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Cellulose

Cellulose

Cellulose is the substance that makes up most of a plant's cell walls. Since it is made by all plants, it is probably the most abundant organic compound on Earth. Aside from being the primary building material for plants, cellulose has many others uses. According to how it is treated, cellulose can be used to make paper, film, explosives, and plastics, in addition to having many other industrial uses. The paper in this book contains cellulose, as do some of the clothes you are wearing. For humans, cellulose is also a major source of needed fiber in our diet.

The structure of cellulose

Cellulose is usually described by chemists and biologists as a complex carbohydrate (pronounced car-bow-HI-drayt). Carbohydrates are organic compounds made up of carbon, hydrogen, and oxygen that function as sources of energy for living things. Plants are able to make their own carbohydrates that they use for energy and to build their cell walls. According to how many atoms they have, there are several different types of carbohydrates, but the simplest and most common in a plant is glucose. Plants make glucose (formed by photosynthesis) to use for energy or to store as starch for later use. A plant uses glucose to make cellulose when it links many simple units of glucose together to form long chains. These long chains are called polysaccharides (meaning "many sugars"

and pronounced pahl-lee-SAK-uh-rydes), and they form very long molecules that plants use to build their walls.

It is because of these long molecules that cellulose is insoluble or does not dissolve easily in water. These long molecules also are formed into a criss-cross mesh that gives strength and shape to the cell wall. Thus while some of the food that a plant makes when it converts light energy into chemical energy (photosynthesis) is used as fuel and some is stored, the rest is turned into cellulose that serves as the main building material for a plant. Cellulose is ideal as a structural material since its fibers give strength and toughness to a plant's leaves, roots, and stems.

Cellulose and plant cells

Since cellulose is the main building material out of which plants are made, and plants are the primary or first link in what is known as the food chain (which describes the feeding relationships of all living things), cellulose is a very important substance. It was first isolated in 1834 by the French chemist Anselme Payen (17951871), who earlier had isolated the first enzyme. While studying different types of wood, Payen obtained a substance that he knew was not starch (glucose or sugar in its stored form), but which still could be broken down into its basic units of glucose just as starch can. He named this new substance "cellulose" because he had obtained it from the cell walls of plants.

Words to Know

Carbohydrate: A compound consisting of carbon, hydrogen, and oxygen found in plants and used as a food by humans and other animals.

Glucose: Also known as blood sugar; a simple sugar broken down in cells to produce energy.

Photosynthesis: Chemical process by which plants containing chlorophyll use sunlight to manufacture their own food by converting carbon dioxide and water to carbohydrates, releasing oxygen as a by-product.

As the chief constituent (or main ingredient) of the cell walls of plants, cellulose performs a structural or skeletal function. Just as our hard, bony skeletons provide attachment points for our muscles and support our bodies, so the rigidity or stiffness found in any plant is due to the strength of its cell walls. Examined under a powerful microscope, fibers of cellulose are seen to have a meshed or criss-cross pattern that looks as if it were woven much as cloth. The cell wall has been likened to the way reinforced concrete is made, with the cellulose fibers acting as the rebars or steel rods do in concrete (providing extra strength). As the new cell grows, layer upon layer of new material is deposited inside the last layer, meaning that the oldest material is always on the outside of the plant.

Human uses of cellulose

Cellulose is one of the most widely used natural substances and has become one of the most important commercial raw materials. The major sources of cellulose are plant fibers (cotton, hemp, flax, and jute are almost all cellulose) and, of course, wood (about 42 percent cellulose). Since cellulose is insoluble in water, it is easily separated from the other constituents of a plant. Cellulose has been used to make paper since the Chinese first invented the process around a.d. 100. Cellulose is separated from wood by a pulping process that grinds woodchips under flowing water. The pulp that remains is then washed, bleached, and poured over a vibrating mesh. When the water finally drains from the pulp, what remains is an interlocking web of fibers that, when dried, pressed, and smoothed, becomes a sheet of paper.

