Key Concepts of Section 18.5

• Cilia and flagella are microtubule-based cell-surface structures with a characteristic central pair of singlet microtubules and nine sets of outer doublet microtubules (see Figure 18-31).

• All cilia and flagella grow from basal bodies, structures with nine sets of outer triplet microtubules that are closely related to centrioles.

• Axonemal dyneins attached to the A tubule on one doublet microtubule interact with the B tubule of another to bend cilia and flagella.

• Cilia and flagella have a mechanism, intraflagellar transport (IFT), in which material is transported to the tip by kinesin-2 and from the tip back to the base by cytoplasmic dynein. IFT regulates the function and length of cilia and flagella.

• Many cells have on their surface a single nonmotile primary cilium, which lacks the normal central pair of microtubules and the dynein arms of motile cilia. The primary cilium functions as a sensory organelle, with receptors for extracellular signals localized to its plasma membrane. Due to its sensory function, many diseases result from defects in receptor localization or in the structure of the primary cilium itself.