Final answer:
Proton pumps actively transport H+ ions across the walls of cytoplasmic organelles and vacuoles to maintain a low pH.
Step-by-step explanation:
The pumps responsible for actively transporting H+ across the walls of cytoplasmic organelles (such as lysosomes) and vacuoles to maintain a low pH are called proton pumps. These pumps work against the concentration gradient, using energy from ATP to move H+ ions from an area of low concentration to an area of high concentration. The presence of these pumps ensures that the lysosome and vacuole maintain an acidic environment, which is necessary for the functioning of enzymes.