How do lysosomes maintain their acidic lumen and why is this important?

• A. Protons are pumped into the lysosomal lumen by V-ATPases, creating an acidic pH essential for hydrolytic enzyme activity.
• B. Lysosomes maintain an alkaline lumen by proton pumps to optimize enzyme function.
• C. The pH of lysosomes is neutral and maintained by buffering systems only.
• D. Lysosome acidity is irrelevant to its degradative functions.

Answer: (A)

The crucial idea is that lysosomes rely on a low pH inside to power their degradative enzymes. This acidic lumen is created by V-ATPases, proton pumps in the lysosomal membrane that use energy from ATP to move protons into the lumen, reducing the pH to about 4.5–5.0. At this acidity, lysosomal acid hydrolases are highly active and can efficiently break down proteins, lipids, nucleic acids, and other macromolecules delivered to the lysosome. If the interior were alkaline or neutral, these enzymes would work poorly or not at all, so digestion would fail. Maintaining the acidity is therefore essential for the lysosome’s degradative function; buffering alone or a neutral pH wouldn’t support the necessary enzyme activity.