Final answer:
The long, threadlike structures protruding from the surface of bacteria involved in movement are called flagella, acting like propellers to propel the bacteria through liquid environments.
Step-by-step explanation:
The long, threadlike structures that protrude from the surface of bacteria and are involved in cellular movement are called flagella. Bacterial flagella act like propellers, wherein they rotate to move the bacteria forward, giving it mobility to travel through its environment. They are made from flagellin protein subunits and look like a tiny whip. In contrast, cilia are short, hair-like structures that can also be involved in cellular movement or in moving substances along the cell's outer surface but are more numerous and commonly found in eukaryotic cells.
Flagella are critical for the motility of certain bacteria, allowing the cell to roll and tumble in aqueous environments. These structures provide a physical means by which bacterial cells can traverse through liquids. Some bacteria may have a single flagellum (monotrichous), several at one end (lophotrichous), or spread around the cell's surface (peritrichous).