Final answer:
Endosomes become lysosomes when the vesicle merges with an early endosome to form a sorting vesicle, where digestive enzymes catalyze the hydrolysis of the vesicle contents.
Step-by-step explanation:
Endosomes become lysosomes when the vesicle that remains, now a lysosome, merges with an early endosome to form a sorting vesicle. In this vesicle, digestive enzymes catalyze the hydrolysis of the vesicle contents. The digest products are then released for cellular use.