Microbiology, 2021

122 3 • The Cell
Figure 3.56
The extracellular matrix is composed of protein and carbohydrate components. It protects cells from physical stresses and
transmits signals arriving at the outside edges of the tissue to cells deeper within the tissue.
Flagella and Cilia
Some eukaryotic cells use flagella for locomotion; however, eukaryotic flagella are structurally distinct from
those found in prokaryotic cells. Whereas the prokaryotic flagellum is a stiff, rotating structure, a eukaryotic
flagellum is more like a flexible whip composed of nine parallel pairs of microtubules surrounding a central
pair of microtubules. This arrangement is referred to as a 9+2 array (Figure 3.57). The parallel microtubules
use dynein motor proteins to move relative to each other, causing the flagellum to bend.
Cilia (singular: cilium) are a similar external structure found in some eukaryotic cells. Unique to eukaryotes,
cilia are shorter than flagella and often cover the entire surface of a cell; however, they are structurally similar
to flagella (a 9+2 array of microtubules) and use the same mechanism for movement. A structure called a basal
body is found at the base of each cilium and flagellum. The basal body, which attaches the cilium or flagellum
to the cell, is composed of an array of triplet microtubules similar to that of a centriole but embedded in the
plasma membrane. Because of their shorter length, cilia use a rapid, flexible, waving motion. In addition to
motility, cilia may have other functions such as sweeping particles past or into cells. For example, ciliated
protozoans use the sweeping of cilia to move food particles into their mouthparts, and ciliated cells in the
mammalian respiratory tract beat in synchrony to sweep mucus and debris up and out of the lungs (Figure
3.57).
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