Carbohydrates are named for their characteristic content of carbon, hydrogen, and oxygen (CH2O). Short chains containing from three to seven carbons form the monosaccharides, the individual building blocks of carbohydrates. Of these, trioses, pentoses and hexoses are most common in cells. All monocsaccharides occur in linear form and each carbon atom in the chain, except one, carries an -OH group. The remaining carbon carries a -C=O (carbonyl) group. In monosaccharides all other available binding sites of carbon are occupied by hydrogen atoms. The cabonyl oxygen of a linear sugar may be located at the end of the carbon chain as an aldeyde group or inside the chain as a keton group.
Monosaccharides with five or more carbons can form a ring as well as a linear configuration. The rings form through a reaction between two functional groups in the same molecule. In the six-carbon monosaccharide (glucose), a covalent bond can form through a reaction between the aldehyde at the 1-carbon and the hydroxyl at the 5-carbon. The reaction produces either of the two closely related glucopyranose ring structures. The aldehyde at the 1-carbon can also react with the hydroxyl at the 4-carbon to produce a glucofuranose ring. Monosaccharide rings such as that of glucose are frequently depicted as a Haworth projection a diagram that suggests the three-dimension orientation of the attached -H, OH, and -C2OH groups in relation to the ring. However, the glucopyranose ring does not actually lie in a flat plane as suggested by this projection. Instead, the ends of the ring are bent up or down, most frequently in the "chair" conformation. More over, the side groups attached to the ring extend at the various angles, not at right angles as depicted in the Haworth projection.