Raw cotton is 91 percent cellulose, and its fiber cells are found on the surface of the cotton seed. There are thousands of fibers on each seed, and as the cotton pod ripens and bursts open, these fiber cells die. Because these fiber cells are primarily cellulose, they can be twisted to form thread or yarn that is then woven to make cloth. Since cellulose reacts easily to both strong bases and acids, a chemical process is often used to make other products. For example, the fabric known as rayon and the transparent sheet of film called cellophane are made using a many-step process that involves an acid bath. In mixtures if nitric and sulfuric acids, cellulose can form what is called guncotton or cellulose nitrates that are used for explosives. However, when mixed with camphor, cellulose produces a plastic known as celluloid, which was used for early motion-picture film. However, because it was highly flammable (meaning it could easily catch fire), it was eventually replaced by newer and more stable plastic materials. Although cellulose is still an important natural resource, many of the products that were made from it are being produced easier and cheaper using other materials.

Importance to human diet

Despite the fact that humans (and many other animals) cannot digest cellulose (meaning that their digestive systems cannot break it down into its basic constituents), cellulose is nonetheless a very important part of the healthy human diet. This is because it forms a major part of the dietary fiber that we know is important for proper digestion. Since we cannot break cellulose down and it passes through our systems basically unchanged, it acts as what we call bulk or roughage that helps the movements of our intestines. Among mammals, only those that are ruminants (cudchewing animals like cows and horses) can process cellulose. This is because they have special bacteria and microorganisms in their digestive tracts that do it for them. They are then able to absorb the broken-down cellulose and use its sugar as a food source. Fungi are also able to break down cellulose into sugar that they can absorb, and they play a major role in the decomposition (rotting) of wood and other plant material.

[See also Plant ]

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Cellulose

Cellulose


Cellulose is the most abundant organic molecule in nature. It is a polysaccharide assembled from glucose monomer units, and it (together with other materials such as hemicellulose and lignin) is the main constituent of plant cell walls. Along with several undigestible polysaccharides, cellulose constitutes the main part of dietary fiber. Specifically cellulose is one of the components of insoluble fiber.

The glucose units in cellulose are combined in a way that results in the formation of very linear, flat molecules that can, in turn, form sheets that possess extensive networks of hydrogen bonds . The hydrogen bonds are both within individual sheets and between successive sheets. As a result of these bonds, sheets of cellulose are particularly stronga property critical to the function of plant cell walls. Cellulose shows a variable degree of polymerization, with anywhere from 1,000 to 14,000 glucose residues comprising a single cellulose polymer. Because of its high molecular weight and crystalline structure, cellulose is insoluble in water and has a poor ability to absorb water.

Human beings lack the enzyme cellulase and are therefore unable to break cellulose down to individual glucose molecules. Although many fungi are able to break down cellulose to glucose, only a few types of bacteria have this ability. In the rumina of cows, sheep, and goats, two different types of bacteria produce the enzyme that breaks down cellulose.

Cellulose and its derivatives are used in a number of food products to modify those foods in different ways (e.g., as a thickener, stabilizer, or texturizer). The fibrous form is a basic material that is used to make both textiles and paper. Cellulose is also used to make nitrocellulose (an ingredient in explosives and lacquers) and as a binder in the manufacture of medicinal tablets.

see also Fibers; Polymers, Natural; Polysaccharides.

Matthew A. Fisher

Bibliography

Atkins, Peter W. (1987). Molecules. New York: W. H. Freeman.

Internet Resources

Department of Polymer Science, University of Southern Mississippi. "Cellulose." Available from <http://www.psrc.usm.edu/macrog/>.

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cellulose

cellulose, chief constituent of the cell walls of plants. Chemically, it is a carbohydrate that is a high molecular weight polysaccharide. Raw cotton is composed of 91% pure cellulose; other important natural sources are flax, hemp, jute, straw, and wood. Cellulose has been used for the manufacture of paper since the 2d cent. Insoluble in water and other ordinary solvents, it exhibits marked properties of absorption. Because cellulose contains a large number of hydroxyl groups, it reacts with acids to form esters and with alcohols to form ethers. Cellulose derivatives include guncotton, fully nitrated cellulose, used for explosives; celluloid (the first plastic), the product of cellulose nitrates treated with camphor; collodion, a thickening agent; and cellulose acetate, used for plastics, lacquers, and fibers such as rayon.

