Membranes are cellular structures which separate the interior and exterior compartments of a cell or individual organelle, whereas the cell wall is required for structural support in only selective cell types. Cell membranes are found in both eukaryotic and prokaryotic cells and are associated with different organelles, such as the mitochondria.
The cell membrane is a vital organelle which allows for compartmentalization. This is critical for all eukaryotic organisms which are multicellular because it keeps all cells separate from one another. Within the cytosol, membrane-enclosed organelles such as the golgi apparatus and endoplasmic reticulum (ER) have distinctive positions which isolate these organelles from other cellular units. This separation is possible because they have membranes.
The cell membrane is a structure which surrounds the outside of a cell and it is composed of two materials, a lipid bilayer and proteins. The lipid bilayer is comprised largely of phospholipids and smaller quantities of other lipids which are called steroids and fatty acids. This membrane is considered to be semi-permeable due to the arrangement of the phospholipids: they have a polar head group and nonpolar tails. This semi-permeability controls the movement of substances, such as ions, into and out of the cell. Molecules which cannot pass through the cellular membrane freely by passive diffusion travel using different mechanisms requiring ATP.
In terms of the proteins, they can either be imbedded directly into the membrane or they can be peripheral proteins. These proteins are critical for regulating biological activities. For example, proteins are utilized for cell to cell communication.
On the other hand, cell walls are only found in plant and fungi cells, along with prokaryotes. These structures are located outside of the cell membrane and provide cellular support, along with protection. In terms of support, the rigidity of the cell wall maintains cellular integrity because without this wall, the cell would collapse under the pressure. For example, when water enters the cell, over-expansion is prevented by the cell wall.