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Glucose Carbohydrate

Glucose is the most common simple carbohydrate.

Image credit: Steven Berg


Carbohydrates are one of the four major classes of organic compounds in living cells. They are produced during photosynthesis and are the main sources of energy for plants and animals. The term carbohydrate is used when referring to a saccharide or sugar and its derivatives. Carbohydrates can be simple sugars or monosaccharides, double sugars or disaccharides, composed of a few sugars or oligosaccharides, or composed of many sugars or polysaccharides.

Carbohydrates: Monosaccharides

A monosaccharide or simple sugar has a formula that is some multiple of CH2O. For instance, glucose (the most common monosaccharide) has a formula of C6H12O6. Glucose is typical of the structure of monosaccharides. Hydroxyl groups (-OH) are attached to all carbons except one. The carbon without an attached hydroxyl group is double-bonded to an oxygen to form what is known as a carbonyl group. The location of this group determines whether or not a sugar is known as a ketone or an aldehyde sugar. If the group is not terminal then the sugar is known as a ketone. If the group is at the end, it is known as an aldehyde. Glucose is an important energy source in living organisms. During cellular respiration, the breakdown of glucose occurs in order to release its stored energy.

Carbohydrates: Disaccharides

Two monosaccharides joined together by a glycosidic linkage is called a double sugar or disaccharide. The most common disaccharide is sucrose. It is composed of glucose and fructose. Sucrose is commonly used by plants to transport sugar from one part of the plant to another. Disaccharides are also oligosaccharides. An oligosaccharide consists of a small number of monosaccaharide units (from about two to ten) joined together. Oligosaccharides are found in cell membranes and assist other membrane structures called glycolipids in cell recognition.

Carbohydrates: Polysaccharides

Polysaccharides can be composed of hundreds to thousands of monosaccharides combined together. These monosaccharides are joined together through dehydration synthesis. Some examples of polysaccharides include starch, cellulose and glycogen. Polysaccharides have several functions including structural support and storage.

Organic Polymers

For information on other types of biological polymers, see:
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