The Cytoskeleton

The cytoskeleton is a collection of protein filaments and cylinders that determine the shape of a cell, lend it structural support, organize its contents, move substances through the cell, and contribute to movements of the cell as a whole. It can form a very dense supportive scaffold in the cytoplasm (fig. 3.31). It is connected to integral proteins of the plasma membrane, and they in turn are connected to protein fibers external to the cell, so there is a strong structural continuity from extracellular material to the cytoplasm. Cytoskeletal elements may even connect to chromosomes in the nucleus, enabling physical tension on a cell to move nuclear contents and mechanically stimulate genetic function.

The cytoskeleton is composed of microfilaments, intermediate filaments, and microtubules. Microfilaments are about 6 nm thick and are made of the protein actin. They form a network on the cytoplasmic side of the plasma membrane called the membrane skeleton. The phospholipids of the plasma membrane spread out over the membrane skeleton like butter on a slice of bread. It is thought that the phos-pholipids would break up into little droplets without this support. The roles of actin in supporting microvilli and producing cell movements were discussed earlier. In conjunction with another protein, myosin, microfilaments are also responsible for muscle contraction.

Intermediate filaments (8-10 nm in diameter) are thicker and stiffer than microfilaments. They resist

Figure 3.29 A Mitochondrion.

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Physiology: The Unity of Function Companies, 2003 Form and Function, Third Edition

Figure 3.30 Centrioles. (a) Electron micrograph of a centriole as seen in cross section. (b) A pair of perpendicular centrioles. How many microtubules does a centriole have?

Chapter 3 Cellular Form and Function 121

stresses placed on a cell and participate in junctions that attach some cells to their neighbors. In epidermal cells, they are made of the tough protein keratin and occupy most of the cytoplasm.

A microtubule (25 nm in diameter) is a cylinder made of 13 parallel strands called protofilaments. Each protofilament is a long chain of globular proteins called tubulin (fig. 3.32). Microtubules radiate from the centrosome and hold organelles in place, form bundles that maintain cell shape and rigidity, and act somewhat like railroad tracks to guide organelles and molecules to specific destinations in a cell. They form the axonemes of cilia and flagella and are responsible for their beating movements. They also form the mitotic spindle that guides chromosome movement during cell division. Microtubules are not permanent structures. They come and go moment by moment as tubu-lin molecules assemble into a tubule and then suddenly break apart again to be used somewhere else in the cell. The double and triple sets of microtubules in cilia, flagella, basal bodies, and centrioles, however, are more stable.

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