The Chemistry of Biology: Carbohydrates


Carbohydrates are organic compounds that are organized as ring structures and are always composed of the elements carbon, hydrogen, and oxygen. Carbohydrates are truly hydrates of carbon because the ratio of hydrogen atoms to oxygen atoms is always nearly 2:1, as in H2O.

They also have many functions. Most of the energy you receive comes from the carbohydrates that you eat. Plants make carbohydrates such as wheat, corn, and potatoes. Carbohydrates are normally consumed by animals either by eating the plant that manufactured it or by eating other animals. Humans also receive carbohydrates from whole grains, fruits, vegetables, milk, candy, soft drinks, and pasta.

Insects manufacture the carbohydrate chitin as a tough exoskeleton for protection, and lobsters and crabs use chitin for their shells. Finally, cellulose is probably the most widely used carbohydrate compound, comprising wood and wood products, such as paper.


The simplest biologically important carbohydrates are monosaccharides, meaning one sugar (mono = one, saccharide = sugar). The general formula for any carbohydrate is (CH2O)x where x is any number between three and eight. The most common monosaccharides (hexoses) are glucose, galactose, and fructose.

Glucose is the simplest monosaccharide and probably the most familiar sugar, especially if you have been in the hospital. In nature, glucose is the sugar that green plants produce during photosynthesis. It is also the main source of energy for cells. Medical procedures often require a glucose IV for recovering patients to regain their strength more quickly. Galactose is found in milk, and fructose gives fruit a sweet flavor. Although the chemical structure of each sugar differs, the chemical formula is the same: C6H12O6.


Monosaccharides are joined together by dehydration synthesis to form disaccharides, or double sugars (di = two). The dehydration synthesis reaction releases water dehydration as a by-product. The most common disaccharide is sucrose, also known as table sugar, C12H22O11. Other common disaccharides include maltose (malt sugar) and lactose (milk sugar).


Further dehydration adds more sugar molecules together to form long chains known as polysaccharides. A polysaccharide generally refers to a carbohydrate polymer consisting of hundreds, even thousands of monosaccharides covalently bonded together. Cells use polysaccharides for a number of reasons, including the storage of excess glucose as starch in plants and glycogen in animals. The large polysaccharide cellulose is a structural component found in plants that gives them their rigidity and flexibility.

Excerpted from The Complete Idiot's Guide to Biology © 2004 by Glen E. Moulton, Ed.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.

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