Cell: The Unit of Life

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ÇËLL ØRGÅÑËLLËS ÏÑ ËÜKÅRÝØTÏÇ ÇËLLS

1.

Cell membrane

Chemical studies on human RBCs show that cell membrane is

composed of a lipid bilayer, protein & carbohydrate. (AIPMT 2010)

Lipids (mainly phosphoglycerides) have outer polar head and

the inner hydrophobic tails. So the non-polar tail of saturated

hydrocarbons is protected from the aqueous environment. It also

contains cholesterol.

Ratio of protein and lipid varies in different cells. E.g. In human RBC,

membrane has 52% protein and 40% lipids.

Based on the ease of extraction, membrane proteins are of 2 types:

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Integral proteins: Partially or totally buried in membrane.

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Peripheral proteins: Lie on the surface of membrane.

Fluid mosaic model of cell membrane: Proposed by Singer &

Nicolson (1972). According to this, the quasi-fluid nature of lipid

enables lateral movement of proteins within the overall bilayer. This

ability to move within the membrane is measured as its fluidity.

Functions:

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Transport of the molecules. The membrane is selectively permeable to

some molecules present on either side of it.

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Due to the fluid nature, the plasma membrane can help in cell growth,

formation of intercellular junctions, secretion, endocytosis, cell division

etc.