VISUAL ANALOGY

HOW CELLS MOVE LIKE BOATS

FIGURE 21–5 The forward motion provided by cilia or some flagella is similar to two ways by which oars propel a boat.

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Cilia and Flagella Many protists move by means of cilia (SIL ee uh) and flagella (fluh JEL uh), structures supported by microtubules. Cilia and flagella have nearly identical internal structures, but they produce cellular motion differently. Cilia (singular: cilium) are short and numerous, and they move somewhat like oars on a boat. Flagella (singular: flagellum) are relatively long and usually number only one or two per cell. Some flagella spin like tiny propellers, but most produce a wavelike motion from base to tip. Compare these two types of motion in Figure 21–5. Protists that move using cilia are known as ciliates, and those that move with flagella are called flagellates.

Passive Movement It may surprise you to learn that some of the most important protists are nonmotile—they depend on air or water currents and other organisms to carry them around. These protists form reproductive cells called spores that can enter the cells of other organisms and live as parasites. Spore-forming protists include Plasmodium, which is carried by mosquitoes and causes malaria, and Cryptosporidium, which spreads through contaminated drinking water and causes severe intestinal disease.

In Your Notebook Look up the word roots for cilia and flagella, and write an explanation of how each term relates to its root.

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