Biologists have long known that plant cells and the cells of fungi and most bacteria have strong, thick walls containing much carbohydrate. But only in recent years have they come to realize that most animal cells, too, have a carbohydrate coat on the outer surface of their plasma membranes, and that this coat plays an important role in de¬termining certain properties of the cells, as seen under digital compound microscopes. The presence of carbohydrate materials on their outer surfaces appears to be a general property of cells. Nonetheless, the conspicuous, thick, relatively rigid walls of plant, fungal, and bacterial cells, on the one hand, and the inconspic¬uous, thin, nonrigid coats of animal cells, on the other, are among the most striking differences between these groups which were examined under digital compound microscopes.

Plant cell walls

Located outside the cell membrane, the plant cell wall is generally not considered part of the cellular material, although it is a product of the cell, as seen under digital compound microscopes. The principal structural component of the cell wall is the complex polysaccharide cellulose, which is generally present in the form of long threadlike structures called fibrils. The cellulose fibrils are cemented together by a matrix of other organic compounds. Spaces between the fibrils that are not entirely filled with matrix generally allow water, air, and dissolved materials to pass freely through the cell wall.

The first portion of the cell wall, as seen under a digital compound microscope,laid down by a young growing cell is the primary wall. As long as the cell continues to grow, this wall, which is stretchable, is the only one formed. Where the walls of two cells come together, an intercellular layer between them, known as the middle lamella, binds them together. Pectin, a complex polysaccharide gen¬erally present in the form of calcium pectate, is one of the principal constituents of the middle lamella. If the pectin is dissolved away, the cells become less tightly bound to each other. That is what happens, for example, when fruits ripen. The calcium pectate is partly con¬verted into other more soluble forms, the cells become loose and the fruit becomes softer.

Cells of the soft tissues of the plant have only primary walls and intercellular middle lamellae, as studied under digital compound microscopes. After ceasing to grow, the cells that eventually form the harder, more woody portions, of the plant add further layers to the cell wall, forming what is known as the second¬ary wall. Since this wall, like the primary wall, is deposited by the cytoplasm of the cell, it is located inside the earlier-formed primary wall, lying between it and the membrane. The secondary wall is often much thicker than the primary wall and is composed of a succession of compact layers oriented at angles to one another. This arrangement gives added strength to the cell wall. In addition to cel¬lulose, secondary walls usually contain other materials, such as lignin, which make them stiffer. Once deposition of the secondary wall is completed, many cells die, leaving the hard tube formed by their walls to function in mechanical support and internal transport for the body of the plant. The cellulose of plant cell walls is commercially impor¬tant as the main component of paper, cotton, flax, hemp, rayon, cellu¬loid, and, obviously, wood itself.

Plant cell walls generally do not form completely uninterrupted boundaries around the cells, as seen under a microscope. There are often tiny holes in the walls through which delicate cytoplasmic connections between adjacent cells may run. These connections are called plasmodesmata. Thus the cytoplasm of an individual cell in a multicellular plant body is not isolated, but is in contact and communication with the cytoplasm of other cells by way of the plasmodesmata. The cy¬toplasm of cells interconnected by plasmodesmata constitutes a con¬tinuous system called the symplast. A large portion of the intercellular exchange of such materials as sugars and amino acids probably takes place through the plasmodesmata of the symplast. The cell walls of both fungi and bacteria differ from those of plant cells in that they are not composed of cellulose. In fungi the main structural component of the wall is chitin, a polymer that is a deriva¬tive of the amino sugar glucosamine. In bacteria the cell wall is composed of a compound called murein, which consists of polysaccharide chains linked together by short chains of amino acids.

Cell walls permit the cells of plants, fungi, and bacteria to with¬stand very dilute (hypotonic) external media without bursting. In such media, the cells tend to take up water by osmosis, as a result of the high osmotic concentration of the cell contents. The cell swells, building up turgor pressure against the cell walls. The walls exert an equal op¬posing pressure against the swollen cell. The cell wall of a mature cell can usually be stretched only by a minute amount. Equilibrium is reached when the resistance of the wall is so great that no further increase in the size of the cell is possible and, consequently, no more water can enter the cell. Thus the cells of plants, fungi and bacteria are not as sensitive as animal cells to the difference in osmotic con¬centration between the cellular material and the surrounding me¬dium. Because of their walls, such cells can withstand much wider fluctuations in the osmotic makeup of the surrounding medium than animal cells which was discovered when they were studied under the microscope.