Question

Why all protozoan not performing phagocytosis while all of them have cell membrane

Answer 1

All Protozoans does not perform phagocytosis because they do not have any kind of phagocytic enzyme.  For phagocytosis the organism must have some hydrolytic enzyme so it can perform phagocytosis

Answer 2

EXPLANATION:

All protozoans do not perform phagocytosis even though all of them possess cell membrane is because in addition to cell membrane a lot of other elements are required to perform phagocytosis.  

The elements are certain chemicals and enzymes which act as triggers. Also, some bacteria find it unnecessary to perform phagocytosis and have lost it over time due to evolution. Phagocytosis is the process by which bacteria engulfs foreign particles, organisms or food material.