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Cellular Organelles 41 protofilament. Microtubules are hollow rod-like, may contain 10–15 protofilaments (Fig. 2.12). In addition to its other functions, it is also responsible for rhythmic movement of cilia and flagella. a– Tubulin a– Tubulin b– Tubulin 25 nm Heterodimers Proto▯lament b– Tubulin Fig. 2.12: Schematic representation of microtubules 2. Actin Filament— It is observed in skeletal muscle and plays a very important role in muscle contraction. It is richly found in the cytoplasm closer to the plasma membrane. Its main function is to provide strength to the cell, facilitating cytokinesis and cell movement. 3. Intermediate Filaments— These are strong filaments resembling a rope, primarily involved in providing mechanical strength to the cell. 2.9 C ilia and F lagella Cilia (Singular: cilium) and flagella (Singular: flagellum) are hair-like, microscopic, filamentous protoplasmic structures. Both cilia and flagella are involved in cell motility. Although cilia and flagella are morphologically and physiologically identical, but it can be differentiated on the basis of their size, number and function (Table 2.1). Cilia are smaller in size and present in membrane largenumberinaciliatedcellwhereas, flagella are longer in size and typically Peripheral vary from one or two in number. The microtubules (doublets) body of a Paramecium (unicellular Central protozoan) is fully covered by a few sheath Interdoublet thousands of cilia. Cells present in the bridge upper respiratory tract of mammals bear cilia to expel particulate matter present in inhaled air. A mammalian microtuble spermatozoancontainssingleflagellum, Radial while a unicellular Chlamydomonas, a spoke Fig. 2.13: Structure of eukaryotic cilium/flagellum green alga has two flagella. Reprint 2026-27