The ability to move, either toward, or away from, an increasing chemical concentration, is a coordinated activity that many single-cell organisms can do. Single-cell animals, and bacteria, typically have some mechanical means of movement. Some bacteria use long external whip-like filaments called flagella. Flagella are rotated by a molecular motor to cause propulsion through water. The larger single-cell animals may use flagella similar to bacteria, or they may have rows of short filaments called cilia, which work like oars, or they may move about as amebas do.
Amebas move by extruding themselves in the direction they want to go.
This pattern of movement is followed until the presence of food is detected. The bacterium can detect molecules in the water that indicate the presence of food. When the bacterium moves in a straight line, it continues longer in that direction if the concentration of these molecules is increasing. Conversely, if the concentration is decreasing, it stops its movement sooner, and changes direction. Eventually, this strategy gets the bacterium to a nearby food source.