Download Flagellar Movement Although the precise mechanism by which

Flagellar Movement
Although the precise mechanism by which bacterial flagella move is not completely understood,
we do know that they rotate 360° like boat propellers rather than whipping from side to side. The
flow of hydrogen ions (H ) or of sodium ions (Na ) through the cytoplasmic membrane near the
basal body powers the rotation, propelling the bacterium through the environment at about 60
cell lengths per second—equivalent to a car traveling at 670 miles per hour! Flagella rotate at
more than 100,000 rpm and can change direction from counterclockwise to clockwise. Bacteria
move with a series of “runs” punctuated by “tumbles.” Counterclockwise flagellar rotation
produces runs, which are movements of a cell in a single direction for some time. If more than
one flagellum is present, the flagella align and rotate together as a bundle. Tumbles are abrupt,
random changes in direction resulting from clockwise flagellar rotation where each flagellum
rotates independently. Both runs and tumbles occur in response to stimuli. Receptors for light or
chemicals on the surface of the cell send signals to the flagella, which then adjust their speed and
direction of rotation. A bacterium can position itself in a more favorable environment by varying
the number and duration of runs and tumbles. The presence of favorable stimuli increases
the number of runs and decreases the number of tumbles; as a result, the cell tends to move
toward an attractant (Figure 3.9). Unfavorable stimuli increase the number of tumbles, which
increases the likelihood that it will move randomly in another direction, away from a repellant.
Movement in response to a stimulus is termed taxis. The stimulus may be either light
(phototaxis) or a chemical (chemotaxis). Movement toward a favorable stimulus is positive taxis,
whereas movement away from an unfavorable stimulus is negative taxis. For example,
movement toward a nutrient would be positive chemotaxis
Flagellar Movement
Although the precise mechanism by which bacterial flagella move is not completely understood,
we do know that they rotate 360° like boat propellers rather than whipping from side to side. The
flow of hydrogen ions (H ) or of sodium ions (Na ) through the cytoplasmic membrane near the
basal body powers the rotation, propelling the bacterium through the environment at about 60
cell lengths per second—equivalent to a car traveling at 670 miles per hour! Flagella rotate at
more than 100,000 rpm and can change direction from counterclockwise to clockwise. Bacteria
move with a series of “runs” punctuated by “tumbles.” Counterclockwise flagellar rotation
produces runs, which are movements of a cell in a single direction for some time. If more than
one flagellum is present, the flagella align and rotate together as a bundle. Tumbles are abrupt,
random changes in direction resulting from clockwise flagellar rotation where each flagellum
rotates independently. Both runs and tumbles occur in response to stimuli. Receptors for light or
chemicals on the surface of the cell send signals to the flagella, which then adjust their speed and
direction of rotation. A bacterium can position itself in a more favorable environment by varying
the number and duration of runs and tumbles. The presence of favorable stimuli increases
the number of runs and decreases the number of tumbles; as a result, the cell tends to move
toward an attractant (Figure 3.9). Unfavorable stimuli increase the number of tumbles, which
increases the likelihood that it will move randomly in another direction, away from a repellant.
Movement in response to a stimulus is termed taxis. The stimulus may be either light
(phototaxis) or a chemical (chemotaxis). Movement toward a favorable stimulus is positive taxis,
whereas movement away from an unfavorable stimulus is negative taxis. For example,
movement toward a nutrient would be positive chemotaxis