What cell structure is primarily involved in movement in single-celled organisms?

The structure primarily involved in movement in single-celled organisms is flagella. Flagella are long, whip-like projections that can rotate or whip back and forth, allowing the organism to propel itself through liquid environments, such as water. Many single-celled organisms, such as certain types of protozoa and bacteria, use flagella as their primary means of locomotion, enabling them to navigate toward nutrients or away from harmful conditions.

While cilia also play a role in movement, particularly in organisms like paramecia, they are typically shorter and cover the cell surface, working in a coordinated manner to create currents or aid in feeding, rather than driving forward movement in the same way flagella do. Pseudopodia are temporary projections that some amoeboid cells utilize for movement and engulfing food, but they are not as efficient for locomotion as flagella. The cell wall, found in many single-celled organisms like bacteria and algae, provides structural support and protection but does not contribute to movement.

Therefore, flagella are the defining structure for movement in many single-celled organisms, solidifying their importance in motility.