The lysosomal lumen is the aqueous interior of a lysosome, separated from cytosol by its surrounding lipid bilayer. A membrane pump couples ATP hydrolysis to inward transport of hydrogen ions, maintaining an acidic environment for acid hydrolases. Membrane transport proteins export digestion products, including amino acids and sugars, for reuse in metabolism. Compartmentalization combines restricted access with controlled exchange.[ref]
Transport from the Trans Golgi Network to Lysosomes. Molecular Biology of the Cell, 4th edition by Bruce Alberts and others; Lysosomes Are the Principal Sites of Intracellular Digestion, Figure 13-31: ATP-driven acidification and export of digestion products
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