Which structure is primarily used for motility in bacteria?

• A. Endospore
• B. Capsule
• C. Flagella
• D. Pili

Answer: (C)

Motility in bacteria is primarily achieved by flagella, which are long, whip-like appendages that rotate like propellers to push the cell through liquid. The flagellum is powered by a flow of ions across the cell membrane (usually protons or sometimes sodium ions) through a motor in the basal body, driving rotation. This rotation enables smooth swimming and, by changing direction, a run-and-tumble type of movement that helps the bacterium navigate toward favorable environments or away from harmful ones.

Endospores are dormant structures formed for survival under stress, not for movement. Capsules are protective, slimey layers outside the cell wall that aid protection and adhesion, not propulsion. Pili mainly function in attachment, DNA transfer between cells, and, in some cases, limited surface movement known as twitching motility, but the classic, primary means of motility across many bacteria is the flagellum.