Overview

Overview

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!

Procedure

Carbohydrates, in their familiar form, include simple sugars and more complex polymers called polysaccharides. Molecularly, they are defined as containing carbon and the components of water, a hydroxyl group, hydrogen and oxygen. Usually in a one-two-one ratio like glucose which has six carbons, twelve hydrogens and six oxygens.

This single sugar molecule is a monosaccharide. Categorized in part by the number of carbons. With six, it's considered a hexose. While another common sugar ribose is called a pentose with five carbon molecules. And depending on the positioning of their carbonyl group they can be further categorized. An aldose like galactose has a carbonyl group at the end of the sugar.

And a ketose such as fructose has a carbonyl group in the middle.

Now a fructose molecule in its ring form as most sugars are in aqueous solutions can combine with a glucose molecule. And via dehydration synthesis, the two monosaccharides form a disaccharide like sucrose the common sweetener, table sugar. Instead, if glucose bonded together with many glucose molecules, different polysaccharides like cellulose or the starch amylose can be formed. Ultimately, simple sugars and polysaccharides serve as energy reserves and structural components of cells.

Instead, if glucose bonded together with many glucose molecules, different polysaccharides like cellulose or the starch amylose can be formed. Ultimately, simple sugars and polysaccharides serve as energy reserves and structural components of cells.