Here are some links to chemistry sites describing redox reactions with some exercises for practice. Note: these sites are designed for chemistry students and go into more detail than we need here. However, they do describe these reactions well and should be helpful.


A. oxidation is the removal of electron(s) from a substance

B. reduction is the addition of electron(s) to a substance

C. examples

(1) H2 --> 2 e- + 2H+ (oxidation)

(2) 1/2 O2 + 2 e- + 2H+ --> H2O (reduction)

E. substance oxidized (H2) is electron donor while substance reduced (O2) is electron acceptor

(1) e donor is always reduced and e acceptor is always oxidized (state prior to reaction)

F. must always have a reduction with an oxidation

(1) each redox reaction requires both half reactions

(2) the ability of a reaction to proceed depends on which half reactions are involved

(3) substances vary in their tendency to give up or accept electrons

(a) reflected in the reduction potential (Eo)

(4) all half reaction are written as reductions with the reductant on the right and vice versa

(a) e.g., 2H+ + 2e- --> H2

(5) Eo’ (pH 7) can be used to determine if a reaction will go forward and how much energy can be gained

(a) each half reaction can be thought of as a redox pair, i.e., an oxidized and reduced half

i) e.g., 1/2O2/H2O

(1) the oxidized half is on the left always even though one of the pairs will have to go in reverse to have a complete redox reaction

(b) the reduced substance of a redox pair whose reduction potential is more negative donates electrons to the oxidized substance of a pair whose reduction potential is more positive

i) in the case of H2 and O2, the 2H+/H2 pair is very negative so the H2 has a strong tendency to donate electrons. The 1/2O2/H2O couple is positive so H2O has a very low tendency to donate electrons, but the O2 has a strong tendency to accept them

(c) reduction tower (from Brock Biology of Microorganisms (Prentice Hall))

i) list of Eo with most negative at top and least at bottom. Therefore, determine which direction reaction will go if at all

G. Metabolism (generation of ATP from oxidation and reduction reactions)

(1) respiration removes electrons from electron donor and passes eventually to electron acceptor (exogenous) with energy generation (ATP formation) from proton motive force at a charged membrane (oxidative-level phosphorylation)

(2) fermentation usually splits a molecule in which part is oxidized while another is reduced and ATP is generated by tranfer of a phosphate from the substrate to ADP (substrate-level phosphorylation)

(a) fermentation yields much less energy than respiration, but is a key intermediate in anaerobic metabolism

(3) differnt types of metabolism are similar in terms of electron flow and redox, but differ in electron donors and acceptors.

H. Redox zones

(1) since electron acceptors differ in their tendancy to accept electrons, different amounts of energy can be gained with each one. Therefore, those that yield the most energy tend to be used first by respiring organisms, and the rest are used in the order of more to less energy. This results in a vertical (or temporal) zonation of electron acceptor use.