Biological Physics: Energy, Information, Life

434 Chapter 11. Machines in membranes[[Student version, January 17, 2003]]

marker

outer segment

inner segment

b

0.2

0.4

0.6

0.8

1

1.2

1.4

-160 -120 -80 -40 0

speed, reovlutions/

s

protonmotive force, mV

c

Figure 11.11: (Photomicrograph; schematic; experimental data.) Experiment to show that the flagellar motor
runs on proton-motive force. (a)Micropipette tip used to study the bacterial flagellar motor. (b)Micropipette with
apartially inserted bacterium. Dashed lines represent the part of the cell wall permeabilized by cephalexin. [Image
kindly supplied by H. C. Berg; see Fung & Berg, 1995.] (c)Flagellar motor speed versus the proton-motive force
across the part of the membrane containing the motor.

second; each revolution requires the passage of about 1000 protons. The Excursion to this chapter
describes a remarkable experiment showing directly the relation between proton-motive force and
torque generation in this motor.

11.4 Excursion: “Powering up the flagellar motor” by H. C. Berg and D. Fung

C. Berg and D. Fung

Flagellar rotary motors are driven by protons or sodium ions that flow from the outside to the
inside of a bacterial cell.E. coliuses protons. If the pH of the external medium is lower than that
of the internal medium, protons move inward by diffusion. If the electrical potential of the external
medium is higher than that of the internal medium, they are driven in by a transmembrane electric
field. We thought that it would be instructive to power up the flagellar motor with an external
voltage source, for example a laboratory power supply.^10 E. coliis rather small, less than 1μm
in diameter by about 2μmlong. And its inner membrane, the one that needs to be energized, is
enclosed by a cell wall and porous outer membrane. Thus, it is difficult to insert a micropipette
into a cell. But one can put a cell into a micropipette.
First, we grew cells in the presence of a penicillin analog called cephalexin: This suppresses
septation (formation of new cell walls between the halves of a dividing cell). The cells then just
grow longer without dividing—they become filamentous, like snakes. Then we attached inert mark-
ers (dead cells of normal size) to one or more of their flagella. We learned how to make glass
micropipettes with narrow constrictions (Figure 11.11a). Then by suction we pulled a snake about
half way into the pipette, as shown schematically in panel b of the figure. The pipette contained

(^10) Actually, we used a voltage clamp; see Section 12.3.1 on page 465.