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Animal cells move. Even in a complex multicellular organism such as a human, many cells are on the move all the time. During development, for example, cells produced in one region of the embryo crawl to other locations before settling down and specializing. After the individual has matured, scavenging macrophages constantly crawl around the spaces in our body and between our cells looking out for dangerous bacterial or other invading organisms. At the time of reproduction, sperm cells swim fantastic distances in search of an egg to fertilize. All of these activities, and others, require cells to have the power of movement.

To move, cells must change shape, and to change shape requires the force of shape changing molecules. Filaments within cells like Amoeba run the length of the cell and carry vesicles of material to the leading edge. In a burst of activity the front of the cell fuses with these microfilament containing vesicles and cause an outward movement and a thrusting forward. The membrane attaches to the surface beneath and back at the trailing edge the membrane is released from the surface. Here, vesicles are pinched off, pulling the membrane inwards and pulling the cell forwards. The net effect is to move the whole cell in the required direction .

More rapid movements can be accomplished by using specialized organelles which extend from the surface of a cell.
Cilia are short projections from the cell surface that are filled along their length with microtubules. Sliding microtubules past one another provides a bending action that causes the whole cilia projection to change its shape. Thousands of cilia sticking out of the surface of a cell beat like oars, propelling the water over the cell surface. This can either result in the cell moving rapidly in the opposite direction (as seen in Paramecium, or in a cleansing action such as that seen when cilia sweep unwanted particles out of our lungs.

But the champion of movement inducing organelles is the flagellum. These projections from the cell surface can extend for great distances and are powered by central 9 + 2 arrays microtubules arranged in circles. Using an enzyme called dynein, energy is taken from ATP and used to change shape. As this dynein molecule is connected across two microtubules, this shape change causes the microtubules to bend and hence the flagellum to bend. Acting like a long whip, the bending flagellum can either spin like a corkscrew or thrust in a wave motion. Either action forces the water in a single direction, thus moving the cell in the opposite direction.

Science@a Distance
© 2002, Professor John Blamire