Final answer:
The substance responsible for activating digestive enzymes in lysosomes is an acidic pH, which provides an optimal environment for the lytic enzymes to become active and perform their function of breaking down various cellular components and invaders.
Step-by-step explanation:
Acidic pH is responsible for activating the digestive enzymes inside lysosomes. Lysosomes contain hydrolytic enzymes known as acid hydrolases, which require an acidic environment to become active. The acidic pH within lysosomes is maintained by proton pumps, which pump hydrogens ions (H+) into the lumen.
Unlike in the cytoplasm, where the pH is much less acidic, the internal environment of lysosomes allows the enzymes to remain in an inactive state until they are within the lysosome. Once inside this acidic environment, they are activated and able to break down a variety of biological macromolecules and organelles. This compartmentalization is essential as many reactions that occur in the cytoplasm could not proceed at such a low pH, hence the cellular advantage of having specialized organelles like lysosomes.