In the proton motive force equation (PMF) there are two terms: a delta psi (membrane potential) and a delta pH (indirectly, chemical proton gradient). If we take the two terms as an electrical one, directly expressed in millivolts, and a chemical one, converted to millivolts, we would be able to just add them together to get the full contribution to the pmf. Still, there is a difference in these two quantities, aside from the fact that one is electrical and one is chemical. The change in the electrical potential is an absolute quantity; it does not depend on where you start. That is, a change of 20 mV say starting from 200 mV is the same as a change in 20 mV starting at 100 mV. However, it is not the same situation with pH. If we drop the pH one unit starting with pH 8, the result is considerably different from starting with pH 7; about 10 fold.
I realize the changes may not be that much in practice as the mitochondrial pH is fairly constant at pH 7.5 or so, but chloroplast changes are dramatic.
The questioning of the meaning of pH stems from reading Peter Stewart's 1981 "How to Understand Acid–Base: A Quantitative Acid–Base Primer for Biology and Medicine". The issue is not that pH is itself in the equation; this is expected as it is the potential of the hydrogen ion concentration. It is the problem of a delta pH.
Delta pH is logarithm of the ratio of the proton concentrations on the two sides of the membrane. For example, if the ratio is 10-fold, then the common logarithm is 1 (∆pH = 1). It doesn't matter (mathematically) whether the two pHs are 8 and 7, or 4 and 3.
pH = -log[H+]
log(A/B) = log A - log B
Both electric and chemical potential differences contibute to change in free energy. It is this free energy that is used to drive chemical processes. It does not matter where it comes from but biological systems are wired to first create a chemical potential that can then be used to create high energy compounds.
Thanks for the comment. It is true that the derivation of the pmf equation is beyond doubt and that the gradient of protons is the driving force for establishing the chemical portion of the potential. However, there is a big difference if the starting pH values are different. The driving force for exchange is the gradient, but the exchange itself depends on the concentration; this would be enormous at pH 4 compared to pH 8. The other complication addressed in the Stewart work is that hydrogen ion is not an independent variable; it must always satisfy equilibration with weak acids and most importantly water. So once [H] increases chemically by the proton expulsion, its value will very rapidly drop. Bioenergetics was an aspect of protons that was not addressed by Stewart but should be.
- Badges
- Science topic
- Similar topics
- Geoscience
- Atmospheric Sciences
- Climate Science
- Climate Change