Cell membrane

 

 

Fluid-mosaic model of cell membrane structure

            Cell membrane consist of all the membranes within the cell such as plasma membrane (cell membrane proper)  membranes of Golgi, mitochondria and nucleus etc.  They essentially have similar structure of double unit membrane.

            Cell membrane is about 7.5 nm thick. The modern thought or model of the structure of the cell membrane is thought to be easily explained by the fluid-mosaic model of membrane structure which recognizes that the membrane is made up of mosaic structure of lipid bilayer containing both integral and peripheral proteins sticking out at both the outer (which is hydrophilic) and inner, which is hydrophobic (cytoplasmic) sides.  The lipids contain phospholipids such as phosphotidylcholine and phosphotidylethanolamine. Some protein structures extend through the whole length of the lipoid bilayer. They are called integral protein while the peripheral proteins are found only in the periphery. 

There are five different types of proteins in the membrane and they are all useful for functions of the cell membrane which include cell transport. They are as follows

  • Structural proteins act as integral proteins

  • Pump proteins which help in active transport

  • Channel proteins assist the membrane for facilitated transport

  • Receptor proteins. They act as receptors on surface of cell membrane

  • Enzymes. These are the enzymes on surface of membranes which are different from the membrane enzymes on mitochondrial membranes.

Glycocalyx moiety is added to the lipid bilayer at the external aspect of the cell membrane.  In this model of membrane structure, the whole entity is thought to be fluid in nature, hence the name, fluid-mosaic model. The hydrophilic ends are surrounded by fluid in the exterior of the cell, while the hydrophobic ends are inside the cell  and have little water surrounding it. This is a neat arrangement.

When examined under the electron microscope, the membrane is three layered when stained with osmium tetroxide (triple layered) but quintuple (5) layered when stained with potassium permanganate.

Cell membrane Junctions

 

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