Carbohydrates; As the name indicates, they consist of a Carbon atom (Carbo-) attached to a Hydrogen and an Oxygen atom in the ratio of 2:1, similar to water H 2 O (-hydrate from hydra in latin meaning water). They have a general chemical formula of (CH 2 O) n where n is usually any number ranging from 2 onwards.

The most popular carbohydrates have n= 3 (triose) or 5 (pentose) or 6(hexose). Since most carbohydrates are sweet and sugary, sugar nomenclature end with “ose.”

They can be classified as single unit sugars which are calledmonosaccharides (mono means one, saccharide means sugar), ordisaccharide (“di” means two therefore two unit sugars joined together), or oligosaccharides (3 to 50 unit sugars joined together) or polysaccharides (“poly” means many and it is above 50 units of sugar joined together). The bond that holds the saccharides together to form carbohydrates is called glycosidic bond and is formed by the loss of a water molecule when two carbohydrates come together and are subsequently joined by the oxygen atom of one of the two saccharides molecules.

Glycosidic bond being formed by the proximity of two monosaccharides.

Carbohydrates are usually used as a food source since sugars are used to convert into energy . Example: Glucose (C 6 H 12 O 6 ) is a monosaccharide or single unit sugar and is a common source of energy for the body. However, glucose is generally stored as an aggregated giant molecule starch in plants or glycogen in animals. They are polysaccharides or polymers (macromolecules). Actually “poly” means many, and “mer” means molecules therefore it means many molecules.

Three important disaccharides are maltose, lactose, and sucrose which are used as fodder to either make the storage macromolecules or to break-down into monosaccharides for converting the sugar into energy.

In addition to it’s role as energy storage, carbohydrates are also used in plant cell wall in the form of the polysaccharide cellulose to give the cell structure, and is an important signal receptor on the plasma membrane of cells where the signal will induce the cell to perform specific functions. This is done when oligosaccharides linked to proteins on the plasma membrane work as signal receptors or markers for cell recognition and interaction.

All monosaccharides contain hydroxyl groups (OH) and are therefore alcohols. Sugar structures can be represented in many ways. Example: Ribose (the most common 5-carbon sugar can be shown as a linear molecule containing 4 hydroxyl groups and one aldehyde group.

Ribose- Fischer Projection.

This linear representation is called a Fischer projection. In its usual bio chemical form, however, the structural ribose is a ring with a covalent bond between the carbon of the aldehyde group (C-1) and the oxygen of the (C-4) hydroxyl group. This ring form is known as Haworth projection. The ring is not actually flat, it can adopt about twenty different conformations in which certain ring atoms are out-of-plane.

Ribose- Haworth’s Projection. Click on image for credit.

Another example is Glucose, which is the most abundant 6-Carbon sugar. (Insert Diagram) It is the monomer of the polysaccharide Cellulose and the storage polysacchride Glycogen and Starch.

Formation of Glycogen from Glucose. Click on image for credit.

In these polysacchrides, each glucose residue is joined covalently to the next bio covalent bond between C-1 of one glucose molecule and a hydroxyl group of another. This bond is called the glycosidic bond. In cellulose, C-1 of each glucose residue is joined to the C-4 hydroxyl group of the next residue. The hydroxyl groups on adjacent chains of cellulose interact non-covalently, creating strong insoluble fibers.