Locomotion In protozoa handwritten nots .
Handwritten notes of locomotion in protozoa .
Locomotion in protozoa
Protozoans exhibit diverse modes of locomotion across the various groups, but the modes of locomotion can be broadly divided into flagellar, ciliary, and amoeboid movement. Only the ciliates among the three major motility groups of protozoans, however, represent a truly monophyletic group (or single evolutionary line). (Some non-ciliates, such as those of group Opalinata, possess cilia-like organelles that are fundamentally different from true cilia.) In contrast, flagella and pseudopodia are present in a wide variety of distantly related taxa.
Flagellar propulsion
Flagellar propulsion is employed during some stages in the life cycles of certain amoebae, including the vegetative phase of some genera, such as Mastigamoeba and Mastigella. The eukaryotic flagellum is a membrane-bound, whiplike structure found not only in protozoans but in animals as well (such as in sperm, the male reproductive cells of animals). The structure of the eukaryotic flagellum consists of a cylinder (axoneme) made up of a pair of central microtubules surrounded and joined by cross-bridges to a circle of nine pairs of microtubules. This “nine-plus-two” arrangement of the microtubules in the axoneme is surrounded by cytoplasm and ensheathed in cell membrane. The flagellum arises from the basal body, or kinetosome, within the cell.
The undulating motion of the flagellum is normally generated at its base. The waves move along the flagellum to produce a force on the water acting along the long axis of the organelle in the direction of the wave. The speed of movement is determined by the length of the flagellum and by the size of, and distance between, the waves it generates. Species of monophyletic stramenopiles (heterokonts) have tripartate tubular hairs (mastigonemes) arising at right angles to the flagellum along its length, whereas other groups, such as the dinoflagellates and euglenids, have slender, simpler hairs called flimmer filaments. Either structure improves the effectiveness of the flagellar stroke, altering the movement of water produced by undulations of the flagellum by reversing its flow toward the flagellar base. Swimming speeds achieved by flagella are relatively low.
Cilium structure and beat
Ciliates have an increased number of beating flagella on the cell surface, thereby enabling greater power and speeds to be developed against viscous forces. The structure of a cilium is identical to that of a flagellum, but the cilium is considerably shorter. Cilia are a type of flagella arranged in closely aligned longitudinal rows called kineties. A complex system of fibres and microtubules arising from the basal bodies, or kinetosomes, of each cilium connects it to its neighbouring cilia in the kinety and to adjacent ciliary rows. In some species the body cilia may be reduced to specialized cirri, where the kinetosomes are not arranged in the usual rows but instead have a hexagonal pattern interlinked at several levels by fibres and microtubules.
The effective stroke of the cilium is usually planar, but in the recovery stroke the cilium sweeps out to the side, creating an overall beat with a three-dimensional pattern. The cilium performs work against the viscous force of the water during both the effective and the recovery strokes. To be effective, each cilium must beat in a coordinated manner with its neighbouring cilia. A synchronized beat is passed along a ciliary row by means of a hydrodynamic linkage between the cilia. During a beat, each cilium displaces a layer of surrounding water. Displaced water layers overlap between cilia and, as a consequence, interference occurs between the movements of adjacent cilia, creating a hydrodynamic linkage.
Amoeboid movement
Amoeboid movement is achieved by pseudopodia and involves the flow of cytoplasm as extensions of the organism. The process is visible under the light microscope as a movement of granules within the organism. The basic locomotory organelle is the pseudopodium. The way in which movement is effected can vary slightly among groups but generally involves the polymerization of cytoskeletal proteins (actin and myosin) at the leading edge of the pseudopod, followed by the flow of cytoplasmic material into the vacancy produced through the polymerization process. The flow of cytoplasm provides the momentum necessary to propel the organism further in its direction of movement. Additional forces driving the amoeboid movement involve the “eupodium,” which extends into a potential substrate for a grab-like traction, similar to a tank tread. Pushing force is also generated in the posterior end of the organism by contractions of the cytoskeletal proteins.
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