3.4: What are Carbohydrates?
Carbohydrates are essential biological molecules made of carbon, hydrogen, and oxygen atoms, often in the ratio of 1:2:1. They occur as simple or complex structures and are essential for energy metabolism and storage.
Naming Convention of Carbohydrates
All carbohydrates are sugars, also called saccharides. However, depending on their length and complexity carbohydrates can be classified as monosaccharides, disaccharides, and polysaccharides. Monosaccharides are also called simple sugars. Polysaccharides are referred to as complex carbohydrates. They are polymers, as they are built from repeating units of simple sugars.
One of the simplest sugars is glucose. It is made of six carbons, 12 hydrogens, and six oxygens (i.e., C6H12O6). Glucose has a single sugar unit and is, therefore, a monosaccharide. Even such a simple molecule has several variants (isomers), depending on the orientation of individual atoms in space. For instance, if the hydroxyl (-OH) group on carbon number five points to the right, we speak of D-Glucose, if it points to the left, it is L-Glucose. The two molecules are enantiomers, mirror images of each other.
The representation of a molecule as a ring structure is called Haworth projection. It reveals another option to arrange the atoms in a glucose molecule. Identify the carbon that previously carried the carboxyl group (1 in glucose, 2 in fructose). If the hydroxyl group on that carbon points down, it is said to be the α-form. If the hydroxyl group points up, it is the β-form.
Monosaccharides are also classified based on the number of carbons. For instance, pentoses have five carbon atoms and hexoses six. Furthermore, monosaccharides are classified by the placement of their carbonyl group (a carbon-oxygen double bond). An aldose has a single terminal aldehyde group (-CH=O) whereas a ketose has a single carbonyl group positioned in the middle of the molecule.
These different classification systems and naming conventions can be combined. For instance, fructose is a ketohexose — a sugar with five carbons, and the carboxyl group is located at a carbon that is not at the end of the molecule.
A disaccharide forms when two monosaccharides linked by dehydration synthesis. A common disaccharide is sucrose. It consists of two monosaccharides, α-glucose and β-fructose. Sucrose is the ordinary household sugar, usually derived from sugar cane or sugar beets.
When more than two monosaccharides link, it forms a polysaccharide. Cellulose is a common polysaccharide that is built from glucose monomers. It is insoluble and the building block of cell walls and fibers in plants. Your cotton t-shirt is made of sugar!