What are the small hairs on a cell's surface that move back and forth to enable movement?

• A. Cilia
• B. Pseudopods
• C. Flagella
• D. Microvilli

Answer: (A)

Motile surface projections that are short and numerous are cilia. They beat in a coordinated back-and-forth motion, either propelling the cell or moving liquids across the cell surface. This rhythmic movement comes from dynein motor proteins that cause the microtubules inside each cilium to slide past one another, producing a bending wave. The description in the question—small hairs on a cell’s surface that move back and forth to enable movement—matches cilia.

Pseudopods, by contrast, are temporary, blob-like extensions that cells extend to crawl or engulf prey; they aren’t hair-like or regularly beating. Flagella are longer, whip-like tails used by some cells to swim, and there are usually only one or a few of them. Microvilli are tiny projections that increase surface area for absorption, not for movement.