Answer;
-Proton gradients across a membrane were used by cells that were the common ancestor of all three domains of life.
Step-by-step explanation;
-The proton gradient generated by proton pumping during the electron transport chain is a stored form of energy. When protons flow back down their concentration gradient (from the intermembrane space to the matrix), their only route is through ATP synthase, an enzyme embedded in the inner mitochondrial membrane.
-Proton pumps are used in various ways by members of every domain of organisms: bacteria, archaea, and eukarya, which means that proton gradients across a membrane were used by cells that were the common ancestor of all three domains of life.