Carbohydrates

Carbohydrates are a major source of energy for life and are important strucutral molecules in many organisms. They are molecules that are made of oxygen, carbon and hydrogen, with the empirical formula for most carbohydrates being (CH₂O)n. All carbohydrates have at least one hydroxyl group (OH).

The most simple carbohydrates are monosaccharides. These are aldehydes or ketones with one or more hydroxyl groups. The smallest monossacharide is composed of three carbon atoms. These are called trioses. Other simple monosaccharides are tetroses (4 carbon), pentoses (5 carbon), hexoses (6 carbon), and heptoses (7 carbon).

Many common sugars exist in cyclic forms. This occurs when the aldehyde or ketone group of the carbohydrate reacts with one of its own hydroxyl group to form either a hemiacetal or hemiketal. A 6-membered (6 carbon) cyclic hemiacetal/hemiketal is called a pyranose. A 5-membered ring is called a furanose.

The most frequently occurring of these is glucose and is important in energy metabolism. When glucose is not immediately required it is synthesised into glycogen. This is known as the Glycogen Synthase Reaction and it involves glucose donating a glucosyl residue to the non-reducing end of a glycogen branch.

Disaccharides are carbohydrates made up of two monosaccharides which are joined by an O-glycosidic bond. Carbohydrates can increase in size more by further addition of monosaccharides, complex carbohydrates containing more than one molecule are called oligosachharides .