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Cell membrane and its structural hypothesis
Cell has a thin boundary membrane made of Plasmalema and Cell Cement. This membrane is known as cell membrane. Cell membrane is made up of phospholipid layers. After the discovery of cell membrane, many scientists have proposed some structural models to describe how phospholipid layers are organised to make cell membrane. In this article I will discuss the models and hypothesises proposed by various scientists time to time.
The cells are bounded by a thin membrane which is not visible under the light microscope and is called plasmalema. Plasmalema along with the cell cement is referred to as cell membrane. Cell membranes are chemically lepoproteins and have polysacharides associated with them. Membrane proteins are made up of structural protein constitute the backbone of the membranes. Enzymes constitute catalytic proteins. The commonest phospholipids of membranes are lecithin and cholesterol. In addition to this organic compounds, sometimes small amount of ions like Ca, Zn, Mg are also present in the biomembranes.
Cell membrane theories :
During the past several years different hypothesis have been put fourth to account for the physical and biological features of the membrane. These are -
- Bilayer model.
- Subunit model.
- Fluid Mosaic model.
Bilayer Models :
We have two different bilayer models on cell membrane structure. First one was proposed by Gorter and Grendel in the year 1925 and second one was proposed by Danielli and Davson in the year 1935. These are -
Lipid membrane model :
Two scientists, Gorter and Grendel in the year 1925 proposed that cell membranes were composed of a double layer of lipid molecules whose polar groups were located towards the outside.
Figure : Lipid Bilayer model (Image source : Wikipedia, Licenced under Creative Commons Attribution Share Alike 3.0)
Protein-Lipid-Protein or Sandwich model :
The first important hypothesis of the structure of biological membrane was proposed by H. Davson and J. Danielli in the year 1935. Hence this model is also known as Danielli-Devson model.
According to this model, two layers of phospholipid molecules are arranged with thin hydrophobic tails facing towards the inner side. The protein sandwich the phospholipids bilayer.
Figure : Danielli and Davson's sandwich model (Image source : Captured from self drawn diagram)
Subunit Model :
Based on Electron Microscopy and functional criteria, J. D. Robertson modified the model of Danielli. He suggested a single layer of protein molecules over the inner and outer surfaces of the lipid bilayer.
Figure : Robertson's Subunit model (Image source : Captured from self drawn diagram)
Initially, Robertson model was widely accegued but lateron several objections were raised. The unit membrane was proposed to consists of a bila of mixed polar lipids with their hydrocarbon chains oriented inward to form a continuous hydrocarbon phase and their hydrophyllic heads oriented outward. Each surface was thought to be coated with a mono molecular layer of protein molecules, with the polypeptide chains in extended form.
Fluid Mosaic model :
The most satisfactory model of cell membrane structure to date appears to be the fluid mosaic model. This model was postulated by S. J. Singer and G. L. Nicolson in the year 1972.
Figure : Singer and Nicolson's Fluid Mosaic model (Image source : Wikipedia, Licenced under Public Domain)
This model postulate that the phospholipids of membranes are arranged in a bila to form a fluid, liquid, crystalline matrix ore core. In this bilayer individual lipid molecules can move lateraly, endowing the bilayer with fluidity, flexibility and a characteristically high electrical resistance relative impermiability to highly polar molecules. Globular proteins form a mosaic on the matrix. View may be partially embedded on either side or may completely penetrate the membrane. This mosaic is not fixed mr static, because the proteins are free to defuse laterally in two dimensions.
Fluid mosaic model accounts satisfactorily for many features and properties of Biological membranes. These are as follows -
- It explains the widely different protein content per unit area.
- It explains the variation in the thickness of different types of membranes.
- It accounts for the electrical properties and permiability of membrane.
- It also accounts for the movement of some proteins in the cell membrane.
So, fluid mosaic model is the widely accepted hypothesis on the structural model of cell membrane.
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