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Inner Cytoplasmic Membrane

	Inner Cytoplasmic Membrane - Prokaryote cells contain relatively fever membranes than eukaryote cells. The cytoplasmic membrane of prokaryote cells there - fore carries out multiple functions which are distributed among the various cell organelles of eukaryote cells, and is an important centre of metabolic activity. (1) The membrane contains the enzymes of the biosynthetic pathways leading to the synthesis of components of the bacterial cell wall such as phospholipids, peptidoglycans, teichoic acid lipopolysaccharides and simple polysaccharides.
(ii) The permeases are responsible for the transport of organic and inorganic nutrients through the membrane. (iii) The membrane of aerobic bacteria contains the components of the electron transport chain and oxidative phosphorylation (iv) The photosynthetic apparatus of purple bacteria is located within the cytoplasmic membrane. (v) The cytoplasmic membrane contains the attachment sites for chromosomal and plasmid DNA (vi) The components for the control of chemotaxis appear to be located in the membrane.
The major features of the structure of the cytoplasmic membrane is explained by the fluid mosaic model of Singer and Nicolson (1972) ,The essential feature of the fluid-mosaic model is that biological membranes are considered to be quasifluid structures in which the lipids and integral proteins are arranged in a mosaic manner. The fluid-mosaic model of Singer and Nicolson (1972) is now widely accepted as best explaining the properties of the cell membrane .
This model assumes that 1here is a continuous bilayer of phospholipid molecules in which are embedded globular proteins. The proteins have been compared to icebergs floating in a sea of the phospolipid bilayer. Thus biological membranes are considered to be quasifluid 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 hydrophilic and hydropbobic 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. They are entirely outside the lipid bilayer. The internal proteins are relatively insoluble and dissociate with difficulty.

Some may partially penetrate either surface of the lipid bilayer, while others penetrate right through. The latter are in contact with the aqueous solvent on both sides of the membrane.

The integral proteins are amphipatic. 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 for m a lipid bilayer. The polar heads of the lipid molecules project into the aqueous phase.

The hydrophobic chains aggrega1e together. Studies with nuclear magnetic resonance (n. m. r.) and electron spin resonance techniques indicate that the lipid bilayer has many dynamic motional properties.

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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