Cell Membrane Structure Related Theories : A Development

There are many theories related to the cell membrane structure, which is given below :-

1.  Lipid & Lipid Bilayer Model
2.  Lipid Bilayer Protein Model Or  Danielli & Davson Model
3.  Micellar Theory
4.  Unit Membrane Model
5.  Protein Crystal Model
6.  Fliud – Mosaic Model
7.  Lipid-Globular Protein Mosaic Theory

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1.  Lipid & Lipid Bilayer model

•  By Charles Overton , 1895 –  The cell membrane is composed of a very layer Lipids.
• By Gorter & Grandel , 1926 – The plasma membrane is composed of a dual layer of Lipids. Each layer’s polar end are positioned outward and tails are facing inward position. This theory puts the base for the theories related to the cell membrane structure.

 2. Protein  - Lipid Bilayer Model Or Danielli & Davson Model , 1935

• This theory is also known as Sandwitch model of plasma membrane or Lamellar theory.
• The plasma membrane consists of two layers of Lipid molecules arranged radially with their hydrophobic hydrocarbon chains ( tails ) towards each other.
• The polar ends are arranged ( heads ) outwardly.
• The polar ends of the Lipid molecules are associated with a monomolecular layer of polar globular protein molecules.
• Thus the entire structure consists a double layer of Lipid molecules sandwiched between two continuous layers of proteins. So that it is called as Sandwitch model.
• Drawback of This Model : Lots of weakness are present in this model such as -

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• This model is failed to explain the movement of water and water soluble substances through phospholipid.
•  Failed to explain the active transport of substances through cell membrane.

Danielli & Davson Model

• Modifications of Danielli & Devson Model
• The model was modified to include polar regions which is coated with protein. These polar regions are extended through the lipid billyer and forms small pores 0.7 – 1.0 nm thick.
• Polar linked polar pores are present in the membrane, measuring about 7 A^o in diameter and permits passage of  small ions and water molecules.

Modifications of Danielli & Devson Model

3.  Micellar Theory

• Micellar theory was proposed by Hilleir and Hoffman, 1953.
• According to this model, Plasma membrane consists of a mosaic of globular subunits or micelles.
• If fatty acid molecules are completely surrounded by water, they form aggregate, called as micelles.
• In micelles the hydrophobic groups ( tails ) are oriented towards the interior side and the hydrophilic groups ( heads ) are oriented towards the exterior side.
• Micelles may be present in the form of small spheres of bimolecular layers, closely packed together to form central core while polar groups present outer side

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• Each lipid micelles measures about 40 – 70 A^o in diameter.
• Protein component forms a monolayer on either side of the lipid micelles.

Micellar Theory

4.  Unit Membrane Model

• Unit membrane model is presented by J. David Robertson , 1959. It supports the Danielli & Devson model.
• Based on his study under electron microscope , he told that cell membrane consists three layers in its structure.
• Two layers are dark stained and one layer is light stained.

Unit membrane model showing width of membrane

• Dark stained layer are present outside of the light stained layer.
• Dark stained outer layers are the protein layers, while light stained inner bilayer shows Lipids.
• According to Unit membrane model – the cell in which bilayer of phospholipid are surrounded by the two unit layers of protein , commonly referred as Unit membrane.

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• Total thickness of cell membrane is about 75 – 100 A^o.
• The thickness of light stained middle two layered phospholipid is about 35 – 50 A^o.
• The thickness of light stained outer protein layer is 20 – 25 A^o each.
• Drawback of This Model : this model also has the same weakness as present in sandwitch model.

Unit membrane model showing protein & lipid layer

5.  Protein Crystal Model

• This model was proposed by David Green and his Coworkers.
• In this model, membrane protein polymerize to form arrays of two layers of loosely packed globular proteins, 3 – 4 nm in diameter.
• The proteins have polar ( head ) and non-polar ( tail ) regions on their surface.
• The non polar region exists hydrophobic nature binds with the non polar groups of phospholipid molecules that fills the cavities between the globular protein unit.
• The polar heads of the phospholipids molecules are remain at the membrane’s surface.
• Modified Concept : Green has modified its concept.
• According to this, the proteins and lipids are considered to move in the plane of the membrane.
• The lipid bilayer forms essentially a continuum and any protein – protein interactions are temporary.

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Protein Crystal Model

6.  Fliud – Mosaic Model

• This concept was presented by S. J. Singer and G. Nicholson, 1972  which is widely accepted.
• This is similar to Green’s modified concept.
• According to this theory, the phospholipids and proteins are arranged in a mosaic form in the cell membrane.
• The cell membrane tends to be semi-liquid rather than solid.
• A dence fluid matrix is occur in plasmalemma .
• Fluid matrix is composed of bilayer of phospholipids having protein molecules on each sides.
• According to fluid mosaic model, the bimolecular lipid layer is present in the center of the cell membrane.
• The rounded polar ends of the lipid are oriented towards outside, while the non polar tails are oriented inward position.
• The extrinsic or peripheral proteins are found on the peripheral side of the bimolecular lipid layer , while intrinsic or integral proteins are embedded in the lipid layer.

Fliud – Mosaic Model

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• Extrinsic or Peripheral Proteins : Extrinsic proteins covers the phospholipids layer fully or partially.
• Extrinsic proteins forms about 20 -30 % the whole membrane proteins.
• Spectrin protein is found as a extrinsic proteins.
• Extrinsic proteins are water soluble and can be easily separated.
• Intrinsic or Integral Proteins : Intrinsic or integral proteins forms 70 – 80 % of the whole membrane proteins.
• Glycophorin are found as a intrinsic protein.
• The intrinsic proteins are densely bound with the hydrophobic ( tails ) and hydrophilic ( heads ) ends of the lipids. Due to this it is not separated easily from phospholipid layer.
• Intrinsic proteins are unable to dissolve in water.
• Intrinsic protein and phospholipids are present in mosaic form.
• Due to the presence of extrinsic & intrinsic proteins,The cell membrane has the ability to perform special activities.
• These protein layers provides elasticity & mechanical support to the matrix of Lipid.
• Fluid mosaic model of cell membrane shows the selective permeability of protoplasmic membrane.
• It has seen that the molecules of branched oligosaccharides are present in plasma membrane.
• These protein & lipid linked molecules of oligosaccharides are known as glycoprotein and glycolipids.
• On the basis of specific characteristic of glycoprotein and glycolipids , the cells recognizes the same cells.

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8.  Lipid-Globular Protein Mosaic Theory

• This model was proposed by Roderick and Capaldi, 1974.
• According to this theory , the lipid molecules are arranged in the form of a thin bilayer and form a structural framework of cell membrane.the protein molecules are arranged in two different manners.
• Some proteins are located exclusively adjacent to the outer and inner surfaces of the membrane, which is called as extrinsic proteins.
•  Other remaining proteins are lying inside of the lipid, called as intrinsic proteins.

Lipid-Globular Protein Mosaic Theory

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