Intermediate filaments are between eight and twelve nanometers in diameter. They are made from proteins like keratin, lamin, desmin, and vimentin. Lamins are found in the nucleus but not in the cytoplasm, helping to support the nuclear envelope. These are the structures that give the cell its basic shape. Microtubules are the biggest and thickest of the fibers of the cytoskeleton, being about 23 nanometers thick. These are hollow tubes that consist of both alpha and beta tubulin. It is the microtubules that make the flagella of the cell and the cilia. The centrosome is an organelle that has microtubules sticking out of it. The centrosome is a microtubule organizing center. It separates the sister chromatids during the process of cell division. They also participate in transporting molecules inside the cell. They help to form cell walls in plant cells. In general, the cytoskeleton functions in several ways. Cells get their shape—even those without cell walls. Cells can move because of the cytoskeleton. Cilia and flagella could not exist and participate in movement without the cytoskeleton. Cells are organized because of the organelle placement by the cytoskeleton. Endocytosis, for example, happens because of the pull of microfilaments that take in the vesicle. Chromosomes move because of the cytoskeleton.
MICROFILAMENTS Microfilaments are also called actin filaments because they are mostly made of actin. These filaments extend throughout the cell so as to provide structure for the cell and to keep organelles in place. They participate in organelle movement, cell division, cell movement, and muscle contraction. Figure 54 shows what microfilaments look like:
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