Electron transport chains ( most known as ETC ) produce energy in the form of a transmembrane electrochemical potential gradient.
2.
The concentration difference creates an electrochemical potential gradient that is used to catalyze the uptake of organic substrates ( in this case leucine ).
3.
This change in both the pH and electrochemical potential gradient between the inside of the cell and the outside produces a proton motive force, as the protons will want to naturally flow back into the area of low concentration and with a voltage closer to zero from their current situation of being in an area of high concentration of positively charged protons.