Essential Cell Biology 5th edition

Microtubules
591
Figures 17–26 Flagella propel a cell through fluid using
repetitive wavelike motion. The movement of a single
flagellum on an invertebrate sperm is seen in a series of images
captured by stroboscopic illumination at 400 flashes per second.
(Courtesy of Charles J. Brokaw.)
the entire cell, rather than moving fluid across the cell surface. Flagella
propagate regular waves along their length, propelling the attached cell
along (Figure 17–26).
Despite these slight differences in operation, cilia and flagella share a
similar internal structure. The microtubules in both cilia and flagella are
arranged in an elaborate and distinctive pattern, which was one of the
most striking revelations of early electron microscopy. A cross section
through a cilium shows nine doublet microtubules arranged in a ring
around a pair of single microtubules (Figure 17–27A). This “9 + 2” array
is characteristic of almost all eukaryotic cilia and flagella—from those of
protozoa to those in humans.
The movement of a cilium or a flagellum is produced by bending that
takes place as its microtubules slide against each other. The microtubules
are associated with numerous accessory proteins (Figure 17–27B),
which project at regular positions along the length of the microtubule
bundle. Some of these proteins serve as cross-links to hold the bundle of
microtubules together; others generate the force that causes the cilium
or flagellum to bend.
The most important of the accessory proteins is the motor protein ciliary
dynein, which generates the bending motion of the structure. Ciliary
dynein is attached by its tail to one microtubule in each outer doublet,
while its globular heads interact with the adjacent microtubule to generate
a sliding force between the two microtubules. Because of the multiple
links that hold the adjacent microtubule doublets together, the sliding
force between adjacent microtubules is converted to a bending motion
(Figure 17–28). Other ciliary components, including the central pair,
inner sheath, and radial spokes, control dynein activity, leading to the
complex wave forms seen in cilia and flagella.
outer dynein arm
ECB5 e17.25/17.26
25 µm
radial spoke
inner dynein arm
inner sheath
central singlet
microtubule
linking
protein
plasma membrane
(A)
100 nm
(B)
A microtubule B microtubule
outer doublet microtubule
Figure 17–27 Microtubules in a cilium or flagellum are arranged in a “9 + 2” array. (A) Electron micrograph of a flagellum of the
unicellular alga Chlamydomonas shown in cross section, illustrating the distinctive 9 + 2 arrangement of microtubules. (B) Diagram of
the flagellum in cross section. The nine outer microtubules (each a special paired structure) carry two rows of dynein molecules. The
heads of each dynein molecule appear in this view like arms reaching toward the adjacent doublet microtubule. In a living cilium, these
dynein heads periodically make contact with the adjacent doublet microtubule and move along it, thereby producing the force for
ciliary beating. The various other links and projections shown are proteins that serve to hold the bundle of microtubules together and to
convert the sliding force produced by dyneins into bending, as illustrated in Figure 17–28. (A, courtesy of Lewis Tilney.)