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Cillia

Microtubules

Cillia

Microtubules

figure 4-19

A SEM of a paramecium shows cilia on the surface of the cell (a). A TEM of a cross section of those cilia (b) reveals the internal structure of the cilia. The characteristic 9+2 configuration of microtubules can be clearly seen (c).

Cilia and Flagella

Cilia (SIL-ee-uh) and flagella (fluh-JEL-uh) are hairlike structures that extend from the surface of the cell, where they assist in movement. Cilia are short and are present in large numbers on certain cells, whereas flagella are longer and are far less numerous on the cells where they occur. Cilia and flagella have a membrane on their outer surface and an internal structure of nine pairs of micro-tubules around two central tubules, as Figure 4-19 shows.

Cilia on cells in the inner ear vibrate and help detect sound. Cilia cover the surfaces of many protists and "row" the protists through water like thousands of oars. On other protists, cilia sweep water and food particles into a mouthlike opening. Many kinds of protists use flagella to propel themselves, as do human sperm cells.

Centrioles

Centrioles consist of two short cylinders of microtubules at right angles to each other and are situated in the cytoplasm near the nuclear envelope. Centrioles occur in animal cells, where they organize the microtubules of the cytoskeleton during cell division, as shown in Figure 4-20. Plant cells lack centrioles. Basal bodies have the same structure that centrioles do. Basal bodies are found at the base of cilia and flagella and appear to organize the development of cilia and flagella.

figure 4-19

A SEM of a paramecium shows cilia on the surface of the cell (a). A TEM of a cross section of those cilia (b) reveals the internal structure of the cilia. The characteristic 9+2 configuration of microtubules can be clearly seen (c).

figure 4-20

During cell division, centrioles organize microtubules that pull the chromosomes (orange) apart. The centrioles are at the center of rays of microtubules, which have been stained green with a fluorescent dye.

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