Cell Membrane

Plasma membrane or cell membrane is the outer most boundaries of the animal cells but in plant cells, it is covered by a cell wall.

Cell membrane is chemically composed of:

• a.        60-80 % Protein.
• b.      20-40 % Lipids.

In addition there is a small quantity (2-10%) of carbohydrates.

Structure:

Structurally, cell membrane is composed of layers of different materials and has extremely small pores in it through which only small molecules can pass. There are different theories about the structure of plasma membrane.

•          i.            In 1935, Danielle and Davson proposed that cell membrane consists of three layers.
•        ii.            In 1938, Harvey and Danielle suggested that plasma membrane is composed of liquid bilayer sandwiched between two protein layers.
•       iii.            Fluid Mosaic Model: In i972, Singer and Nicolson that lipid bilayer is not sandwiched between two protein layers. Instead proteins are embedded in the lipid in a mosaic manner. This discovery led to the proposed of “Fluid Mosaic Model”. This model is most accepted model. Cell membrane also contains charged pores through which movement of materials takes place both by active and passive transport.

Some proteins are embedded and some are present on the outside of cell. Small amounts of carbohydrates are also found. These may be joined with proteins (glycoprotein) or with lipids (glycolipids). Both these forms act as fingerprints of the cell.

Functions:

•          i.            Plasma membrane provides mechanical support and external form to the protoplasm (living matter inside the cell).
•        ii.            Plasma membrane offers a barrier between the cell contains and their environment, allowing only selective substance to pass through it, thus it is known as differentially permeable or selectively permeable membrane.
•       iii.            Transport of Materials: Transport of materials is one of the vital roles it plays of the cell.