How do microvilli and cilia differ in structure and function?

• A. Microvilli are motile and move mucus
• B. Microvilli and cilia are both primarily for structural support
• C. Cilia increase surface area for absorption
• D. Microvilli increase surface area for absorption; cilia are motile to move mucus and fluids

Answer: (D)

Microvilli and cilia serve distinct roles on epithelial surfaces. Microvilli are tiny, fingerlike projections of the plasma membrane supported by actin filaments; their main job is to increase surface area to boost absorption and secretion, as seen in the intestinal lining and kidney tubules. They are not motile.

Cilia are longer, hairlike structures that beat in coordinated waves to move mucus, fluids, or debris across the surface; their movement is powered by microtubule-based axonemes with dynein motors. This motion helps clear mucus from airways and keep pathways open.

So the best statement is that microvilli increase surface area for absorption, while cilia are motile to move mucus and fluids. The other options misstate either the role of microvilli, the function of cilia, or both.