Modes of Nutrition in Protozoa

Modes of Nutrition in Protozoa

Protozoa obtain their nourishment in many ways. All types of nutrition are found in Protozoa, such as holophytic, holozoic, saprozoic, mixotrophic and parasite, etc.

[I] Holophytic Nutrition

All the phytoflagellates that possess chloroplasts or chromatophores synthesize their food by photosynthesis. As the energy is supplied by sunlight to carry out photosynthesis, this method involves self-feeding which is also referred to as autotrophic phototrophy. Carbon dioxide and water, acts as raw materials, and enter into a complex cycle of chemical reactions and produce dextrose sugar. From dextrose paramylum may be formed which is special characteristic of euglenoid flagellates.

[II] Holozoic Nutrition

Majority of free-living Protozoa derive nourishment by ingesting other organisms, both animals and plants. Such protozoa are called holozoic and mode of nutrition is called as holozoic nutrition. All Sarcodina are holozoic except for the some parasitic species. This mode of nutrition involves development of organelles for food capture, ingestion, digestion and egestion of indigestible residues.

  1. Food and feeding:

    Food of holozoic Protozoa consists of microorganisms like other protozoans, bacteria, diatoms, rotifers, crustaceans, larvae, etc. The method involved in ingestion of these organisms is referred to as phagocytosis or

  2. Digestion:

    Food in Protozoa is digested within food vacuoles, which usually keep on circulating in the endoplasm. Within food vacuoles, the reaction is at first acidic which later on becomes alkaline. Proteolytic and carbohydrate splitting enzymes have been observed in many protozoans. In the acidic medium, proteins are converted into dipeptides and in the alkaline medium, dipeptides are converted into amino acids. Hydrolysis of carbohydrates takes place in alkaline medium. Certain Protozoa are able to digest fats. Digestive enzymes are furnished by lysosomes which fuse with the food vacuoles.

  3. Absorption:

    Digested food gets diffused into endoplasm, where it is assimilated into protoplasm. Excess food may be stored as glycogen, paramylum, lipids, chromatoid bodies, etc.

  4. Egestion:

    Indigestible residue of food is expelled from the hinder part of body in case of moving amoeba at any point. For this purpose, ciliates often possess an anal opening, the cytopyge or ectoproct.

[III] Pinocytosis

Pinocytosis is also called as cell-drinking. In addition to phagocytosis, pinocytosis has been reported in Amoeba and certain certain flagellates and ciliates. This involves ingestion of liquid food by invagination through surface of body. Pinocytosis channels are formed at some parts of body surface to enclose the fluid food from the surrounding medium. Lower ends of channels are pinched off as food vacuoles which circulate into the ectoplasm. Pinocytosis is induced only by certain active substances in the medium surrounding the cell, such as some proteins and many salts. Its physiological significance seems to be the absorption of high molecular compounds from the external medium.

[IV] Saprozoic Nutrition

Saprozoic nutrition involves absorption of food by osmosis, i.e., through general surface of body. This method of obtaining food is referred to as osmotrophy. Food consists in the form of solution of dead organic matter, rendered so by the decomposing bacteria. This mode of nutrition is found in Mastigamoeba and in some colourless flagellates (e.g., Chilomonas, Astasia, Polytoma). Dissolved food materials, upon which the saprozoic protozoans subsist, are proteins and carbohydrates.

[V] Myxotrophic Nutrition

This is a combination of more than one mode of nutrition. Many Protozoa using photosynthesis as a means of food-synthesis also take place in some part of their diet in dissolved form by osmotrophy or solid form by phagotrophy. Flagellates like Euglena and Peranema nourish themselves by this method.

[VI] Nutrition of Parasites

The food obtaining mechanisms used by parasitic Protozoa are generally same as those of their non-parasitic relatives. Many intestine inhabiting Zoomastigophora (Trichomonas) have a distinct mouth or cytosome through which food particles are ingested by phagotrophy. Many parasitic ciliates like Nyctotherus and Balantidium have the same process too. Parasitic Sarcodina of the genus Entamoeba feed by phagotrophy, at least at certain stage of their life cycle. Zooflagellates inhabiting blood (e.g. Trypanosome) feed by osmotrophy. Osmotrophic forms may be either coelozoic or histozoic. Opalina, which is found in the rectum of frog, is coelozoic and absorbs all its food through the cell surface. The young trophozoite of Monocystis is histozoic within the sperm morula and it feeds upon the sperms substance by osmotrophy. Parasitic saprozoic forms may also use directly the serum of their hosts’s blood.

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