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Transcript
Objectives Describe the role of the cytoskeleton in cell movement. Compare and contrast the functions of flagella and cilia. Explain why a cell can be described as a coordinated unit. Key Terms microtubule microfilament flagella cilia Some cells are capable of moving by extending parts of themselves and "oozing" from one place to another. Just as you have an internal skeleton that serves several functions in your body, a cell has its own kind of internal support system that enables it to move, support organelles, and maintain shape. The Cytoskeleton Biologists once thought that the organelles of a cell drifted about freely in the cytoplasm. However, improvements in microscopes and research techniques revealed a cytoskeleton (cyto means "cell"), a network of fibers extending throughout the cytoplasm. Unlike your body's skeleton, the skeleton of most cells does not keep the same structural pattern all the time. It is always changing, with new extensions building at the same time that others are breaking apart. Different kinds of fibers make up the cytoskeleton. Straight, hollow tubes of proteins that give rigidity, shape, and organization to a cell are called microtubules. As protein subunits are added or subtracted from the microtubules, these structures lengthen or shorten. One function of microtubules is to provide "tracks" along which other organelles can move. For example, a lysosome might reach a food vacuole by moving along a microtubule. Thinner, solid rods of protein called microfilaments enable the cell to move or change shape when protein subunits slide past one another. This process contributes to the oozing Staff Wednesday, October 19, 2011 9:15:48 AM CT movements of an amoeba and some white blood cells. Flagella and Cilia Unlike an amoeba that moves as changes occur to microfilaments in its cytoplasm, many other kinds of cells move as a result of the action of specialized structures that project from the cell. Flagella (singular, flagellum) are long, thin, whip-like structures, with a core of microtubules, that enable some cells to move. A flagellum usually waves with an "S"-shaped motion that propels the cell. Cilia (singular, cilium) are generally shorter and more numerous than flagella. Like flagella, cilia also contain bundles of microtubules, but cilia have a back-and-forth motion—something like the oars of a rowboat—that moves a cell through its surroundings. Cilia or flagella can also extend out from stationary cells that are held in place as part of a layer of tissue in a multicellular organism. Here, their motion moves fluid over the surface of the tissue. For example, the cells lining your windpipe have cilia that sweep mucus with trapped debris out of your lungs. This sweeping action helps keep your respiratory system clean and allows air to flow through it smoothly. The Cell as a Coordinated Unit From the overview of a cell's organization to a close-up inspection of each organelle's architecture, this tour of the cell has provided many opportunities to connect structure with function. As you study the parts of a cell, remember that none of its organelles works alone. Consider the white blood cell's role in helping defend the body against infections by ingesting bacteria. The cell moves toward the bacteria using thin cytoplasmic extensions created by the interaction of parts of the cytoskeleton. After the cell engulfs the bacteria, they are destroyed by lysosomes that were produced by the ER and Golgi apparatus. Ribosomes made the proteins of the cytoskeleton and the enzymes within the lysosomes. And the production of these proteins was programmed by messages dispatched from the DNA in the nucleus. All these processes require energy, which mitochondria supply in the form of ATP. The cooperation of cellular organelles makes a cell a living unit that is greater than the sum of its parts. Concept Check 6.6 1. How do microfilaments function in the cytoskeleton of a cell? 2. How do flagella differ in structure and function from cilia? 3. Give an example of coordination within a cell. Staff Wednesday, October 19, 2011 9:15:48 AM CT Copyright © 2006 by Pearson Education, Inc., publishing as Pearson Prentice Hall. All rights reserved. Staff Wednesday, October 19, 2011 9:15:48 AM CT