What is the term used to describe the build-up of a higher concentration of H + on one side of the inner membrane of the mitochondrion than on the other side during the electron transport chain?

electrochemical proton gradient

Explanation:

The electrochemical proton gradient is defined as the difference in the proton concentration or hydrogen ions (H+) concentration across a inner mitochondrial membrane during the electron transport chain.

The electrochemical proton gradient is build due to the active transport of ions across the cell membrane which is useful for ATP synthesis during the electron transport chain by the coupling of the electrochemical gradient across the membrane.

Hence, the term used is electrochemical proton gradient.