The bacterial cell wall is a unique structure which surrounds the cell membrane. Although not present in every bacterial species, the cell wall is very important as a cellular component. Structuraly, the wall is necessary for:
The cell walls of all bacteria are not identical. In fact, cell wall composition is one of the most important factors in bacterial species analysis and differentiation. There are two major types of walls: Gram-positive and Gram-negative. The cell wall of Gram-positive bacteria consists of many polymer layers of peptidoglycan connected by amino acid bridges. A schematic diagram provides the best explanation of the structure. The peptidoglycan polymer is composed of an alternating sequence of N-acetylglucosamine and N-acetyl-muraminic acid. It's a lot easier to just remember NAG and NAMA. Each peptidoglycan layer is connected, or crosslinked, to the other by a bridge made of amino acids and amino acid derivatives. The particular amino acids vary among different species, however. The crosslinked peptidoglycan molecules form a network which covers the cell like a grid. Also, 90% of the Gram-positive cell wall is comprised of peptidoglycan.
The cell wall of Gram-negative bacteria is much thinner, being comprised of only 20% peptidoglycan. Gram-negative bacteria also have two unique regions which surround the outer plasma membrane: the periplasmic space and the lipopolysaccharide layer. The periplasmic space separates the outer plasma membrane from the peptidoglycan layer. It contains proteins which destroy potentially dangerous foreign matter present in this space. The lipopolysaccharide layer is located adjacent to the exterior peptidoglycan layer. It is a phospholipid bilayer construction similar to that in the cell membrane and is attached to the peptidoglycan by lipoproteins. The lipid portion of the LPS contains a toxic substance, called Lipid A, which is responsible for most of the pathogenic affects associated with harmful Gram-negative bacteria. Polysaccharides which extend out from the bilayer also contibute to the toxicity of the LPS. The LPS, lipoproteins, and the associated polysaccharides together form what is known as the outer membrane.
Keep in mind that the cell wall is not a regulatory structure like the cell membrane. Although it is porous, it is not selectively permeable and will let anything pass that can fit through its gaps.