Carbohydrates are among the most abundant compounds on earth. They are normally broken down into five major classifications of carbohydrates:

  1. Monosaccharides
  2. Disaccharides
  3. Oligosaccharides
  4. Polysaccharides
  5. Nucleotides


The word monosaccharide is derived from mono, meaning "one", and saccharide, meaning "sugar". The common monosaccharides are glucose, fructose, and galactose. Each simple sugar has a cyclic structure and is composed of carbon, hydrogen and oxygen in ratios of 1:2:1 respectively. Although each sugar mainly exists as a cyclic compound, it is important to note that they are all in equilibrium to a small extent with their linear forms.

Figure %: Monosaccharides
While galactose and glucose are composed of six-membered rings, fructose has only five carbon atoms bonded to each other in ring form.


Glucose is the main sugar metabolized by the body for energy. The D-isomer of glucose predominates in nature and it is for this reason that the enzymes in our body have adapted to binding this form only. Since it is an important energy source, the concentration of glucose in the bloodstream usually falls within a narrow range of 70 to 115mg/100 ml of blood. Sources of glucose include starch, the major storage form of carbohydrate in plants.


Galactose is nearly identical to glucose in structure except for one hydroxyl group on carbon atom number four of the six-sided sugar. Since it differs in only one position about all six asymmetric centers in the linear form of the sugar, galactose is known as an epimer of glucose. Galactose is not normally found in nature in large quantities, however it combines with glucose to form lactose in milk. After being absorbed by the body, galactose is converted into glucose by the liver so that it can be used to provide energy for the body. Both galactose and glucose are very stable in solution because they are able to adopt chair and boat conformations.

Figure %: Chair and Boat Conformations
These conformations are most stable because their OH groups are pointed away from the structure, preventing steric hindrance.


Fructose is a structural isomer of glucose, meaning it has the same chemical ormula but a completely different three-dimensional structure. The main difference is that fructose is a ketone in its linear form while glucose is an aldehyde. Through an intramolecular addition reaction with the C-5 OH group, glucose forms a six-membered ring while fructose forms a five-membered ring as seen in Figure 1. Upon consumption, fructose is absorbed and converted into glucose by the liver in the same manner as lactose. Sources of fructose include fruit, honey and high-fructose corn syrup.


Disaccharides, meaning "two sugars", are commonly found in nature as sucrose, lactose and maltose. They are formed by a condensation reaction where one molecule of water condenses or is released during the joining of two monosaccharides. The type of bond that is formed between the two sugars is called a glycosidic bond.

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