What is the structure of the cell membrane?

The cell membrane is a thin (ranging from 5 nm to 10 nm in thickness), semi-permeable, dynamic membrane that completely envelopes the cytoplasm of the cell. It is composed of lipids, proteins, and carbohydrates that occur in fluid-mosaic arrangement. The fluid-mosaic model describes the cell membrane as a mosaic of its individual components, which give it a fluid character. 

Phospholipids make up the major component of a cell membrane. A phospholipid molecule consists of glycerol, a hydrophilic phosphate head and two hydrophobic fatty acid tails. Phospholipid molecules form a lipid bilayer in which their hydrophilic heads are attached to each other and face the aqueous cytosol on one side and extracellular fluid on the other. The hydrophobic tails are sandwiched between the two hydrophilic head areas, facing away from both the cytosol and extracellular fluid. This arrangement gives the cell membrane its characteristic semi-permeable nature, allowing only certain molecules to diffuse across it. 

Cholesterol molecules, the other lipid component of animal cell membranes, are dispersed selectively among the membrane phospholipids. This helps to keep some distance between the phospholipid molecules, which prevents the membrane from becoming stiff. 

The cell membrane is studded with proteins that may cross the membrane entirely, extend partially into the membrane, or attach loosely to the inner or outer face of the membrane. 

Carbohydrates are present only on the outer surface of the membrane, where they may be attached to lipids, forming glycolipids, or attached to proteins, forming glycoproteins.