FLUID MOSAIC SYSTEM IN PLASMA MEMBRANE(CELL MEMBRANE)

Fluid Mosaic System- Plasma Membrane

The Fluid Mosaic Model states that membranes are composed of a Phospholipids Bilayer with various protein molecules floating around within it. The 'Fluid' part represents how some parts of the membrane can move around freely, if they are not attached to other parts of the cell. The 'mosaic' part illustrates the 'patchwork' of proteins that is found in the Phospholipids Bilayer.

All cells are surrounded by a plasma membrane. Eukaryotic cells also contain internal membranes and membrane bound organelles. This was originally proposed by Davison and Daniele and later In 1972, S. Singer and G. Nicolson described the Fluid Mosaic Model of membrane structure. „According to this model, cell membranes are composed of a lipid bilayer (two layers) with globular proteins embedded in the bilayer.

The membrane’s lipid bilayer is mainly 2 layers of phospholipids; the non-polar tails point (hydrophobic) inward and the polar heads(Hydrophilic) are on the outer surface. 

As the cell membrane consists of a mosaic of protein and lipids it is called as a fluid mosaic model.

„No cell exists as a closed system. In order to survive, materials must be transported into and out of the cell, across the plasma membrane. „In addition, because different processes take place in different parts of the cell, materials must be transported from one part of the cell to another.

The phosphor lipids are arranged in bilayers with the polar head groups oriented towards the extra cellular side and the cytoplasmic side with a hydrophobic core. Each leaflet is 25A thick and the total thickness is 50 to 80 A.

Some proteins in the membrane are called 'Intrinsic'. This means that they completely span the Bilayer. Others are called 'Extrinsic' - they are partly embedded in the Bilayer.

Here choline containing phospholipids are in the external layers and ethanolamine and serine containing phospholipids are in the inner layer. The lipid bilayer is said to be fluid in nature because it shows free lateral movement of its component and their fluidity helps the membrane to perform endocytosis(import into the cell and exocytosis(export out of the cell). („In endocytosis, part of the plasma membrane envelops small particles or fluid, then seals on itself to form a vesicle which enters the cell)

But flip-flop movement is restricted that means movement of particle from inner to outer layer , or outer to inner layer is restricted. Flip-flop movement occurs only during apoptosis. The fluidity of the membrane is maintained by the length of hydrocarbon chain, and nature of polar head group. Membrane also contains cholesterol. As cholesterol concentration increases ,membrane breaks and loss of fluid in the outer surface , but more fluid in the core.effect of cholesterol on fluidity is vary with different temperature.

Membrane protein

The peripheral or extrinsic protein exists on the surface of the bilayer. They are attached by ionic and polar bonds to polar heads of lipids and they can be easily separated.

The integral (intrinsic) are deeply embedded in the bilayer and they cannot be easily separated. Eg.  Hormones receptors.