Complexes I, III and IV each contributed toward generation of at H+ gradient such that the pH is lower in the inter membrane space than it is in the inner matrix.
Unequal concentrations are unstable and would prefer to diffuse back to a condition in which the pH is the same on both sides, but the inner membrane will not allow passive diffusion of charged molecules through it.
Enter Complex V. This protein complex will allow H+ to "flow" through it (and only it) back toward the inner matrix.
David B. Hicksa, Jun Liua, Makoto Fujisawab, Terry A. Krulwich Biochimica et Biophysica Acta (BBA) - Bioenergetics Volume 1797, Issue 8, August 2010, Pages 1362.
This flow, like a river, can be used to do work. Think back about a century.... water wheels were used as the primary mechanism for turning a grind stone for converting grain into flour. Complex V is going to do much the same thing. It will use the "flow" of H+ back into the inner matrix through a portion of complex V to physically rotate a portion of it.
Consider the cartoon of complex V (The inter membrane space is at the top of the picture the inner matrix is at the bottom). There are effectively three portions:
Through the "middle" of the F1 portion runs the "axle" or γ subunit. As H+ "flow" through F0 it causes a rotation of the γ subunit "an axle" that runs through the middle of the F1. The F0 and F1 portions remain stationary.
The Knob portion (F1) is made of three each two different types of subunits called the α and β subunits which are arranged in alternating fashion, like sections of an orange. These are the actual ATP synthesis subunits.
As the γ subunit rotates into one of three positions it causes physical changes in the shape of each pair of α, β pair of subunits. At one position one pair bind ADP and PO4= in active site that is in an open configuration (sort of like an open hand). As the "axel rotates one postition the active site closes with ADP and PO4= bound (much like making a fist). This has the effect of excluding waster from the active site so that the condnesation con proceed in a low water concentration environment. AS the γ subunit clicks one more position, the active site opens up again to release the synthesizes ATP back into solution. The other two pair of α, β are in complementary configurations.
so now the picture is complete. Glucose has been oxidized to CO2 and the electrons have been disposed to generate water. The energy of this electron transfer is used to make a proton gradient which is then used to make ATP.
Earlier, it was found that each pait of electrons can theoretically generate up to 7 ATP. It was also indicated that nothing is 100% efficient. The question still remains: