the length of the tumbles is reduced, allowing overall movement toward the higher concentration of the attractant.
When no chemical gradient exists, the lengths of runs and tumbles are more equal, and overall movement is more
random (Figure 3.35).
Figure 3.34 Bacteria achieve directional movement by changing the rotation of their flagella. In a cell with
peritrichous flagella, the flagella bundle when they rotate in a counterclockwise direction, resulting in a run. However,
when the flagella rotate in a clockwise direction, the flagella are no longer bundled, resulting in tumbles.
Figure 3.35 Without a chemical gradient, flagellar rotation cycles between counterclockwise (run) and clockwise
(tumble) with no overall directional movement. However, when a chemical gradient of an attractant exists, the length
of runs is extended, while the length of tumbles is decreased. This leads to chemotaxis: an overall directional
movement toward the higher concentration of the attractant.
112 Chapter 3 | The Cell
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