What extension of the cell membrane aids in propelling substances across cell surfaces?

The extension of the cell membrane that aids in propelling substances across cell surfaces is primarily represented by cilia, particularly in the respiratory system. Cilia are tiny, hair-like structures that protrude from the surface of certain cells. They move in a coordinated manner, creating a wave-like motion that effectively sweeps mucus, trapped particles, and pathogens out of the lungs and toward the throat. This function is essential for maintaining clear airways and ensuring proper respiratory function.

Cilia are specifically adapted for this purpose, allowing the movement of substances over the epithelial surface, which is crucial for functions such as mucociliary clearance in the respiratory tract. In contrast, microvilli primarily serve to increase surface area for absorption rather than propulsion. Flagella are typically associated with motility in sperm cells and are not involved in propelling substances across surfaces. Filopodia, on the other hand, are slender projections that play a role in sensing the environment and cell movement, but they do not have the same function as cilia in the context of propulsion across surfaces.