What part of the dead leaf cells are digested by the enzyme cellulase

The part of the dead leaf cells where the cellulase acts is in the cell wall.

Explanation:

Cellulase enzymes are used to break down the cellulose of plant cell walls into simple sugars that can be transformed (fermented) by microbes to fuels, primarily ethanol, as well as to chemicals, plastics, fibers, detergents, pharmaceuticals, and many other products.