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Cellulose

Cellulose

Cellulose is a major structural component of the cell walls of all land plants, including trees, shrubs, and herbaceous plants. The cell wall is a complex polysaccharide layer that surrounds each cell within a plant. Chemically, cellulose is a polysaccharide made up of long, unbranched chains of glucose linked end to end, making a very flat chain. (Starch is also made up of glucose, but linked such that it curls, resulting in very different properties.) Many cellulose chains associate side by side to make a cellulose ribbon, or microfibril, that has exceptional mechanical strength and chemical stability. Cellulose microfibrils, which are approximately 5 to 10 nanometers thick and many micrometers long, make cell walls strong and able to resist large forces, such as those generated internally by turgor pressure or externally by the weight of the plant or by wind. Economically, cellulose is important as a major component of wood products and of fibers used to make paper and textiles, such as cotton and linen. For industry, cellulose is dissolved and spun as a thread (called rayon) or formed into a thin sheet (cellophane). Cellulose is also chemically modified to make many kinds of films (such as cellulose acetate), thickeners used in foods and paints, and coatings such as nail polish (which contains cellulose nitrate).

see also Carbohydrates; Cell Walls; Fiber and Fiber Products.

Daniel Cosgrove

Bibliography

Brett, C. T., and K. Waldron. Physiology and Biochemistry of Plant Cell Walls, 2nd ed. London: Chapman and Hall, 1996.

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cellulose

cellulose A polysaccharide of glucose units which is not hydrolysed by mammalian digestive enzymes; the main component of plant cell walls. It is digested by the bacterial enzyme cellulase, and only ruminants and animals that have a large caecum have an adequate population of intestinal bacteria to permit them to digest cellulose to any significant extent. There is little digestion of cellulose in the human large intestine; nevertheless, it serves a valuable purpose in providing bulk to the intestinal contents, and is one of the major components of dietary fibre or non‐starch polysaccharides.

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cellulose

cellulose A polysaccharide that consists of a long unbranched chain of glucose units (see formula). It is the main constituent of the cell walls of all plants, many algae, and some fungi and is responsible for providing the rigidity of the cell wall. It is an important constituent of dietary fibre. Cellulose occurs typically as microfibrils, each consisting of long parallel arrays of 50–60 cellulose molecules. The fibrous nature of extracted cellulose has led to its use in the textile industry for the production of cotton, artificial silk, etc.

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cellulose

cel·lu·lose / ˈselyəˌlōs; -ˌlōz/ • n. 1. an insoluble substance that is the main constituent of plant cell walls and of vegetable fibers such as cotton. 2. paint or lacquer consisting principally of cellulose acetate or nitrate in solution. DERIVATIVES: cel·lu·lo·sic / ˌselyəˈlōsik; -ˈlōzik/ adj.

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cellulose

cellulose Polysaccharide, carbohydrate (C6H10O5)n that is the structural constituent of the cell walls of plants and algae. Consisting of parallel unbranched chains of glucose units cross-linked together into a stable structure, it forms the basic material of the paper and textile industries. The first synthetic cellulose was produced in 1996 by Japanese chemists.

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cellulose

cellulose A straight-chain, insoluble polysaccharide that is composed of glucose molecules linked by beta-1, 4 glycosidic (see GLYCOSIDE) bonds. It is the principal structural material of plants, and as such is the most abundant organic compound in the world. It has also been found in certain sea squirts.

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cellulose

cellulose A straight-chain, insoluble polysaccharide composed of glucose molecules linked by (β-1, 4 glycosidic bonds. It is the principal structural material of plants, and as such is the most abundant organic compound in the world. It has also been found in certain sea squirts (Urochordata).

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cellulose

cellulose A straight-chain, insoluble polysaccharide that is composed of glucose molecules linked by beta-1,4 glycosidic bonds. It is the principal structural material of plants, and as such is the most abundant organic compound in the world. It has also been found in certain sea squirts.

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cellulose

cellulose adj. consisting of cells XVIII; sb. a carbohydrate, main constituent of plant-cell walls XIX. As adj. — modL. cellulōsus; as sb. — F. cellulose; see prec. and -OSE.
Hence celluloid XIX; the use of -OID is arbitrary.

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cellulose

cellulose (sel-yoo-lohz) n. a carbohydrate consisting of linked glucose units. It is an important constituent of plant cell walls. Cellulose cannot be digested by humans and is a component of dietary fibre (roughage).

