The right answer is B. They were used by the common ancestor of all three domains of life.
The proton pump is a transmembrane protein, and more specifically an active membrane transporter, which displaces protons against their concentration gradient using the energy released by the hydrolysis of an adenosine triphosphate (ATP) molecule. Its operation results in the enrichment of a cell compartment (lysosome) or a vesicle (acetylcholine vesicle) in H + ions (ie a proton alone), that is to say to a decrease of the pH in these vesicles or acidification.
Answer;
-Proton gradients across a membrane were used by cells that were the common ancestor of all three domains of life.
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.
