Far from being a passive shell, the plasma membrane is an active structure responsible for maintaining vital chemical differences between the cytoplasm and the extracellular medium, and mediates several key functions in the cell’s interaction with its environment. Cells generally exist in aqueous environments, and the bulk of their internal constituents are water-soluble chemicals. This imposes the need for a boundary that is impervious to water. Not surprisingly, the bulk of the plasma membrane consists of fatty molecules.

The chief constituents of the plasma membrane are phospholipids, which have two components: a hydrophilic ‘head’ group, and two hydrophobic hydrocarbon ‘tails’. In the plasma membrane, the phospholipids are arranged in the form of a bilayer. In each layer, the hydrophilic head groups are oriented towards the outside watery environment, and the hydrophobic tails are oriented towards the interior of the bilayer. It is important to note that the bilayer is ‘fluid’, meaning that the individual molecules are free to move along the 2-dimensional surface of the bilayer. Additional structural integrity is provided mainly by cholesterol molecules interspersed among the phospholipid molecules.

While the lipid bilayer provides the required aqueous impermeability, the plasma membrane must perform a host of other functions involving the cell’s interaction with its environment. These functions are mediated by membrane associated proteins and glycolipids, which are intermixed among the phospholipids to form a fluid mosaic. Through associated proteins called transport proteins, the plasma membrane regulates the movement of essential molecules into and waste products and secretions out of the cell.

Membrane proteins are responsible for the adhesion of the cell to other cells or the extracellular matrix. The interaction of membrane proteins with the cytoskeleton is what gives each cell its characteristic shape.