In an average human diet, carbohydrates make up approximately half of the caloric intake and serves as the major energy source. About half of these carbohydrates come from foods such as cereals and vegetables and are in forms of polysaccharides such as starch or dextrins. The other half are consumed in the form of simple sugars, including but not limited to sucrose, lactose, maltose and glucose. Atoms in all these compounds are arranged in such a way that they all can be considered to be hydrates of carbon, thus giving them the name of carbohydrates. There are currently three major classes of carbohydrates, and they are the monosaccharides, the oligosaccharides and the polysaccharides.
Essentially the simplest form of carbohydrates, monosaccharides cannot be reduced any further by hydrolysis, and thus it should not be surprising that they are grouped as simple sugars. Among all monosaccharides, glucose, a six carbon sugar, is the most abundant and nutritionally most important.
Monosaccharides can each contain three to seven carbon atoms and they not only occur in nature, but can also be formed as intermediate products in digestion. Each monosaccharide molecule involves a hydroxyl group as well as a functional carbonyl group that could be an aldehyde or a ketone. Therefore, they can be further subdivided into two groups, sugars that have an aldehyde group are aldoses, while sugars that have a ketone group are ketoses.
A monosaccharide is classified by which of these two groups it possess as well as the number of carbon molecules it has. For example, a four carbon sugar with a ketone group would be called a ketotetrose while a five carbon sugar with an aldehyde group would be called an aldohexose.
Oligosaccharides are also simple sugars that are generally of 2 to about 10 sugar units in length. The monosaccharides are attached to each other by acetal bonds, or otherwise known as glycosidic bonds. The bond is formed between the hydroxyl group of one monosaccharide and the next in the polymer, and one water molecule is eliminated in the process.
Among oligosaccharides, disaccharides, composing of only 2 sugar units, are the most common. Sucrose, made up of glucose and fructose is the most nutritionally important as it makes up about a third of the total carbohydrate intake in an average diet. It comes from cane sugar and beet sugar and is common used as natural sweeteners. Sucrose is not a reducing sugar.
Other examples of disaccharides include maltose and lactose. Maltose is found in malt beverages including beers and malt liquors and is usually formed from the partial hydrolysis of starch. Lactose, on the other hand, is found naturally in milk and milk products.
They are long chains that can contain several to hundreds and thousands of monosaccharides. For animals, glycogen is the most important while for plants, starch and cellulose are the most important. Glycogen and starch are respectively the major storage forms of carbohydrates in plants and animal tissues.
Polysaccharides are high molecular weight polymers. The structure is called a homopolysaccharide if it is only made up of a single type of monomers and is called a heteropolysaccharide if it involves two or more types of monosaccharides. Among the two, homopolysaccharides are much more nutritionally important as they are abundant in many natural foods.
A polysaccharide molecule can weight from a few thousand to about 500,000 in molecular weight. A linear polysaccharide has a reducing and a non reducing end. This is what designates at which end of the polymer that certain enzymatic reactions occur.