Protozoans ingest food by phagocytosis, and consequently, the ingested food is surrounded by a plasma membrane creating a food vacuole, or phagosome. Digestion does not start until phagosomes fuses with a lysosome that contains the digestive enzymes. Everything being digested, and enzymes digesting it, are contained inside a plasma membrane permeable only to small molecular weight compounds, products of digestion. Enzymes are too big to cross the membrane, so there's no danger that they might mistakenly digest the cytoplasm of cell. Initially the pH inside the food vacuole is acidic and as digestion proceeds, contents of the vacuole become more alkaline. To be sure that nutrients get to every part of the cell, food vacuole is swept around the cell by the cytoplasmic currents and nutrients diffuse out supplying the different parts of the cell. Once there is only undigested food left, food vacuole fuses with the cell membrane, dumping its contents to the outside of the cell.