A carbohydrate is an organic compound that is composed of atoms of carbon, hydrogen and oxygen in a ratio of 1 carbon atom, 2 hydrogen atoms, and 1 oxygen atom. Some carbohydrates are relatively small molecules, the most important to us is glucose which has 6 carbon atoms. These simple sugars are called monosaccharides.

The primary function of carbohydrates is for short-term energy storage (sugars are for Energy). A secondary function is intermediate-term energy storage (as in starch for plants and glycogen for animals). Other carbohydrates are involved as structural components in cells, such as cellulose which is found in the cell walls of plants.

Two common Monosaccharides, (single sugars) Glucose and Fructose

Hooking two monosaccharides together forms a more complex sugar, such as the union of glucose and fructose to give sucrose, or common table sugar. Compounds such as sucrose are called Disaccharides (two sugars). Both monosaccharides and disaccharides are soluble in water.

Larger, more complex carbohydrates are formed by linking shorter units together to form long or very long sugar chains called Polysaccharides. Because of their size, these are often times not soluble in water. Many biologically important compounds such as starches and cellulose are Polysaccharides. Starches are used by plants, and glycogen by animals, to store energy in their numerous carbon-hydrogen bonds, while cellulose is an important compound that adds strength and stiffness to a plant's cell wall.

Sugars are most often found in the form of a "RING". The glucose molecule in the image above and the one in the image below (Glc) are really the same molecule, just arranged differently. The corners of the "stop sign" represent Carbon atoms even thought they are not labeled with a "C" (its chemistry shorthand). To form these rings, the Carbonyl (C=0) Carbon of the straight-chain form (above) forms a bond with the next to last Carbon in the chain, making the ring.

The image on the left shows two monosaccharides, Glucose and Galactose (Gal). Examine their structure and you will notice there is very little difference. Their molecular formulas, C6H1206, are even the same. Molecules with the same chemical formula, but different molecular structures are called Isomers.

The sugar subunits can be linked by the reaction, dehydration synthesis, to form larger molecules. The disaccharide, Sucrose, is formed from two monosaccharides, Glucose and Fructose.

The disaccharide Lactose is a dimer (two subunits) of Glucose and Galactose, the disaccharide Maltose is a dimer of Glucose.

Large polymers of sugars are called Carbohydrates. Carbohydrates can be 100's of sugars long and either straight or branched. The term Complex Carbohydrate, or sometimes even just Carbohydrate refers to long chains of sugars. Three common types of complex carbo's we will examine are: Starch, Cellulose, and Glycogen. All three are composed only of Glucose. They differ only in the bonding arrangements between the Glucose subunits. Not all complex carbs are composed of glucose alone, many have highly unusual sugars in their chains.

Starch is a long (100's) polymer of Glucose molecules, where all the sugars are oriented in the same direction. Starch is one of the primary sources of calories for humans.

Cellulose is a long (100's) polymer of Glucose molecules. However the orientation of the sugars is a little different. In Cellulose, every other sugar molecule is "upside-down". This small difference in structure makes a big difference in the way we use this molecule.

Glycogen is another Glucose polymer. Glycogen is a stored energy source, found in the Liver and muscles of Humans. Glycogen is different from both Starch and Cellulose in that the Glucose chain is branched or "forked".

More Carbohydrate structure and function