Cell Membrane or Plasma Membrane or BIOMEMBRANE

Biochemical investigation clearly revealed that the cell membranes possess lipid, protein and carbohydrate. The ratio of protein and lipid varies considerably in different cell types. In human beings, the membrane of the erythrocyte has approximately 52 per cent protein and 40 per cent lipids

Average composition of cell membrane:

Lipids = 40% (Phospholipid, Cholesterol, Glycolipids)

 Proteins 58-59% (Arginine, Lysine rich)

Carbohydrates = 1-2% 

• It is a universal structure and structurally cell membrane of prokaryotes is similar to the eukaryotes.

STRUCTURE OF BIOMEMBRANES :

(1) Sandwich or Trilamellar model :- By Davson & Danielli (1935) 

• According to this model, the plasma-membrane is made up of three layers in which a bimolecular layer of lipid is sandwiched between two single layers of proteins.
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• According to this model each protein layer is 20A thick and bilayer of phospholipid is 35A thick. Thus total thickness is 75A (PLLP - structure, range 75-100A)
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• Phospholipid molecule called as amphipathic molecule due to presence of two type of parts (hydrophile head and hydrophobic tail).
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• Hydrophilic head of the phospholipid binds with protein layer by hydrogen and ionic bonds.
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• Hydrophobic tail of phospholipid are attached to each other by vanderwal force.

fig: Structure of Biomembrane

(2)  Unit membrane model - By Robertson 1959.

•  According to this model all the cellular and organelle membranes are structurally & functionally similar difference in chemically & size).
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• Both of the above models are rejected because they fails to explain the selective permeability of plasmalemma.
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• The detailed structure of the membrane was studied only after the advent of the electron microscope in the 1950s. Meanwhile,chemical studies on the cell membrane, especially in human red blood cells RBCS). mabied the scientists to deduce the possible structure of plasma membrane Fluid mosaic model : By Singer & Nicolson (1972 G.L Telfon This is latest & most widely accepted model for the structure of plasmalemma.

(3) Fluid mosaic model: By Singer and Nicolson (1972).

• This is latest and most widely accepted model for the structure of plasmalemma.
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•  According to fluid mosaic model proteins are arranged in phospholipid lager as mosaic pattern. Thus membrane is termed as "protein iceberg in a sea of phospholipid" or "Gulab Jamun (protein) in a concentrated solution (phospholipid) of sugar"

(i) Phospholipids:

• Phospholipid is the main component of cell membrane because it forms continuous structural frame of all membrane.
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• The studies showed that the cell membrane is composed of lipids that are arranged in a bilayer. Also, the lipids are arranged within the membrane with the polar head towards the outerside and the hydrophobic tails towards the inner part. This ensures that the nonpolar tail of saturated and unsaturated hydrocarbons is protected from the aqueous environment. The lipid component of the membrane mainly consists of phosphoglycerides (phospholipids).
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• Phospholipid layer provides fluidity to plasma membrane because phospholipids are rich in unsaturated fatty acid which are liquid in nature.
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• The QuasiFluid nature of lipid enable lateral movement of protein with in the overall bilayer. This ability to move within the membrane is measured as its fluidity.
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• The fluid nature of the membrane is also important in various function like cell growth, formation of intercellular junction, endocytosis, secretion, cell division etc.
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• Cholesterol is also present in plasma membrane. Cholesterol are more rigid than phospholipid. So, it helps in stability of membrane structure.
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• Cholesterol is absent in membrane of prokaryote. Thus Hopanoids (Pentacyclic sterol provides stability to prokaryotic cell membrane.

(ii) Proteins:

Two types of protein are present in plasma membrane. (On the basis of ease of extraction) 

(a) Integral or intrinsic protein.

•  These protein are tightly binds with phospholipid. Thus, they can not easily removed from membrane.
• Integral proteins are of 2 types:  
• i) Partially buried   ii) Totally buried.
•  Some integral proteins which are totally buried through the complete thickness of membrane These type of protein are called as tunnel (channel) protein which provide a passage for movement of water soluble material across the membrane.

(b) Peripheral or extrinsic protein. 

These are superficially arranged on the surface of lipid layer and can be separated easily. These protein have enzymatic activity.

Oligosaccharides (sugar) of the glycolipids & glycoproteins on the outer surface of plasma membranes are involved in cell to cell recognition mechanism. Best example of cell recognition is fertilisation. (where sperm & egg recognize to each other) and blood - Antigens.

fig: Fluid mosaic model of plasma membrane

fig: Ultrastructure of plasma membrane