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cellulose

celluloseadiós, chausses, Close, Davos, dose, engross, gross, Grosz, jocose, morose, Rhos, verbose •grandiose • religiose • otiose •globose • viscose • bellicose • varicose •vorticose • cellulose • lachrymose •lactose • comatose • siliquose

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Cellulose

Cellulose

Structure of cellulose

How cellulose is arranged in plant cell walls

Cellulose digestion

Resources

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 celluloseit is also a polysaccharidethe 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 plants 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 plants plasma membrane is an enzyme, called cellulose synthetase, which synthesizes cellulose. As cellulose is synthesized, it spontaneously forms microfibrils that are deposited on the cells 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 Earths 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.

Resources

BOOKS

Hon, David N. S., and Nobuo Shiraishi. Wood and Cellulosic Chemistry. New York: Marcel Dekker, 2001.

Koshijima, Tetsup. Association between Lignin and Carbohydrates in Wood and Other Plant Tissues. Berlin and New York: Springer, 2003.

OTHER

Martin Chaplin, London South Bank University. Water

Structure and Behavior: Cellulose. <http://www.lsbu.ac.uk/water/hycel.html> (accessed October 4, 2006).

Kathleen Scogna

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Cellulose

Cellulose

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 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, the 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 walls of plants perform several functions, each related to the rigidity of the cell wall. The cell wall 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 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. 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 (singlecelled 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 then 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 by-products 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.

Resources

books

Brett, C.T. Physiology and Biochemistry of Plant Cell Walls. London: Unwin Hyman, 1990.

Van Soest, Peter J. Nutritional Ecology of the Ruminant. 2nd ed. Ithaca: Comstock Press, 1994.


Periodicals

Benedict, C. R., et al. "Crystalline Cellulose and Cotton Fiber Strength." Crop Science 24 (January-February 1994): 147.

Dunkle, Richard L. "Food Science Research: An Investment in Health." Agricultural Research 41 (December 1993): 2.

Dwyer, Johanna. "Dietary Fiber and Colorectal Cancer Risk." Nutrition Reviews 51 (May 1993): 147.

Kleiner, Susan M. "Fiber Facts: How to Fight Disease with a High-fiber Diet." The Physician and Sportsmedicine 18 (October 1990): 19.

Slavin, Joanne L. "Dietary Fiber: Mechanisms or Magic on Disease Prevention?" Nutrition Today 25 (December 1990): 6.

Young, Stephen. "How Plants Fight Back." New Scientist 130 (June 1, 1991): 41.


Kathleen Scogna

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.

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Cellulose

Cellulose

OVERVIEW

Cellulose (SELL-you-lohs) is a colorless to white, tasteless, odorless, polysaccharide fiber found in the cell walls of all land plants and some bacteria, seaweeds, algae, and fungi. Polysaccharides (the term means "many sugars") are polymers consisting of monosaccharide (simple sugar) monomers joined together in very large molecules. The monomer of which cellulose is made is glucose, also known as blood sugar, dextrose, or grape sugar. The subscript "n" at the end of the chemical formula indicates that a large number of these monomers combine to make the polymer. Cellulose provides the structural support for plants and other organisms in which it occurs. It is generally regarded as the most common organic compound found in nature.

KEY FACTS

OTHER NAMES:

None

FORMULA:

(C6H10O5)n

ELEMENTS:

Carbon, hydrogen, oxygen

COMPOUND TYPE:

Polysaccharide (carbohydrate polymer; organic)

STATE:

Solid

MOLECULAR WEIGHT:

Very large, in excess of 100,000 g/mol

MELTING POINT:

Decomposes at temperatures above 260°C (500°F)

BOILING POINT:

Not applicable

SOLUBILITY:

Insoluble in water and all common organic solvents

Cellulose never occurs in nature in a pure form. Cotton is the purest natural form, consisting of about 90 percent cellulose. Flax (linen fiber) consists of about 70 to 75 percent cellulose, wood of about 40 to 50 percent cellulose, and seaweeds and algae of about 25 to 30 percent cellulose.

Cellulose was first discovered in 1819 by the French naturalist Henri Braconnot (1781–1855). The compound was first isolated and analyzed fifteen years later by the French botanist Anselme Payen (1795–1871), who gave it its modern name of cellulose based on its origin ("cell" ) plus the suffix -ose. The earliest chemical studies of cellulose were conducted by a team of English chemists, Charles Frederick Cross (1855–1935), Edward John Bevan (1856–1921), and Clayton Beadle (1868–1917), who identified the compound we now know as cellulose and reported on its structure and properties in the 1890s and early 1900s.

