The Role of Oxidation and Reduction in Metabolism
Oxidation–reduction reactions, also referred to asredoxreactions, are those inwhich electrons are transferred from a donor to an acceptor. Oxidation is the loss of electrons, and reduction is the gain of electrons. The substance that loses electrons (the electron donor)Ñthat is, the one that is oxidizedÑis called the reducing agent or reductant. The substance that gains electrons (the electronacceptor)Ñthe one that is reducedÑis called the oxidizing agent or oxidant. Both an oxidizing agent and a reducing agent are necessary for the transfer of electrons (an oxidation-reduction reaction) to take place.
An example of an oxidation-reduction reaction is the one that occurs when a strip of metallic zinc is placed in an aqueous solution containing copper ions. Although both zinc and copper ions play roles in life processes, this particular reaction does not occur in living organisms. However, it is a good place to start our discussion of electron transfer because, in this comparatively simple reac-tion, it is fairly easy to follow where the electrons are going. (It is not always quite as easy to keep track of the details in biological redox reactions.) The experimental observation is that the zinc metal disappears and zinc ions go into solution, while copper ions are removed from the solution and copper metal is deposited. The equation for this reaction is
Zn(s) + Cu2+(aq) - > Zn2+(aq) + Cu(s)
The notation (s) signiÞes a solid and (aq) signiÞes a solute in aqueous solution.
In the reaction between zinc metal and copper ion, the Zn lost two electrons to become the Zn2+ ion and was oxidized. A separate equation can be written for this part of the overall reaction, and it is called the half reaction of oxida-tion:
Zn - > Zn2+ + 2e-
Zn is the reducing agent (it loses electrons; it is an electron donor; it is oxidized).
Likewise, the Cu2+ ion gained two electrons to form Cu and was reduced. An equation can also be written for this part of the overall reaction and is called the half reaction of reduction.
Cu2+ + 2e- - > Cu
Cu2+ is the oxidizing agent (it gains electrons; it is an electron acceptor; it is reduced).
If the two equations for the half reactions are combined, the result is an equation for the overall reaction:
This reaction is a particularly clear example of electron transfer. It will be useful to keep these basic principles in mind when we examine the ßow of electrons in the more complex redox reactions of aerobic metabolism. In many of the biological redox reactions we will encounter, the oxidation state of a carbon atom changes. Figure 15.2 shows the changes that occur as carbon in its most reduced form (an alkane) becomes oxidized to an alcohol, an aldehyde, a carboxylic acid, and ultimately carbon dioxide. Each of these oxidations requires the loss of two electrons.
In catabolism, large molecules are broken down to smaller products, releasing energy and transferring electrons to acceptor molecules of vari-ous sorts. The overall process is one of oxidation.
In anabolism, small molecules react to give rise to larger ones; this process requires energy and involves acceptance of electrons from a variety of donors. The overall process is one of reduction.