Free energy is available to fuel the cell processes of living things. All living things need a constant flow of energy into and through their systems. Many of the chemical reactions that transfer energy in living things involve the transfer of electrons. These reactions occur most easily in the polar enviornment of a water solution. Organic compounds contain carbon atoms that are covalently bonded to other carbon atoms and other elements like hydrogen, oxygen, and nitrogen.
Carbon atoms are special for two reasons.
First, carbon atoms easily bond with other carbon atoms. This is not the case with most atoms. Second, small clusters of atoms combined with carbon, known as monomers, have the ability to link in repeating units called polymers . Large polymers are known as macromolecules . It is these macromolecules that make the complexity of life possible.