A cell wall is a tough, semirigid case that surrounds a cell. Both plants and some single-celled organisms have cell walls. Cell walls are outside the cell membrane and are not part of the living cell. They protect the cell and provide it with support.
Cell walls are found only in some single-celled organisms like fungi and bacteria, but they are found in all plants. They are one of the characteristics that separate plant cells from animal cells (which do not have cell walls). A cell wall is different from a cell membrane, since all cells have plasma membranes that are a part of the living cell. Membranes are also semipermeable and only allow substances of a certain size to pass in and out of the cell. A cell wall in a plant is a structure that is just outside the membrane and provides a plant with protection and rigidity. In plants, it is made up of a complex carbohydrate called cellulose that, although it is very tough, also allows water and solutions to reach the plasma membrane. Since cellulose is both light and strong, it provides the ideal material for a cell wall, acting as a kind of external skeleton that gives the cell (and therefore the plant) its shape and strength. The stem of a plant is able to hold itself up despite gravity by having thousands of cells lined up next to and on top of each other. As the cells take in water, they expand like a balloon and exert pressure against their own walls and against the stem walls. It is their pressure that holds the stem up. When a plant droops, it is because its cells lack water to push against the walls, and the cells begin to shrink.
The cell walls of a green plant are made of cellulose, making it the most abundant organic compound on Earth. The cellulose in a plant's cell walls is formed by fibers that are very strong because they are linked in a criss-cross mesh pattern. Herbivores or animals who eat nothing but green plants must have special digestive systems since the tough cell walls of a plant make it very difficult to digest. This is why herbivores have a much longer and more elaborate digestive tract than do carnivores (meateaters) who consume mostly easy-to-digest proteins. Herbivores must also consume enormous amounts of plant material since each mouthful of vegetation contains a relatively small amount of energy (compared to a protein diet).
A plant's cell wall helps protect the important membrane and gives the plant cell and the plant its shape and support. Fungi and bacteria also have cell walls, but they are not made of cellulose. Most fungi have a cell wall made of chitin, while yeast (a type of bacteria) cell walls are made of a different complex of carbohydrates.
Gale Document Number: GALE|CX3437100052