the fluid-mosaic model is that biological membranes are considered to be quasi fluid structures in which the lipids and integral proteins are arranged in a mosaic manner. The fluid mosaic model of Singer and Nicolson (1912) is now widely accepted as best explaining the properties of the cell membrane. This model assumes that there is a continuous bilayer of phospholipid molecules in, which are embeded globular proteins
The proteins have been compared, to icebergs floating in a sea of the phospholipid bilayer. Thus biological membranes are considered to be quasi fluid structures in which lipids and integral proteins are arranged in a mosaic manner While the Danielli Davson model assumes hydrophilic bonding between lipids and proteins, the Singer-Nicolson model considers the lipid protein association to be hydrophobic
The fluidity of the membrane is the result of this hydrophobic interaction. It should be noted that the phospholipids and many intrinsic proteins are amphipatic molecules, i.e. both hydrophobic groups occur within the same molecule. The globular proteins of the membrane are considered to be of two different types, extrinsic (peripheral) proteins and intrinsic (integral) proteins. The peripheral proteins are soluble and readily dissociate from the membrane.
Their hydrophilic polar heads protrude from the surface of the membrane, while the non polar regions are embedded in the interior of the membrane. The integral proteins are capable of lateral diffusion in the lipid bilayer. When phospholipids are dispersed in water they from a lipid bilayer. The polar heads of the lipid molecules project into the aqueous phase. The hydrophobic chains aggregate together.
	Studies with nuclear magnetic resonance (n.m.r,) and electron spin resonance (e.s.r.) techniques Indicate that the lipid bilayer has many dynamic motional properties. (i) Firstly, it is possible that there is rapid internal motion involving flexing within each lipid molecule. (ii) Secondly, a rapid lateral diffusion of the lipids is possible. (iii) Thirdly, a slow 'flip-flop' motion, i.e. a transfer of lipid molecules from one side of the bilayer to the other, is also possible.

(iv)Lastly, the lipid molecules might rotate about their axes.

Because of, the rapid movement of the lipid and protein molecules the Singer-Nicolson membrane is considered to be highly fluid.

This contrasts with the static picture of the membrane in the Danielli-Davson model.

The proteins of the membrane are concerned with the enzymatic activity of the membrane, with transport of molecules and with a receptor function. The lipid bilayer provides the permeability barrier.

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