Which structure is responsible for motility in prokaryotic cells?

The structure primarily responsible for motility in prokaryotic cells is flagella. Flagella are long, whip-like appendages that extend from the surface of the cell membrane and rotate to propel the cell through liquid environments, allowing for movement toward nutrients or away from harmful substances. Their unique structure, often comprised of a protein called flagellin, enables the flagellum to function effectively as a motor.

In contrast, the cell wall provides shape and protection to the cell but does not facilitate movement. The cytoplasm is the gel-like substance within the cell where various cellular processes occur, but it is not directly involved in propulsion. Pili, while they can aid in attachment to surfaces and in some instances in movement (like in twitching), are not the primary structures used for swimming motility. Thus, flagella are the key structures that grant prokaryotic cells their ability to move through their environments.