Which whip-like structure on some cells is used for movement?

• A. Flagella
• B. Cilia
• C. Pseudopod
• D. Trichocyst

Answer: (A)

Whip-like cellular projections used for movement are flagella. A flagellum is a long, slender extension that propels the cell by bending or rotating, acting like a propeller to push the organism through fluid. In many bacteria, flagella rotate via a motor at the base, spinning the tail to move forward. In many eukaryotic cells, flagella beat in a whip-like fashion because of microtubule structures arranged in a 9+2 pattern and powered by dynein motors. This movement is distinct from cilia, which are shorter and present in large numbers that beat in coordinated waves to move the cell or fluids around it. A pseudopod, by contrast, is a temporary cytoplasmic extension used for crawling, not a whip-like projection. Trichocysts are defense-related organelles, not structures for locomotion. So the correct movement-propelling whip-like structure is the flagellum.