Which are membrane-enclosed intracellular organelles, need an acidic lumen in order to function. Acidification is achieved by an H⁺ pump in the endosomal membrane, which also contains Cl^– channels. If the channels do not function properly (e.g., because of a mutation in the genes encoding the channel proteins), acidification is also impaired.

A. Can you explain how Cl^– channels might help acidification?

B. According to your explanation, would the Cl^– channels be absolutely required to lower the pH inside the endosome?