BioPHYSICAL chemistry

The second contribution arises from the difference in charge for the two

sides of the membrane. The difference in the Gibbs energy for that con-

tribution is given by the voltage difference ΔVacross the cell membrane

(eqn 6.5) using n=1 for a proton charge:

(ΔG)rec=−nFΔV=−FΔV (6.21)

Together, the total Gibbs energy available due to the difference in the

concentration of protons on the two sides of the cell membrane is termed

the protonmoti 9 e force,Δp, and can be written as:

Δp=−FV−2.3RT(ΔpH) (6.22)

Experiments have established that only the total value of Δpis critical for

the synthesis of ATP, as different relative fractions of the two components

result in the same outcome. In thylakoids, the membrane potential is small

and so Δpis due primarily to the pH difference across the membrane,

although in intact plants the membrane potential may be larger.

Research direction: respiratory chain

The cellular pathway for the development of the protonmotive force is

carried out by four integral membrane proteins, identified as complexes

126 PARTI THERMODYNAMICS AND KINETICS



4H

4H

H

Intermembrane

space

Q

I II III

IV

Cyt c

H 2 O

F 0

F 1

O 2  2H

2H



NADH  H NAD ADP ^ Pi

Fumarate

Succinate

Matrix

ATP

    

 

     











^ 







  ^ 









1

2

Figure 6.10A schematic representation showing the involvement of four protein complexes,
identified as complexes I–IV, and ATP synthase in the chemiosmotic hypothesis.