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Starch Grains - Carbohydrates
This image shows starch grains (green) in the parenchyma of a Clematis sp. plant. Starch is synthesized from the carbohydrate sucrose, a sugar produced by the plant during photosynthesis, and used as a source of energy. It is stored as grains in structures called amyloplasts (yellow). STEVE GSCHMEISSNER/Science Photo Library/Getty Images


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. Polysaccharides have several functions including structural support and storage. Some examples of polysaccharides include starch, glycogen, cellulose, and chitin. Starch is a vital form of stored glucose in plants. Vegetables and grains are good sources of starch. In animals, glucose is stored as glycogen in the liver and muscles. Cellulose is a fibrous carbohydrate polymer that forms the cell walls of plants. Chitin is the tough polysaccharide that forms the exoskeleton of arthropods such as spiders, crustaceans, and insects.

Carbohydrates: Digestion

Carbohydrates in the foods we eat must be digested to extract the energy stored in food. As food travels through the digestive system, it is broken down allowing glucose to be absorbed into the blood. The circulatory system transports glucose to the cells and tissues of the body. The release of insulin by the pancreas allows glucose to be taken in by our cells to be used to produce energy through cellular respiration. Excess glucose is stored as glycogen in the liver and muscles for later use. An overabundance of glucose can also be stored as fat in adipose tissue. Undigested carbohydrates or insoluble fiber is eliminated from the body through the colon.

Organic Polymers

For information on other types of biological polymers, see: