observe the sliding of microtubule delts that is responsible for the movement of a sperm cell flagellum. Tiny gold beads, 40 nanometers diameter, were attached to opposite sides of this flagellum, and ATP was added to stimulate movement. Notice how the beads move relative to each other, demonstrating the microtubule sliding action that causes the flagellum to bend. Credit, animation, Charles J. Broca, California Institute of Technology, Broca CJJ CellBile 114-1201-1215-1991.