Early Life, Page 2: [Previous Page] [Next Page] [Class Home Page] Textbook pages 71 - 79

REDOX Reactions

To oxidize an atom or molecule means you have increased its overall positive charge. Removing electrons does this. Atoms or molecules that give up electrons (or become oxidized) are electron donors. For example, in the following reaction, hydrogen gives up its electrons:
H₂ ----> 2 H⁺ + 2 e^-

The term "Oxidation" comes from Oxygen, which is an incredibly powerful acceptor (electron stealer!), oxidizing just about every thing it comes into contact with.

To reduce an atom or molecule means you have reduced its overall positive charge. Adding electrons, resulting in a more negative charge, does this. Atoms or molecules that take on electrons (or become reduced) are called electron acceptors. For example:

1/2 O₂ + 2 e^- + 2 H⁺ ----> H₂O

Oxidation is ALWAYS accompanied by reduction. Reactions involving Oxidation and Reduction are called REDOX reactions. Most important, REDOX reactions generate usable energy for life! Lucky for life, MANY molecules can act either as the electron donor, or as the electron acceptor (some elements can do both, depending on what else is around!). Possible electron donors: H₂, CO, Fe²⁺, Mn²⁺, CH₄, S, NO₂, H₂S, NH₃.

Possible electron acceptors: O₂, CO₂, CO, SO₄, NO₃, Mn⁴⁺, Fe³⁺.

Because of the large number of possible redox reactions (see here), some form of life can make use of almost what ever it is given. Life thus exists in an enormous variety of chemical and physical environments. This is an important concept when we start considering the conditions we might find on extra-terrestrial planets or moons.

1) Can you think of a molecule rich in hydrocarbons that hasn't been on Earth long enough yet for microbes to figure out how to use it as a food source?