Final answer:
Bacteria use runs and tumbles during chemotaxis to move towards more favorable environments: running in straight lines when sensing an attractant and tumbling to change direction when moving away from it.
Step-by-step explanation:
Runs and tumbles are the mechanisms used by bacteria to move towards more favorable environments, a movement known as chemotaxis. When a bacterium senses a chemical gradient of an attractant, it moves in a series of runs and tumbles to find the optimal path towards a higher concentration of the chemical.
During a run, the bacterium moves in a straight line, and its flagella rotate counterclockwise in a coordinated fashion, propelling the bacterium forward.
If the bacterium determines it is not moving toward a more desirable location, it performs a tumble, during which the flagella rotate clockwise and cause the bacterium to stop and change direction.
By alternating between runs, which are longer in the direction of the attractant, and shorter tumbles when moving away from it, the bacterium biases its movement toward areas of higher attractant concentration, effectively moving to a more desirable location.