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|Chapter Two: The Chemistry of Biology|
Organic molecules are any molecules that contain atoms from three elements: carbon, hydrogen, and oxygen. For example, glucose is organic, since its molecular formula is C6H12O6. Carbon dioxide (CO2) is inorganic since it does not contain hydrogen.
All organic molecules have two important parts, the carbon backbone and the functional groups. To understand why carbon is the backbone of most molecules, we have to think back to the topic of bonds between atoms. Carbon becomes more stable when it gains four more electrons; that is, when it makes four bonds. So, every carbon atom in an organic molecule has made four bonds, which allows the molecule to have a complex structure with carbon as the "backbone."
At each end of the molecule is a functional group. Often organic compounds are classified based upon their functional groups because molecules with the same functional groups have similar properties. In general, when organic molecules chemically combine to form larger molecules, the bonding occurs at the functional group. So, certain molecules combine frequently with one another because their functional groups are "compatible." The most important functional groups are shown below. Later, you'll understand how certain pairs of these groups can react with one another to join two molecules together.