Organic compounds are substances containing the element carbon. The carbon atoms found in organic compounds typically bond to hydrogen, oxygen, nitrogen, or other carbon atoms. Four biologically important types of organic compounds are carbohydrates, lipids, proteins, and nucleic acids. Composed of carbon, hydrogen, and oxygen, carbohydrates range in size from the monosaccharides, simple "building-block" molecules possessing from three to six, or even more, carbons and all containing a C=O group in their straight-chain forms, to the polysaccharides, large, complex molecules consisting most commonly of chains of six-carbon monosaccharides. Of the six-carbon sugars, or hexoses, glucose is the most important, since other sugars, and even other types of compounds such as proteins and fats, can be converted to or from glucose. Glucose can exist either in a straight- or circular-chain form. Monosaccharides are joined together to form complex sugars through a process called condensation that involves the removal of one molecule of water from every two monosaccharide molecules being joined. Polysaccharides are then simply many monosaccharides that have been joined together by multiple condensation reactions. Glycogen, the principle polysaccharide found in animals, for example, is made of hundreds of glucose molecules joined in such a way. In turn, complex sugars can be split apart through a process of hydrolysis, which requires adding a molecule of water for every bond between two monosaccharides that is broken.
