Why evolve digestive enzymes which function at PH different from cytosol?
Lysosomes are single membrane bounded organelles, containing digestive enzymes that have the role to process the food in the cell and also to destroy the dead cells. The enzymes contained by the lysosomes are engulfed by the membrane of the cell in order to protect the rest of the cell of being destroyed.
The functions of lysosomes are several, such that: exocytosis, autophagy, heterophagy, biosynthesis, autolysis.
Through exocytosis process, the enzymes are relesed outside of the cell, in order to destroy the materials found in the neighbourhood of the cell. Autophagy represents the process of breaking down the food within the cell. The digestion of materials outside the cell is called heterophagy, which can act through three mechanisms: phagocytic, pinocytic and endocytic.
Biosynthesis represents the recycling function of lysosomes and autolysis represents the break down of the dead cells.
Lysosomal enzymes are very active at a pH of 5, they being proteins are dependent by the pH of environment because their structure can be affected by this pH. Hence lysosomal digestive enzymes do not work at all with the slightly base (not acidic) cytosol; cytosol is protected from the ~pH 5 enzymes because cytosol is a base with pH 7.2. This explains (protective difference in pH) why enzymes evolved with a different pH from cytosol.