Many extracellular particles and molecules ingested by cells end up in lysosomes, which are membranous sacs of hydrolytic enzymes that carry out the controlled intracellular digestion of both extracellular materials and worn-out organelles.
B.
They contain about 40 types of hydrolytic enzymes, including those that degrade proteins, nucleic acids, oligosaccharides, and lipids.
C.
All of these enzymes are optimally active in the acidic conditions (pH ~5) maintained within lysosomes.
D.
The membrane of the lysosome normally keeps these destructive enzymes out of the cytosol (whose pH is about 7.2), but the enzymes’ acid dependence protects the contents of the cytosol against damage should some of them escape.