Similar to all animal cells, protozoans are covered by a plasma membrane which surrounds cytoplasm of the cell, protozoan's integument or skin. Like all membranes, it's permeable; water and low molecular weight substances dissolve in it. Oxygen, is required for metabolism, and metabolic wastes which are generated by metabolism, diffuse across the surface, giving plasma membrane respiratory and excretory functions. Nucleus, with its nucleolus and chromosomes, contains genetic material and would be involved in providing copies of itself for new cells, reproductive system. Golgi apparatus, rough endoplasmic reticulum, and lysosomes which they produce, fuse with ingested food vacuoles, phagosomes.