Carbohydrates were previously thought of as only providing energy. However, it is now known that carbohydrates are involved in many other biochemical processes. Some examples of these processes are intercellular recognition, infection processes, and certain types of cancer.
Part of the difficulty of determining structures for carbohydrates is that the units that they are built from can be connected in many different ways. For example, there are over two-hundred structural possibilities from three connected glucose units.
The monosaccharides traditionally referred to a class of polyhydroxylcarbonyl compounds that had the empirical formula CH2O. This definition has been expanded to include compounds that may contain certain oxidized, reduced, or heteroatom-substituted groups. Carboydrates are built from these monosaccharide units. A sugar may refer to a monosaccharide or to small compounds with more than one monosaccharide. A polysaccharide contains many monosaccharide units.
The suffix -ose indicates a sugar. Hexoses are six-carbon sugars and pentoses are five-carbon sugars. Many monosaccharides, such as glucose, may be in equilibrium between their acyclic and cyclic form. From their acyclic form the terms aldose and ketose are derived. An aldose will contain an aldeyde and a ketose contains a ketone. The monosaccharide may also be referred to its cyclic form. If the ring has five carbons and one oxygen, it is a pyranose. If it has four carbons and one oxygen, it is a furanose.