Cytoskeleton
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Means “cell skeleton”
Internal framework of cell
Has many functions

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Anchoring cell organelles
Provide cell shape
Aids in cell motility
Response to environmental signals
Comprises
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Microtubules
Microfilaments
Intermediate filaments
Microtubules

Hollow tubes made of the protein tubulin
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Alternating dimers of a and b tubulin
Largest of cytoskeleton filaments
Is used for:


Maintenance of cell shape
Motility
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Movement of organelles through cell
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Flagella or cilia
Often involves motor molecule
Often originate from centrosome
Table 6-1a
10 µm
Column of tubulin dimers
25 nm
a
b
Tubulin dimer
Centrioles
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Located in centrosome of animal cells
Occur in perpedicular pair
Have 9 triplets of microtubules
Facilitate microtubule assembly and
chromosome separation in some cells
Fig. 6-22
Centrosome
Microtubule
Centrioles
0.25 µm
Longitudinal section Microtubules Cross section
of one centriole
of the other centriole
Flagellum structure
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Basal body links flagellum or cilia to cell surface
 Basal body looks just like a centriole
9 +2 arrangement of microtubules
Radial spokes prevent dramatic sliding and only bending
Fig. 6-24
Outer microtubule
doublet
0.1 µm
Dynein proteins
Central
microtubule
Radial
spoke
Protein crosslinking outer
doublets
Microtubules
Plasma
membrane
(b) Cross section of
cilium
Basal body
0.5 µm
(a) Longitudinal
section of cilium
0.1 µm
Triplet
(c) Cross section of basal body
Plasma
membrane
Motor molecules


Interact with tubulin or actin
Are fixed at one end and
allowed to move freely at the
other end

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Undulation-used for flagella
and cilia movement

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Movement is directional
Two microtubules moving
relative to one another
Organelle movement is like a
ski lift tram or a monorail
Fig. 6-21
ATP
Vesicle
Receptor for
motor protein
Motor protein Microtubule
(ATP powered) of cytoskeleton
(a)
Microtubule
(b)
Vesicles
0.25 µm
Cell motility

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Cell movement facilitated by flagella or cilia
Unlike in prokaryotes, eukaryotic flagella undulate
Cilia are small appendages and they move like a
swimmers arm-active stroke and return stroke
How cell movement works


Dynein is motor molecule that interacts with
tubulin
Dynein walks along one microtubule, while
bound to another
This results in bending
 If no radial spokes or organelle coat, then
microtubules would walk out of cell

Fig. 6-25
Microtubule
doublets
ATP
Dynein
protein
(a) Effect of unrestrained dynein movement
ATP
Cross-linking proteins
inside outer doublets
Anchorage
in cell
(b) Effect of cross-linking proteins
1
3
2
(c) Wavelike motion
Microfilaments
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Made of two intertwined strands of actin
Helps maintain cell shape
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Actin rearrangements allow engulfment events
Psuedopod formation in ameoba
Promote cytoplasmic streaming in plants
Essential for muscle contraction
Used by invading bacteria to move around cell
Frequently being assembled and disassembled
within cell
Table 6-1b
10 µm
Actin subunit
7 nm
Microfilaments 2


Myosin interacts with actin to
cause contraction
Cytoplasmic streaming and
ameoboid motion are similar
Cortical cytoplasm around the
perimiter of cell contains
perpendicular actin (wind fence)
 Streaming portion has parallel
actin which facilitates cytoplasm
movement
 Plant cell wall prevents
amoeboid movement of plant cell

Intermediate Filaments
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Resemble cable in structure
Are made of protein subunits
Help maintain cell shape
Are durable and not assembled and
disassembled as other cytoskeleton
components
May help maintain organelle position
Table 6-1c
5 µm
Keratin proteins
Fibrous subunit (keratins
coiled together)
8–12 nm