A demonstration of chemiosmosis by Peter Mitchell allowed ................. to move down their concentration gradient by using two different ................ .

Peter Mitchell demonstrated the chemiosmotic mechanism by first immersing mitochondria in a medium at pH 8. All mitochondrial compartments equilibrate at pH 8 and since mitochondria can not set their own proton gradient using the electron transport chain, they starved. This was followed by placing the same mitochondria in an acidic medium with a much higher concentration of protons.  Protons move quickly through the freely permeable outer membrane into the intermembrane space and thereby lowering the pH of this compartment. The established proton gradient across the inner mitochondrial membrane allows the flow of protons from high concentration to low concentration through specialized channels in the inner mitochondrial membrane with simultaneous ATP synthesis. Thus, the correct answer is D.