Carbohydrates

The word monosaccharide is derived from mono, meaning "one", and saccharide, meaning "sugar". The common monosaccharides are glucose, fructose, and galactose. Each simple sugar has a cyclic structure and is composed of carbon, hydrogen and oxygen in ratios of 1:2:1 respectively. Although each sugar mainly exists as a cyclic compound, it is important to note that they are all in equilibrium to a small extent with their linear forms. While galactose and glucose are composed of six-membered rings, fructose has only five carbon atoms bonded to each other in ring form.

Glucose is the main sugar metabolized by the body for energy. The D-isomer of glucose predominates in nature and it is for this reason that the enzymes in our body have adapted to binding this form only. Since it is an important energy source, the concentration of glucose in the bloodstream usually falls within a narrow range of 70 to 115mg/100 ml of blood. Sources of glucose include starch, the major storage form of carbohydrate in plants.

Galactose is nearly identical to glucose in structure except for one hydroxyl group on carbon atom number four of the six-sided sugar. Since it differs in only one position about all six asymmetric centers in the linear form of the sugar, galactose is known as an epimer of glucose. Galactose is not normally found in nature in large quantities, however it combines with glucose to form lactose in milk. After being absorbed by the body, galactose is converted into glucose by the liver so that it can be used to provide energy for the body. Both galactose and glucose are very stable in solution because they are able to adopt chair and boat conformations. These conformations are most stable because their OH groups are pointed away from the structure, preventing steric hindrance.

Fructose is a structural isomer of glucose, meaning it has the same chemical ormula but a completely different three-dimensional structure. The main difference is that fructose is a ketone in its linear form while glucose is an aldehyde. Through an intramolecular addition reaction with the C-5 OH group, glucose forms a six-membered ring while fructose forms a five-membered ring as seen in Figure 1. Upon consumption, fructose is absorbed and converted into glucose by the liver in the same manner as lactose. Sources of fructose include fruit, honey and high-fructose corn syrup.

Disaccharides, meaning "two sugars", are commonly found in nature as sucrose, lactose and maltose. They are formed by a condensation reaction where one molecule of water condenses or is released during the joining of two monosaccharides. The type of bond that is formed between the two sugars is called a glycosidic bond.

Lactose is a disaccharide formed through the condensation of glucose and galactose. The bond formed between the two monosaccharides is called a beta glycosidic bond (). The alpha glycosidic bond, found in sucrose and maltose, differs from the beta glycosidic bond only in the angle of formation between the two sugars. Unfortunately, unlike alpha glycosidic bonds, beta-glycosidic bonds are unable to be digested by some people. Therefore, many people are lactose intolerant and suffer from intestinal cramping and bloating due to the incomplete digestion of the substance.

Sucrose is found in common table sugar and is composed of glucose and fructose linked via a 1-2 alpha glycosidic bond. Sucrose is an excellent preservative because it has no "reducing end" or reactive group like the other sugars. Because glucose is joined to the carbon atom labeled number two on fructose, neither monosaccharide is able to open or react with other compounds in solution. It is for this reason that sucrose is an excellent natural preservative and is found in many jarred foods including jams. Other natural sources of sucrose are found in plants such as sugar cane, sugar beets, and maple syrup.

Maltose is the final disaccharide and consists of two glucose molecules joined by an alpha glycosidic bond. Maltose is an interesting compound because of its use in alcohol production. Through a process called fermentation, glucose, maltose and other sugars are converted to ethanol by yeast cells in the absence of oxygen. Through an analogous process, muscle cells convert glucose into lactic acid to obtain energy while the body operates under anaerobic conditions. Although maltose is uncommon in nature, it can be formed through the breakdown of starch by the enzymes of the mouth.

Carbohydrates that contain more than two simple sugars are called oligosaccharides or polysaccharides, depending upon the length of the structure. Oligosaccharides usually have between three and ten sugar units while polysaccharides can have more than three thousand units. These large structures are responsible for the storage of glucose and other sugars in plants and animals.

Important oligosaccharides are raffinose and stachyose. Composed of repeating units of galactose, glucose and fructose, these oligosaccharides are of nutritional importance because they are found in beans and legumes. Because of their unique glycosidic bonds, raffinose and stachyose cannot be broken down into their simple sugars. Therefore, they cannot be absorbed by the small intestine and are often metabolized by bacteria in the large intestine to form unwanted gaseous byproducts. Commercial enzyme preparations such as Beano can be consumed before a meal rich in beans and legumes in order to aid the small intestine in the breakdown of these oligosaccharides.

Polysaccharides or complex carbohydrates are usually monomers and consist of thousands of repeating glucose units. Naturally, they allow for the storage of large quantities of glucose. Starch is the major storage form of carbohydrate in plants and has two different types: amylose and amylopectin. Although digestible alpha glycoisidic bonds link both types of starch, each type is unique in their branching of glucose. While amylose is a straight chain polymer, amylopectin is highly branched. These differences account for the fact that amylopectin can form stable starch gels which are able to retain water while amylose is unable to do so. Therefore, amylopectin is often used by manufacturers to produce many different kinds of thick sauces and gravies. Sources of starch include potatoes, beans, bread, pasta, rice and other bread products.

Like amylopectin, glycogen is a highly branched polymer of glucose that is the main storage form of carbohydrate in humans. The main chain of the structure is composed of alpha 1, 4 glycosidic bonds, while alpha 1,6 glycosidic bonds give rise to the branch points of the polymer (figure 5). Glycogen is stored in the liver and muscle where it is synthesized and degraded depending upon the energy requirements of the body.

Indigestible forms of polysaccharides are known as dietary fiber and come in many different forms including cellulose, hemicellulose, pectin, gum and mucilage. Cellulose is by far the most abundant biochemical compound on the earth because it forms part of the structure of many plants. It is unique among polysaccharides in that it forms intramolecular hydrogen bonds between adjacent glucose units as well as beta 1,4 glycosidic bonds present in other carbohydrates. These special bonding characteristics allow cellulose to form long, straight chains of glucose and give it strength and rigidity that many plants require for proper growth. Cellulose and most forms of hemicellulose are insoluble fibers while pectin, gum and mucilage are all soluble fibers and readily dissolve or swell when mixed with water.

Other sugars of importance are found in nucleotides such as deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Both RNA and DNA are five sided cyclic sugars; however, RNA has one more hydroxyl group than DNA. Glucose-6-phosphate, an intermediate in the breakdown of glucose for energy, can be used for the synthesis of these compounds.