Chapter 3: Cilia and Microvilli 3.8

Cilia

"eyelashes" whiplike, motile cellular extension that occur, typically in large numbers, on the exposed surfaces of certain cells. _____ry action moves substances in one direction across cell surfaces. For example: _____ted cells that line the respiratory tract propel mucus laden with dust particles and bacteria upward away from the lungs.

Microvilli

"little shaggy hairs" are tiny, fingerlike extensions of the plasma membrane that project from an exposed cell surface. They increase the plasma membrane surface area tremendously and are most often found on the surface of absorptive cells such as intestinal and kidney tubule cells. They have a core of bundled actin filaments that extend into a mat of actin filaments, called the terminal web, near the surface of the cell. ( Actin is often a contractile protein, but in microvilli and the terminal web it acts as a mechanical "stiffener" that shapes the cell.)

basal bodies

centrioles forming the bases of cilia and flagella. The "9+2" pattern of microtubules in the cilium or flagellum itself ( nine doublets, or pairs, of microtubules encircling one central pair) differs slightly from that of centriole (nine microtubule triplets). Additionally, the cilium has flexible "wagon wheels" of cross-linking proteins. Extending from the microtubule doublets are arms composed of the motor protein dynein. The dynein arms produce the movement of cilium and flagellum. Arms of one doublet grip the adjacent doublet and, powered by ATP, push it up, release, and then grip again. Doublets are physically restricted by other proteins, they are forced to bend. The collective bending action of all the doublets causes the cilium to bend.

cilium moves

it alternates rhythmically between a propulsive power stroke, when it is nearly straight and moves in an arc, and a recovery stroke, when it bends and returns to its initial position. with these two strokes, the cilium produces a pushing motion in a single direction that repeats some 10 to 20 times per second. The bending of one cilium is quickly followed by the bending of the next and then the next, creating a current at the cell surface that resembles the traveling waves that pass across a field of grass on a windy day.

Flagella

projections formed by centrioles, but are substantially longer than cilia. the only ______ted cell in the human body is the sperm, which has one propulsive ______um, commonly called the tail. Cilia propel other substances across a cell's surface, where as _____ propels the cell itself.

When a cell is about to form cilia

the centrioles multiply and line up beneath the plasma membrane at the cell's free (exposed) surface. microtubules then "sprout" from each centriole, forming the _____ry projections by exerting pressure on the plasma membrane.