Nutrition occurs in Protozoans

All types of nutrition occur in Protozoa. Some protozoans synthesize their own food from inorganic precursors (carbon dioxide, nitrates or ammonrum salts) and hence are autotrophic; eg. chlorophyll bearing flagellates; others are heterotrophic as they depend on organic and inorganic raw materials from the environment for their food.
Heterotrophs have varied food habits. Some feed on soluble organic nutrients from their environment and are termed saprozoic or osmotrophs, eg. parasitic protozoa; still others are holozoic ingesting rigid or solid organic food, or phagotrophs i.e. they ingest solid food particles of animal or plant origin. Thus they may also feed on bacteria or other small protistans which are ingested whole, and digest them within food vacuoles eg. rhizopods and ciliates.

Autotrophic protozoa use light energy to synthesize their organic molecules but they often become phagotrophic or osmotrophic. Even among the heterotrophic protozoa few follow only one mode of nutrition exclusively. For example in Euglena specles there is considerable variety in modes of nutrition. Some species requlre some preformed organic molecules even though they are autotrophs, some species lose their chloroplasts if kept in the dark and become permanent osmotrophs. Phagotrophic nutrition involves.

Figure: Phagocytosis in Amoeba.

phagocytosis in which the cell membrane infolds the food particle which is then engulfed. The food particle is then enclosed in an intracellular membrane bound vesicle the food vacuole or phagosome. The food is then digested by the action of enzymes contained in lysosomes. These enzymes are implied into the vacuole and the indigested material is thrown out from the body by exocytosis.The digested material is absorbed by the cell. In most ciliates, many flagellates and apicomplexan the site of phagocytosis is a definite mouth structure called the cyfostome. In amnebas phagocytosis occurs at any point by surrounding the food particle with pseudopodia. Flagellates may have a temporary cytosome usually in a characteristic position or the cytosome may be a permanent specialized structure. Saprozoic feeding may take place by pinocytosis or through direct diffusion or facilitated or active transport of solutes through the membrane.