Monosaccharides

Two Ways to Classify a Monosaccharide

Within the monosaccharide group itself, a further split is made along two independent axes: how many carbon atoms the molecule has, and which carbonyl functional group it carries.

By carbon count

Monosaccharides are named according to their carbon-chain length using the standard "-ose" family of terms — a 3-carbon sugar is a triose, a 4-carbon sugar a tetrose, a 5-carbon sugar a pentose, a 6-carbon sugar a hexose, and a 7-carbon sugar a heptose.

By carbonyl group

A monosaccharide's carbonyl group is either an aldehyde or a ketone:

• If it contains an aldehyde group, it is called an aldose.
• If it contains a keto group, it is called a ketose.

Combining both labels

The two classifications are used together, so a monosaccharide's full descriptive name states its chain length and its carbonyl type in one word — for instance, a six-carbon sugar with an aldehyde group is an aldohexose, while a six-carbon sugar with a ketone group is a ketohexose. Glucose (an aldohexose) and fructose (a ketohexose) are the two monosaccharides examined next, and both illustrate this naming directly. …