Redox Potentials

The redox potential is a measure (in volts) of the affinity of a substance for electrons — its electronegativity — compared with hydrogen (which is set at 0).

Substances more strongly electronegative than (i.e., capable of oxidizing) hydrogen have positive redox potentials. Substances less electronegative than (i.e., capable of reducing) hydrogen have negative redox potentials.

Link to a discussion of electronegativity.

Oxidations and reductions always go together. They are called redox reactions.

When electrons flow "downhill" in a redox reaction, they release free energy.

Discussion

We indicate this with the symbol ΔG (delta G) preceded by a minus sign.

It requires an input of free energy to force electrons to move "uphill" in a redox reaction. We show this with ΔG preceded by a plus sign.

The electronegativity of a substance can also be expressed as a redox potential (designated E)

The standard is hydrogen, so its redox potential is expressed as E = 0.

Any substance — atom, ion, or molecule — that is more electronegative than hydrogen is assigned a positive (+) redox potential; those less electronegative a negative (−) redox potential.

The greater the difference between the redox potentials of two substances (ΔE), the greater the vigor with which electrons will flow spontaneously from the less positive to the more positive (more electronegative) substance.

The difference in potential (ΔE) is, in a sense, a measure of the pressure between the two. ΔE is expressed in volts.

If we bring two substances of differing E together with a potential path for electron flow between them, we have created a battery. Although it may be in a mitochondrion, it is just as much a battery as a the lead-acid storage battery in an automobile.

The greater the voltage, ΔE, between the two components of a battery, the greater the energy available when electron flow occurs. It is, in fact, possible to quantify the amount of free energy available. The relationship is:

ΔG = − n (23.062 kcal) (ΔE)

where

n is the number of moles of electrons transferred and
23.062 is the amount of energy (in kcal) released when one mole of electrons passes through a potential drop of 1 volt.

Cellular Respiration

For every molecule of glucose respired, 24 electrons travel down the respiratory chain to the final acceptor: oxygen molecules.
Discussion
Carbon reduced to the extent occurring in carbohydrates like glucose (only partially reduced) has a redox potential of approximately − 0.42 volt.
Oxygen, as the most electronegative substance in the system, naturally has the largest E: + 0.82 volt

The difference (ΔE) is thus 1.24 volts. Allowing 24 moles of electrons to pass through this potential gives us a free energy yield of − 686 kcal:

ΔG = − (24)(23.062)(1.24) = − 686 kcal

Photosynthesis

To synthesize a molecule of glucose by photosynthesis,

24 electrons must be removed from water molecules where they have been held by the redox potential of oxygen (+ 0.82 volt) and pumped "uphill" to
carbon atoms which they partially reduce to carbohydrate with a redox potential of − 0.42 volt.

Once again, the difference is 1.24 volts, so

ΔG = + (24)(23.062)(1.24) = + 686 kcal

ΔG is positive here because electrons are moving against the gradient (from positive to negative) instead of with it as they do in cellular respiration. Thus energy (from light) must be put into the system.

Link to a graphic showing the path taken and the changing redox levels of electrons in photosynthesis.

Link to an analysis of the energy changes occurring as covalent bonds are broken and formed during cellular respiration and photosynthesis.

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23 May 2006