No, in many cells the plasma membrane is encased within an extraneous coat of nonliving material that usually confers some rigidity on the cell it surrounds. Although not affecting permeability, this rigid coat affords considerable protection to the underlying cell, especially in dilute solutions in which the cell might take in water and tend to swell.The coat is known as a cell wall in plant cells, fungi, and bacteria. Thiswall is composed of cellulose in all plant cells. Cellulose is the major component of wood and certain other commercially useful plant products. In fungi,the walls are composed of chitin, a complex carbohydrate rich in amino-containing sugars. Bacterial cell walls consistof complex carbohydrates and linked short peptides, but there is considerable variation in the chemical makeup of these mixed polysaccharide and amino acid chains in different bacterial strains.
In most animal cells, theremaybe a layer of carbohydrate lying outside the plasmamembrane, but this is not a separable coat. It is called a glycocalyx and exists in intimate association with the membrane. The carbohydrates themselves are generally short chains but are covalently bound to the lipids or proteins of the membrane to produce a thin, furlike cover for the cell. The glycocalyx contains receptors for a variety of substances with which the cell may interact. The blood types of humans are based on the antigenic properties of the glycocalyx of the red blood cell. The sites on many cells that signal ‘‘self’’ or ‘‘foreigner’’ to the immune systemof host organisms occur within the glycocalyx as well. Thus, in dealing with the possibility of whether a transplanted tissue or organ will be rejected, a major focus is on the glycoproteins formed within the glycocalyx. Collectively, the chief set of such glycoproteins is known as the major histocompatibility antigens, and these antigens are coded for by a group of genes known as the major histocompatibility complex (MHC).
In many cells, special structures are formed that anchor the cells firmly together. These structures, particularly associated with epithelial tissue, include tight junctions, inwhich there is virtually no intercellular space, and desmosomes, in which a highly layered, narrow space can be discerned. Such structures may also play a role in the transport of materials from one cell to another. Animal cells involved in absorption, such as the cells that line the intestine, often have filaments extending out from the plasma membrane. These filaments, which are rich in carbohydrates, are known as microvilli. They increase the absorptive surface of the cell and may also contain enzymes that function in digestion. Another type of extracellular coat is formed by the layers of polysaccharide that surround the eggs of many vertebrate and invertebrate species. These coats, usually added to the egg proper as it passes along the reproductive tract before hatching, must be penetrated by the sperm at the time of fertilization. The enzymes associated with the acrosome found in the head of most sperm cells aid in carrying out this task.Among many protista, a highly elastic pellicle overlies the plasma membrane, but the function of this structure is not completely clear. In Euglena, it consists of flexible protein strips.