Cellulose

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Cellulose

How cellulose is arranged in plant cell walls

Cellulose is a substance found in the cell walls of plants. Although cellulose is not a component of the human body, it is nevertheless the most abundant organic macromolecule on Earth. The scientific community first observed cellulose in 1833 when it was studied in plant cell walls. The chemical structure of cellulose resembles that of starch, but unlike starch, cellulose is extremely rigid (Figure 1). This rigidity imparts great strength to the plant body and protection to the interiors of plant cells.

Structure of cellulose

Like starch, cellulose is composed of a long chain of at least 500 glucose molecules. Cellulose is, thus, a polysaccharide (Latin for “many sugars”). Several of these polysaccharide chains are arranged in parallel arrays to form cellulose microfibrils. The individual polysaccharide chains are bound together in the microfibrils by hydrogen bonds. The microfibrils, in turn, are bundled together to form macrofibrils (Figure 1).

The microfibrils of cellulose are extremely tough and inflexible due to the presence of hydrogen bonds. In fact, when describing the structure of cellulose microfibrils, chemists call their arrangement crystalline, meaning that the microfibrils have crystal-like properties. Although starch has the same basic structure as cellulose—it is also a polysaccharide—the glucose subunits are bonded in such a way that allows the starch molecule to twist. In other words, the starch molecule is flexible, while the cellulose molecule is rigid.

How cellulose is arranged in plant cell walls

Like human bone, plant cell walls are composed of fibrils laid down in a matrix, or background material. In a cell wall, the fibrils are cellulose microfibrils, and the matrix is composed of other polysaccharides and proteins. One of these matrix polysaccharides in cell walls is pectin, a substance that, when heated, forms a gel. Pectin is the substance that cooks use to make jellies and jams.

The arrangement of cellulose microfibrils within the polysaccharide and protein matrix imparts great strength to plant cell walls. The cell wall of plants performs several functions, each related to the rigidity of the cell wall. It protects the interior of the plant cell, but also allows the circulation of fluids within and around the cell wall. The cell wall also binds the plant cell to its neighbors. This binding creates the

KEY TERMS

Anaerobic— Describes biological processes that take place in the absence of oxygen.

Cell wall— The tough, outer covering of plant cells composed of cellulose microfibrils held together in a matrix.

Cellulose synthetase— The enzyme embedded in the plasma membrane that synthesizes cellulose.

Colon— The terminal portion of the human digestive tract.

Golgi body— The organelle that manufactures, sorts, and transports macromolecules within a cell.

Lignin— A polysaccharide that forms the secondary cell wall in some plants.

Matrix— The material, composed of polysaccharides and protein, in which microfibrils of cellulose are embedded in plant cell walls.

Methane— A gas produced during the anaerobic digestion of cellulose by bacteria in certain animals.

Microfibril— Small fibrils of cellulose; consists of parallel arrays of cellulose chains.

Polysaccharide— A molecule composed of many glucose subunits arranged in a chain.

Ruminant— A cud-chewing animal with a four-chambered stomach and even-toed hooves.

tough, rigid skeleton of the plant body. Cell walls are the reason why plants are erect and rigid. Some plants have a secondary cell wall laid over the primary cell wall. The secondary cell wall is composed of yet another polysaccharide called lignin. For example, lignin is found in trees. The presence of both primary and secondary cell walls makes the tree even more rigid, penetrable only with sharp axes.

Unlike the other components of the cell wall, which are synthesized in the plant’s Golgi body (an organelle that manufactures, sorts, and transports different macromolecules within the cell), cellulose is synthesized on the surface of the plant cell. Embedded within the plant’s plasma membrane is an enzyme, called cellulose synthetase, which synthesizes cellulose. As cellulose is synthesized, it spontaneously forms microfibrils that are deposited on the cell’s surface. Because the cellulose synthetase enzyme is located in the plasma membrane, the new cellulose microfibrils are deposited under older cellulose microfibrils. Thus, the oldest cellulose microfibrils are

outermost on the cell wall, while the newer microfibrils are innermost on the cell wall.

As the plant cell grows, it must expand to accommodate the growing cell volume. However, because cellulose is so rigid, it cannot stretch or flex to allow this growth. Instead, the microfibrils of cellulose slide past each other or separate from adjacent microfibrils. In this way, the cellwall is able to expand when the cell volume enlarges during growth.

Cellulose digestion

Humans lack the enzyme necessary to digest cellulose. Hay and grasses are particularly abundant in cellulose, and both are indigestible by humans (although humans can digest starch). Animals such as termites and herbivores such as cows, koalas, and horses all digest cellulose, but even these animals do not themselves have an enzyme that digests this material. Instead, these animals harbor microbes that can digest cellulose.

The termite, for instance, contains protists (single-celled organisms) called mastigophorans in their guts that carry out cellulose digestion. The species of mastigophorans that performs this service for termites is called Trichonympha, which, interestingly, can cause a serious parasitic infection in humans.

Animals such as cows have anaerobic bacteria in their digestive tracts which digest cellulose. Cows are ruminants, or animals that chew their cud. Ruminants have several stomachs that break down plant materials with the help of enzymes and bacteria. The partially digested material is, then, regurgitated into the mouth, which is again chewed to break the material down even further. The bacterial digestion of cellulose by bacteria in the stomachs of ruminants is anaerobic, meaning that the process does not use oxygen. One of the byproducts of anaerobic metabolism is methane, a notoriously foul-smelling gas. Ruminants give off large amounts of methane daily. In fact, many environmentalists are concerned about the production of methane by cows, because methane may contribute to the destruction of ozone in Earth’s stratosphere.

Although cellulose is indigestible by humans, it does form a part of the human diet in the form of plant foods. Small amounts of cellulose found in vegetables and fruits pass through the human digestive system intact. Cellulose is part of the material called fiber that dieticians and nutritionists have identified as useful in moving food through the digestive tract quickly and efficiently. Diets high in fiber are thought to lower the risk of colon cancer because fiber reduces the time that waste products stay in contact with the walls of the colon (the terminal part of the digestive tract).

See also Rumination.