When cilia (singular = cilium) are present, however, they are many in number and extend along the entire surface of the plasma membrane. When present, the cell has just one flagellum or a few flagella. Flagella and Ciliaįlagella (singular = flagellum) are long, hair-like structures that extend from the plasma membrane and are used to move an entire cell, (for example, sperm, Euglena). However, the exact function of the centrioles in cell division is not clear, since cells that have the centrioles removed can still divide, and plant cells, which lack centrioles, are capable of cell division. The centrosome replicates itself before a cell divides, and the centrioles play a role in pulling the duplicated chromosomes to opposite ends of the dividing cell. Each centriole is a cylinder of nine triplets of microtubules. It contains a pair of centrioles, two structures that lie perpendicular to each other. The centrosome is a region near the nucleus of animal cells that functions as a microtubule-organizing center.

In cilia and flagella, the microtubules are organized as a circle of nine double microtubules on the outside and two microtubules in the center.

They are also the structural components of flagella and cilia. Microtubules guide organelle movement and are the structures that pull chromosomes to their poles during cell division. These are hollow tubes that can dissolve and reform quickly. Microtubules are the thickest of the cytoskeletal fibers. Keratin, the compound that strengthens hair and nails, forms one type of intermediate filament. Intermediate filaments are of intermediate diameter and have structural functions, such as maintaining the shape of the cell and anchoring organelles. These components are also common in muscle cells and are responsible for muscle cell contraction. They also maintain the structure of microvilli, the extensive folding of the plasma membrane found in cells dedicated to absorption.

Microfilaments are the thinnest of the cytoskeletal fibers and function in moving cellular components, for example, during cell division. Microfilaments, intermediate filaments, and microtubules compose a cell’s cytoskeleton.

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