Carbohydrates

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Carbohydrates are a diverse group of compounds composed of the elements carbon (C), hydrogen (H), and oxygen (O) with the empirical formula [CH2O]n, where n represents the number of CH2O units in the compound. The ultimate source of all carbohydrates is photosynthesis, in which the energy of sunlight is used to chemically fix atmospheric CO2 into carbohydrate. Carbohydrates constitute as much as 80 percent of the dry weight of a plant. This is largely due to the presence of cell walls made of complex carbohydrates surrounding each plant cell.

Sugars, also called saccharides, are carbohydrates. Common sugars occurring in nature have from three (glyceraldehyde) to seven (sedoheptu-

lose) carbon atoms bonded together to form the molecule's backbone. Sugars can be classified chemically as either aldehydes or ketones, and contain OH (hydroxyl) groups attached to their carbon backbones. Glucose (an aldehyde sugar) and fructose (a ketose sugar) are two examples of common six-carbon sugars that occur widely in plants. These molecules form ring structures, with glucose forming a six-member ring, and fructose a five-member ring.

ch2oh

H OH D-glucose

OH H D-fructose ch2oh

Individual sugars are called monosaccharides. Two sugars linked together chemically are called disaccharides; three are trisaccharides; four are tetrasaccharides, and so on. A generic term for several sugars linked together is oligosaccharide. Molecules with many sugars linked together to form a polymer are called polysaccharides. Sucrose (table sugar) is the most abundant disaccharide and is comprised of glucose and fructose. A large amount of sucrose is formed in plant leaves from photosynthetic products. Sucrose is translocated over long distances in plants in specialized conductive tissue known as phloem. Sucrose can be broken down, and its components (glucose and fructose) metabolized for energy and as the initial raw material for building cellular components in plant cells distant from pho-tosynthetic leaves. Sucrose also can be stored. High concentrations are found in storage organs such as fruits, sugarcane stems, and enlarged roots of sugar beets. The latter two are commercial sources from which sucrose is refined.

hydroxy! the chemical group -OH

polymer a large molecule made from many similar parts translocate to move, especially to move sugars from the leaf to other parts of the plant polymer a large molecule made from many similar parts translocate to move, especially to move sugars from the leaf to other parts of the plant

CH2OH

CH2OH

ch2oh ch2oh

OH OH

Sucrose

ch2oh

OH OH

Sucrose

Two important plant polysaccharides, cellulose and starch, are composed exclusively of glucose units bonded together. Cellulose is the most abundant polysaccharide in plant cell walls, and thus, the most abundant polysaccharide on Earth. In cell walls, cellulose occurs along with other complex polysaccharides, each of which is composed of more than one type of sugar. Because of the way glucose is linked in cellulose, individual chains, hundreds of glucose molecules long, are able to bond together by hydrogen bonding in a crystalline arrangement to form a cable-like structure known as a microfibril. These microfibrils are interwoven

and give cellulose its strength in plant cell walls and in cotton fabric and paper.

ch2oh ch2oh ch2oh ch2oh ch2oh ch2oh

Cellulose

ch2oh -o

Cellulose

Starch and cellulose differ in the way the glucose molecules are bonded together. Starch is a storage compound in plants, and is broken down as needed into glucose for metabolic use.

Treatment of denim with cellu-lase has largely replaced pumice stone in the production of "stone-washed" jeans.

enzyme a protein that controls a reaction in a cell

CH2OH

ch2oh ch2oh ch2oh

CH2OH

ch2oh ch2oh ch2oh

Starch

Starch

Carbohydrates are important in human nutrition, often constituting the major source of calories in the diet. Glucose, fructose, sucrose, and starch are readily digested by humans. Cellulose and the other complex carbohydrates of the plant cell wall are not readily digested, but constitute useful dietary fiber.

Cellulase, an enzyme that can degrade cellulose, is made by many different fungi and some microorganisms. This enzyme allows these organisms to decompose plant material, an important step in the recycling of materials in food webs. see also Photosynthesis, Carbon Fixation and; Sugar.

D. Mason Pharr and John D. Williamson

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