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Mechanisms of food ingestion


Many ciliates are filter feeders, creating water currents with special ciliary structures associated with the cytostome. The synchronized beating of these ciliary structures pushes a stream of water against a membranelle composed of cilia. The membranelle acts as a collecting sieve, where the food particles become trapped in the free spaces between the cilia. Using this mode of feeding, ciliates can shift considerable volumes of water in relation to their size. Tetrahymena, for example, can filter 3,000 to 30,000 times its own volume in one hour.


The suctorians are ciliate predators that usually possess tentacles of two functional types: feeding tentacles and piercing tentacles. The latter trap and immobilize the prey, usually other ciliates that make chance contact with the outstretched tentacles of the suctorian. The cell contents of the prey are transported up through the feeding tentacles into the suctorian, where digestive vacuoles are formed. The transporting mechanism is mediated by a complex array of microtubules within the tentacle. A single suctorian can often feed on several prey at the same time, and frequently the prey are larger than the predator.

Amoebae, all of which lack a cell mouth, or cytostome, also exhibit a diverse array of feeding mechanisms and diet. Some feed on filaments of cyanobacteria (blue-green algae)—which are composed of long chains of individual cells—by taking in the entire filament at any point on the cell surface and rolling it up into a coil inside a digestive vacuole. Others, such as the testate amoeba Pontigulasia, pierce single cells in algal filaments and remove the contents. The radiolarians and foraminiferans trap a wide range of prey, including protozoans, algae, and small crustaceans, in their complex pseudopodial networks and then transport the food items along the pseudopodia to the main body of the cell for ingestion. This process gives foraminiferans a distinctive appearance under the light microscope; relatively large grains of food particles are scattered across the reticulate pseudopodia, earning the organisms the name granuloreticulosans.

Parasitic protozoans feed in a variety of ways. Many live in the nutrient-rich medium of the body fluids—e.g., the blood or cells of their host. There they take in energy-rich fluids by pinocytosis, in which small amounts of the medium are pinched off into digestive vacuoles either at a specific site, such as the cytostome in ciliates or the flagellar pocket in trypanosomes, or along the surface of the cell in amoebae. Other parasitic protozoans engulf portions of the host tissue through phagocytosis in much the same way that free-living amoebae feed. Plasmodium, for example, engulfs portions of the red blood cells or liver cells in which they live. The hemoglobin in the cytoplasm of the red blood cell is only partially digested by the parasite. The protein portion of the hemoglobin molecule is degraded to its constituent amino acids, but the iron-containing portion is converted into insoluble iron-containing hemozoin, which remains within the parasite’s endosomes until discarded at the next division. This process removes free hematin from the parasite cytoplasm, where it would otherwise prevent further metabolism within the parasite because it inhibits the actions of succinic dehydrogenase, an enzyme in the tricarboxylic acid cycle.

Food vacuoles



Food selection


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