that these compounds could dissipate the proton gradient due to their abil-
ity to diffuse through the membrane in both protonated and deprotonated
forms. In 1966, the hypothesis was dramatically supported by experiments
by Andre Jagendorf and coworkers (see Jagendorf, 1998). Thylakoids were
suspended in a buffer of pH 4 that caused both the interior and exterior of
the cells to equilibrate at that pH (Figure 6.9). A buffer of pH 8 was injected
rapidly into the solution, creating a pH difference of approximately four
units across the thylakoid membrane. The pH difference resulted in large
amounts of ATP being formed from ADP and inorganic phosphate, support-
ing the chemiosmotic hypothesis. Ultimately most skeptics were convinced
and finally in 1978 Mitchell received a Nobel Prize for Chemistry.
The transport of protons across the membrane creates both a concentra-
tion difference and a charge difference, with both effects influencing the
energetics. One contribution to the Gibbs energy difference arises from the
difference in proton concentration for the two sides of the cell membrane.
The proton difference arises from different metabolic processes (Figure 6.10)
or from the action of proton pumps (Chapter 18). For an interior proton
concentration [H+]inand proton concentration outside the membrane [H+]out,
the Gibbs energy difference is given by the ratio of these two concentrations:
(6.19)
This expression for the Gibbs energy difference can be rewritten in terms
of the pH difference:
=ln[H+]in−ln[H+]out=2.3(log[H+]in−log[H+]out) (6.20)=−2.3(pHin−pHout) =−2.3δpH ΔG−ΔG°=+RT(−2.3ΔpH)ln
[]
[]
H
H
in
out+
+δGGG RTln
[]
[]
=−°=+
+
ΔΔ +H
H
in
outCHAPTER 6 REDOX REACTIONS AND BIOENERGETICS 125
pH 4pH 4EquilibrationpH 4
Buffered
mediumThylakoid
membranes
(pH 8)pH 4pH 8Add
ADP
PipH 4pH 8ADP
Pi ATPThylakoids
transferredFigure 6.9Support for the chemiosmotic hypothesis was provided by the formation of ATP in the
experiments of Jagendorf and coworkers.