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:
˝ 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, 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.
Can you think of
a molecule that hasn’t been on Earth long enough yet for microbes to figure out
how to use it as a food source?