cilia

Meaning: Cilia are short, hair-like projections of eukaryotic cells. Their core, the axoneme, is made of microtubules arranged as nine outer doublets around two central microtubules (9+2). Each cilium is anchored by a basal body, a centriole-like structure of nine triplets. Dynein arms use ATP to slide neighboring doublets past each other, which bends the cilium. Motile cilia beat in coordinated waves to move fluid or particles across a cell surface, as in the respiratory tract (moving mucus) and the oviduct (moving the egg). Many cells also have a single nonmotile primary cilium, usually 9+0, that works as a sensory antenna.

On the MCAT: A passage may describe a dynein defect or a drug that disrupts microtubules and ask what fails: ciliary beating, mucus clearance, or sperm motility. You may also be asked to name the cytoskeletal element (microtubules, not actin or intermediate filaments) or to read a cross-section micrograph showing the 9+2 pattern.

Common confusion: Eukaryotic flagella share the 9+2 structure and dynein but are longer and fewer in number, as in the sperm tail. Bacterial flagella are a different structure. They are built of flagellin, have no microtubules or dynein, and rotate using a proton gradient.