Introduction to Carbohydrates

James Richard Fromm


Four classes of compounds are essential to living organisms. They are the carbohydrates, and in particular the sugars; the fats or lipids; the amino acids and their polymers the proteins; and the nitrogenous bases, which together with pentose sugars and phosphates make up nucleosides and nucleotides. These classes of compounds and their reactions are the subject matter of the chemistry of life--biochemistry.

Carbohydrates are those organic compounds which have the empirical formula CnH2nOn, or (CH2O)n, from which they became known as "hydrates of carbon", carbohydrates. In living organisms, carbohydrates serve several different important functions including structural materials such as cellulose and energy sources such as starch. Carbohydrates are the primary products of photosynthesis. The simplified light-driven reaction of photosynthesis is

nH2O + nCO2 -[CH2O]n- + nO2.

The product carbohydrates are used in plant structures or, using the reverse reaction to photosynthesis, consumed as fuel by plants and animals.

Monosaccharides

In more complex carbohydrates the number of carbons and the number of water molecules in the empirical formula may differ, but in the simpler sugars or saccharides the number is the same. The names sugar and saccharide reflect the sweet taste of these molecules, which are actually short hydrocarbon chains with an aldehyde group and several hydroxyl groups. Sugars are classified by the number of carbon atoms in their hydrocarbon chain. This number may vary from three to seven or more, but the most common and the most significant sugars are the pentoses, which have five carbons, and the hexoses, which have six carbons. Sugars contain asymmetric carbon atoms and are optically active. The form found in living organisms is the D (dextro) or right-handed isomer. Dashed lines indicate the location of cyclization to ring structure which occurs with loss of a water molecule. However, in aqueous solution the monosaccharides exist primarily as closed ring structures.

Different ring structures are possible for the same monosaccharide. These structures are distinguished by Greek letters such as alpha and beta. The most common monosaccharide is the hexose glucose, C6H12O6, which is also known as blood sugar and grape sugar. Other common hexoses include galactose, fructose, and mannose. The most significant pentoses are ribose and 2-deoxyribose, which have a crucial role in the chemistry of nucleotides.


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Copyright 1997 James R. Fromm