Definition The cell membrane, which envelops the cell, is a thin, pliable, elastic structure.
Thickness :7.5 to 10 nanometers.
Its basic structure is a lipid bilayer interposed with large globular protein molecules.
a Lipid of the cell membrane :
Composed almost entirely of phospholipids and cholesterol (lipid in nature).
Hydrophilic : The phosphate radicle of the phospholipid is hydrophilic. That part is soluble in water.
Hydrophobic The fatty acid radicle are hydrophobic. That part is soluble only in fat.
b. The cell membrane proteins
These are membrane proteins, most of which are glycoproteins. The cell membrane proteins are of two types:
Integral proteins : Many of the integral proteins provides structural channels (or pores). Others of the integral proteins act as carrier proteins. Still others acts as enzymes.
Peripheral proteins : The peripherals proteins occur mainly on the inside of the membrane, and they often are attached to one of the integral proteins. These peripheral proteins function almost entirely as enzymes or as other types of controllers of intracellular function.
The amounts of protein varies with the function of the membrane but makes up on average 50% of the mass of the membrane; ie, there is about one protein molecule per 50 of the much smaller phospholipid molecules.
c. Membrane carbohydrate :
Membrane carbohydrate occur almost invariably in combination with proteins or lipids in the form of glycoproteins or glycolipids.
The glyco portions of these molecules almost invariably protrude to the outside of the cell dangling outward from the cell surface.
Many other carbohydrate compounds, called proteoglycans, which are mainly carbohydrate substances bound to small protein cores, often are loosely attached to the outer surface of the cell as well. Thus, the entire outside surface of the eel often has a loose carbohydrate coat called the glycocalyx.