Biochemistry. Biochemistry is the study of the chemical substances found in living systems and the chemical interactions of these substances with each other.

Carbohydrates. Carbohydrates are polyhydroxy aldehydes, polyhydroxy ketones, or compounds that yield such substances upon hydrolysis. Plants contain large quantities of carbohydrates produced via photosynthesis.

Carbohydrate classification. Carbohydrates are classified into three groups: monosaccharides, oligosaccharides, and polysaccharides.

Classification of monosaccharides. Monosaccharides are classified as aldoses or ketoses on the basis of the type of carbonyl group present. They are further classified as trioses, tetroses, pentoses, etc. on the basis of the number of carbon atoms present.

Handedness in monosaccharides. Most monosaccharides exhibit handedness - that is, they exist in a "left-handed" form (L form) and a "right-handed" form (D form). Handedness requires a chiral center, an atom that has four different groups tetrahedrally bonded to it. Naturally occurring monosaccharides are "right-handed" molecules.

Important monosaccharides. Important monosaccharides include glucose, galactose, fructose, and ribose. Glucose and galactose are aldohexoses, fructose is a ketohexose, and ribose is an aldopentose.

Cyclic monosaccharides. Cyclic monosaccharides form through an intramolecular reaction between the carbonyl group and an alcohol group of an open-chain monosaccharide. These cyclic forms predominate in solution.

Reactions of monosaccharides. Four important reactions of monosaccharides are (1) oxidation to a polyhydroxy acid, (2) reduction to a polyhydroxy alcohol, (3) glycoside formation, and (4) phosphate ester formation.

Disaccharides. Disaccharides are glycosides formed from the linking together of two monosaccharides. The most important disaccharides are maltose, lactose, and sucrose. Each of these has at least one glucose unit in its structure.

Polysaccharides. Polysaccharides are polymers of monosaccharides. Cellulose and starch are polymers of glucose, but the linkages between glucose units are different. The a linkages in starch are easily broken by enzymatic hydrolysis in humans, thus providing great sources of dietary energy. However, the b linkages in cellulose prevent enzymatic hydrolysis in humans, so cellulose provides no usable energy in our diets. Glycogen is a highly branched glucose polymer utilized for energy storage within the body.