Cellulose, in its natural forms such as wood, cotton, and linen, has countless numbers of uses as food, clothing, construction materials, and other applications. It also serves as the raw material in making derivatives such as cellulose acetate, cellulose nitrate, and cellulose xanthate with many other uses and applications.

HOW IT IS MADE

Cellulose is synthesized in plants and some microorganisms through the process known as photosynthesis. In that process, carbon dioxide (CO2) and water (H2O) are combined in a complex series of reactions to produce glucose (C6H10O5) and oxygen (O2). Glucose molecules are then linked to each other to from successively larger and more complex molecules, eventually resulting in the formation of cellulose.

Commercially, most cellulose is extracted from wood by one of two methods, the kraft (sulfate) process or the steam explosion process. The product of these reactions is wood pulp, which consists primarily of cellulose. In the kraft process, wood chips are treated with a solution of sodium hydroxide (NaOH) and sodium sulfide (Na2S) at temperatures of about 175°C (350°F) for two to six hours. This process usually results in a yield of about 40 to 45 percent wood pulp. The pulp is then treated with a bleaching agent, such as calcium or sodium hypochlorite (Ca(OCl)2 or NaClO) or chlorine dioxide (ClO2) to remove the color of lignin and other impurities.

Interesting Facts

  • Scientists estimate that plants worldwide synthesize up to one trillion metric tons of cellulose annually.

In the steam explosion process, wood chips are saturated with moisture and then exposed to temperatures of 200°C to 250°C (400°F to 500°F) at pressures of one to five atmospheres. In this process, cellulose fibers are physically separated from lignin fibers, which are the other major constituent of wood. In either process, yields of up to 99 percent pure cellulose can be obtained.

COMMON USES AND POTENTIAL HAZARDS

The number of products made from pure cellulose is almost endless. The largest volume of such products include paper and paper products, including (with 2001 production numbers): newsprint (5.8 million metric tons; 6.4 million short tons), printing and writing paper (22.1 million metric tons; 24.4 million short tons), packaging and related uses (3.9 million metric tons; 4.3 million short tons), tissue paper (6.4 million metric tons; 7.0 million short tons), containerboard (26.6 million metric tons; 29.3 million short tons), and boxboard (7.7 million metric tons; 8.5 million short tons). Other uses of cellulose include:

  • Manufacture of cotton products, such as items of clothing, sheeting, and industrial fabrics;
  • Production of other organic products, especially ethanol (ethyl alcohol; grain alcohol) and methanol (methyl alcohol; wood alcohol);
  • As insulation and soundproofing;
  • As a food additive, where it is used to thicken and add bulk to food products;
  • In equipment used in analytical chemistry, such as chromatographic devices used to separate the components of a mixture.

Words to Know

POLYMER
A compound consisting of very large molecules made of one or two small repeated units called monomers.
SYNTHESIS
A chemical reaction in which some desired chemical product is made from simple beginning chemicals, or reactants.

A major industrial use of cellulose is in the preparation of various cellulose derivatives, primarily cellulose acetate, cellulose nitrate, and cellulose xanthate, each of which has a number of applications.

FOR FURTHER INFORMATION

"Cellulose." Fibersource. http://www.fibersource.com/f-tutor/cellulose.htm (accessed on September 30, 2005).

"Cellulose." London South Bank University. http://www.lsbu.ac.uk/water/hycel.html (accessed on September 30, 2005).

"Cellulose Processing." Organic Materials Review Institute. http://www.omri.org/cellulose_final.pdf (accessed on September 30, 2005).

Cross, Charles, Clayton Beadle, Edward Bevan. Cellulose. An Out line of the Chemistry of the Structural Elements of Plants, with Reference to Their Natural History and Industrial Uses. Elibron. Replica of 1895 edition by Longmans, Green, and Co., London. http://www.elibron.com/english/other/item_detail.phtml?msg_id=10006054 (accessed on September 30, 2005).

See AlsoCellulose Acetate; Cellulose Nitrate; Cellulose Xanthate

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