ho—C—H h-c-oh h-c-oh ch2oh d(+)-glucose d(+)-galactose d(+)-ribose d(-)- fructose
One end of monosaccharides such as glucose reacts spontaneously with either the other end or the adjacent carbon to form ring structures. For example, glucose can form a six-membered ring, and fructose, a five-membered ring.
The rings are closed by an oxygen atom. The ring form and the open-chain form freely interconvert. However, for glucose, for example, the equilibrium highly favors the ring form.
A number of monosaccharide derivatives are important. Conversion of ends to —COOH groups produces the sugar acids, such as glucuronic acid. Fermentation may result in the production of sugar acids as intermediates. Amino sugars are formed by replacing one of the hydroxyl groups on certain monosaccharides with a nitrogen-containing amino group (see below under proteins). Another derivative is the deoxy sugar, formed by replacing one of the hydroxy groups with a hydrogen; thus, one of the carbons will have two hydrogens. A very important deoxy sugar is deoxyribose, an important component of DNA.
Monosaccharides can also form a special type of bond with each other or with other molecules. This glycosidic bond forms between a hydroxide on the monosaccharide and a hydroxide on another molecule, which may be another monosaccharide, with the elimination of one molecule of water. Again, the link will be through an oxygen atom.
If the other molecule is another monosaccharide, the result is a disaccharide. Familiar table sugar is sucrose, a disaccharide formed from the monosaccharides glucose and fructose. Lactose, the sugar in milk, is formed from glucose and galactose. The same two monosaccharides can form disaccharides in several ways, depending on orientation and location of the connection. For example, maltose and cellobiose are both made from two glucose residues.
Glycosidic bonds can form longer-chain carbohydrates called oligosaccharides, those with only a few monosaccharides, which include the disaccharides, and the much longer-chain polysaccharides. Starch is a type of polysaccharide produced by plants for energy storage. Humans obtain most of their dietary carbohydrate in the form of starch from grain. Starch is a chain of glucose residues bonded as in maltose. The chain may be straight, as in amylose, which makes up about 20% of potato starch; or it may be branched, as in amylopectin, which forms the other 80% of potato starch.
Animals store carbohydrates in a polysaccharide called glycogen. Glycogen is similar to starch except that the chain is much more highly branched. It is thought that by having more "ends" to the glycogen molecule, it is more available for rapid conversion to glucose for the sudden energy demands of animals. Depletion of glycogen in the muscle may cause the "wall" experienced by marathoners after several hours of running, which prevents them from continuing the race. With glycogen gone, the body switches to fat, which does not provide energy fast enough and produces other physiological stresses. In mammals, skeletal muscle contains about two-thirds of the body's glycogen, and the liver holds most of the rest. The liver uses the glycogen to control glucose levels in the blood. Figure 3.6a shows the basic structure common to both starch and glycogen.
Cellulose is an unbranched polysaccharide also composed exclusively of glucose residues, but with an important difference from starch. The glycosidic bond is reversed, as in the cellobiose disaccharide (see Figure 3.6b). This prevents the molecule from twisting, making it stiffer. As a result, it finds use as a structural material located in the cell walls of plants and forming the major component of wood. (It should be noted that the hardness of wood comes not from cellulose but from lignin, described below.) Another important fact about cellulose is that only a very few animals (such as garden snails) can digest it. Most cannot break it down to glucose to make the stored energy available. However, several animals, such as termites and cows, have developed associations with microorganisms that live in their digestive systems. The microorganisms accomplish the digestion for the animals.
Another structural polysaccharide is chitin, which forms the hard shell of arthropods such as insects and crabs, as well as the cell walls of most fungi. Chitin is a polymer of a sugar amine, N-acetyl glucosamine; crustaceans also include calcium carbonate in their shells. Chitin has been studied for use as an adsorption media for removing heavy metals from water. Other important structural polysaccharides include agar and carrageenan, which are extracted from seaweed. The former is used as a substrate for culturing bacteria, and the latter is used as a food thickener.
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