Biological molecules are organic compounds

   With the exception of water, most molecules that are formed by living organisms and make up their structures contain the element carbon. This fascinating element lends itself to being the basis of living material because of its ability to form four bonds with other atoms and therefore interact with other molecules in myriad ways. Silicon, an atom with similar bonding properties, is abundant in the Earth's crust but not in living things. However, silicon is used by some organisms in building outer skeletons or shells. An example of a group of such organisms is the diatoms (shown below), one of the most abundant protists on this planet.

   The carbon-containing molecules that make up living things are called organic compounds. Organic compounds are often very large and are usually held together by covalent bonds. The study of organic molecules and their interactions is called organic chemistry. The study of molecules and substances not containing carbon is called inorganic chemistry. Inorganic compounds are often quite small and are usually held together by ionic bonds. A few carbon-containing, covalently bonded inorganic molecules exist (such as carbon dioxide and carbonic acid), but they are considered inorganic molecules because they do not make up the structure of living things.

   Although carbon is the underlying component of biological molecules, 10 other elements are common in living organisms. These elements are listed in Table 1 and are compared according to their frequency in the Earth's crust. Notice that the great majority of atoms in living things (96.3%, in fact) are oxygen, carbon, hydrogen, or nitrogen.