Biology  5/e   Raven/Johnson
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Oxidation and reduction: Oxidation involves the removal of electrons from a molecule; reduction involves the addition of electrons to a molecule. The electrons contain energy, so removing or adding electrons also removes or adds energy to the molecule. The atmosphere around the early earth was a reducing one, making it relatively easy for complex, energy-rich organic molecules to form as hydrogen protons and accompanying electrons were added to carbon and other atoms. In today's oxidizing atmosphere, the spontaneous evolution of life from simple inanimate chemicals is no longer possible.

Covalent bonds: Covalent bonds are formed when one or more pairs of electrons are shared between two atoms. The chemistry of life is based on organic molecules, and organic molecules are built on a backbone of carbon. Each carbon atom forms four covalent bonds with other atoms. Life arose on earth as atoms formed covalent bonds with each other and complex organic molecules were formed.

Proteins: Proteins are vital components of living organisms. They play structural roles and control chemical reaction rates. They are made up of chains of amino acids. With the formation of amino acids and then proteins in the "primordial" of the early oceans, one of the key ingredients of living organisms was present and helped lead to the evolution of life.

Nucleic acids: DNA and RNA are the two types of nucleic acids. Hereditary information is stored in DNA as a sequence of nucleotides. RNA is the "working copy" nucleic acid that is responsible for the production of particular proteins based on the hereditary information it is carrying from the DNA. Recent experiments have shown that RNA may have been one of the first macromolecules to form during the evolution of life. A hereditary system is a prerequisite for all living organisms.

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