There are four major classes of organic molecules in cells: carbohydrates, lipids, nucleic acids, and proteins. All of these molecules contain carbon backbones, and almost all of them contain oxygen and hydrogen as well as other elements. Some or all of the members of each class of organic molecules occur as very large molecules, called macromolecules, that are polymers of smaller molecules joined together by covalent bonds. For example, starch and cellulose are carbohydrate polymers of simpler carbohydrates called sugars. Likewise, fats and oils are lipid polymers composed of smaller lipids called fatty acids and the sugar alcohol called glycerol.
See also: Carbohydrates, Lipids, Nucleic Acids, Proteins
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