The Cell Membrane Or Plasma Membrane Structure : A Study

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The cell has a different internal environment from that of external environment. The difference is maintained throughout the life of cell by the thin membrane, known as cell membrane or plasma membrane. It covers the protoplasm. It is also known as Plasma Membrane or Plasmalemma.  In animal, this is outer most layer of the cell while in plants it is present beneath the cell wall.

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Structure of Cell Membrane Or Plasma Membrane

Cell Membrane Or Plasma Membrane : Structure

           i.        It is normally composed of Lipid and protein.

        ii.         The composition is : protein – 60 to 80 % , Lipid – 20 to 40 % , cellulose – few , Enzymes – few , water – about 20% of whole weight.

        iii.          Cell membrane is semi permeable or differentially permeable.

        iv.          It has elasticity nature.

         v.          It is a living structure, which has the ability to heal the injury or wound.

        vi.          Cell membrane is 70 – 100 Ao thick. It possess three layers in there structure –

                                i.     The outer dense layer – about 20 Ao thick – composed of protein.

                              ii.     The inner dense layer – about 20 Ao thick – composed of protein.

                           iii.     The middle layer – about 35 Ao. It is thick and yellow colored – composed of bimolecular layer of phospholipid.

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Ultra Structure of Cell Membrane or Plasma Membrane

It has been found that morphology of plasma membrane generally depends on the fixation medium, stain used and the resolution. Thus on this basis there are two types of ultra structure of cell membrane defined.

A.     Symmetrical Molecular Structure

B.     Asymmetrical Molecular Structure

A.   Symmetrical Molecular Structure

By fixing the cell in osmic acid and embedding in epoxy resin with low resolution , then symmetrical arrangement of micelles in plasma membrane is observed.

·        Plasma membrane is a tripartite structure being made up of three layers. The thickness is about 75 Ao.

o   There are two adelectronic layers, each measuring 25 Ao thick.  

o   A middle dielectronic layer which is also 25 Ao thick.

·        The middle layer is a trimolecular layer of lipids. The non –polar hydrophobic groups are facing inward and the polar hydrophilic groups are facing outward.

·        The hydrophilic polar groups are covered by a protein layer which is also 20 – 25 Ao thick.

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·        The protein chains are lie at the right angles to the lipids.

o   The protein layer is reported as carbohydrate in nature due to its less marked adielectronicity.

Symmetrical Molecular Structure

B.   Asymmetrical Molecular Structure

By fixing the cell in vestopal and embedding in uranyl acetate, then it appear as asymmetrical.

·        It is also tripartite structure having the total thickness is about 90 – 95 Ao.

o   The inner layer is adielectronic whose thickness is about 35 – 40 Ao.

o   The outer layer is narrow and adielectronic whose thickness is 25 Ao.

o   The center layer is dielectronic ( bimolecular layer of lipid ) which is about 30 Ao wide.


Asymmetrical Molecular Structure



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