What structure on the surface of a bacterial cell is used for movement?

The structure on the surface of a bacterial cell that is primarily used for movement is the flagella. Flagella are long, whip-like appendages that rotate in a propeller-like motion, allowing bacteria to move through liquid environments. This type of movement is crucial for bacteria as it helps them to swim toward nutrients, escape from harmful substances, and migrate towards more favorable environments.

Other structures mentioned, such as pili, serve different functions like attachment to surfaces or other cells, and cilia are primarily found in eukaryotic cells for movement or feeding. The capsule, which is a protective layer surrounding some bacteria, plays a role in protecting the cell and in certain ways helps the bacteria to evade the immune response, but it does not function in movement. Understanding the specific roles of these structures clarifies why flagella are the primary means of motility for bacteria.