1. It is composed of three types of molecular structures:
Concept 6.6: The cytoskeleton is a network of fibers that organizes structures and activities in the cell
The cytoskeleton is a network of fibers extending throughout the cytoplasm
It organizes the cell’s structures and activities, anchoring many organelles
It is composed of three types of molecular structures:
Microtubules
Microfilaments
Intermediate filaments
For the Cell Biology Video The Cytoskeleton in a Neuron Growth Cone, go to Animation and Video Files
For the Cell Biology Video Cytoskeletal Protein Dynamics, go to Animation and Video Files.
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2. Microtubule Microfilaments 0.25 µm Fig. 6-20
Figure 6.20 The cytoskeleton
Microfilaments
0.25 µm

3. Roles of the Cytoskeleton: Support, Motility, and Regulation
The cytoskeleton helps to support the cell and maintain its shape
It interacts with motor proteins to produce motility
Inside the cell, vesicles can travel along “monorails” provided by the cytoskeleton
Recent evidence suggests that the cytoskeleton may help regulate biochemical activities
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4. Receptor for motor protein
Fig. 6-21
Vesicle
ATP
Receptor for motor protein
Motor protein (ATP powered)
Microtubule
of cytoskeleton
(a)
Microtubule
Vesicles
0.25 µm
Figure 6.21 Motor proteins and the cytoskeleton
(b)

5. Components of the Cytoskeleton
Three main types of fibers make up the cytoskeleton:
Microtubules are the thickest of the three components of the cytoskeleton
Microfilaments, also called actin filaments, are the thinnest components
Intermediate filaments are fibers with diameters in a middle range
For the Cell Biology Video Actin Network in Crawling Cells, go to Animation and Video Files.
For the Cell Biology Video Actin Visualization in Dendrites, go to Animation and Video Files.
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6. Table 6-1 Table 6-1 10 µm 10 µm 10 µm Column of tubulin dimers
Keratin proteins
Actin subunit
Fibrous subunit (keratins coiled together)
25 nm
7 nm
8–12 nm  
Tubulin dimer

7. 10 µm Column of tubulin dimers Tubulin dimer   25 nm Table 6-1a

8. Table 6-1b
10 µm
Table 6-1b
Actin subunit
7 nm

9. Fibrous subunit (keratins coiled together)
Table 6-1c
5 µm
Table 6-1c
Keratin proteins
Fibrous subunit (keratins
coiled together)
8–12 nm

10. Functions of microtubules:
Microtubules are hollow rods about 25 nm in diameter and about 200 nm to 25 microns long
Functions of microtubules:
Shaping the cell
Guiding movement of organelles
Separating chromosomes during cell division
For the Cell Biology Video Transport Along Microtubules, go to Animation and Video Files.
For the Cell Biology Video Movement of Organelles in Vivo, go to Animation and Video Files.
For the Cell Biology Video Movement of Organelles in Vitro, go to Animation and Video Files.
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11. Centrosomes and Centrioles
In many cells, microtubules grow out from a centrosome near the nucleus
The centrosome is a “microtubule-organizing center”
In animal cells, the centrosome has a pair of centrioles, each with nine triplets of microtubules arranged in a ring
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12. Longitudinal section of one centriole Microtubules Cross section
Fig. 6-22
Centrosome
Microtubule
Centrioles
0.25 µm
Figure 6.22 Centrosome containing a pair of centrioles
Longitudinal section of one centriole
Microtubules
Cross section
of the other centriole

13. Video: Paramecium Cilia
Cilia and Flagella
Microtubules control the beating of cilia and flagella, locomotor appendages of some cells
Cilia and flagella differ in their beating patterns
Video: Chlamydomonas
Video: Paramecium Cilia
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14. Direction of organism’s movement
Fig. 6-23
Direction of swimming
(a) Motion of flagella
5 µm
Direction of organism’s movement
Figure 6.23a A comparison of the beating of flagella and cilia—motion of flagella
Power stroke
Recovery stroke
(b) Motion of cilia
15 µm

15. Animation: Cilia and Flagella
Cilia and flagella share a common ultrastructure:
A core of microtubules sheathed by the plasma membrane
A basal body that anchors the cilium or flagellum
A motor protein called dynein, which drives the bending movements of a cilium or flagellum
Animation: Cilia and Flagella
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16. Cross section of cilium
Fig. 6-24
Outer microtubule doublet
Plasma membrane
0.1 µm
Dynein proteins
Central microtubule
Radial spoke
Protein cross-linking outer doublets
Microtubules
(b)
Cross section of cilium
Plasma membrane
Basal body
0.5 µm
(a)
Longitudinal section of cilium
0.1 µm
Figure 6.24 Ultrastructure of a eukaryotic flagellum or motile cilium
Triplet
(c) Cross section of basal body

17. How dynein “walking” moves flagella and cilia:
Dynein arms alternately grab, move, and release the outer microtubules
Protein cross-links limit sliding
Forces exerted by dynein arms cause doublets to curve, bending the cilium or flagellum
For the Cell Biology Video Motion of Isolated Flagellum, go to Animation and Video Files.
For the Cell Biology Video Flagellum Movement in Swimming Sperm, go to Animation and Video Files.
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18. Figure 6.25 How dynein “walking” moves flagella and cilia
Microtubule
doublets
ATP
Dynein
protein
(a) Effect of unrestrained dynein movement
ATP
Cross-linking proteins
inside outer doublets
Anchorage
in cell
Figure 6.25 How dynein “walking” moves flagella and cilia
(b) Effect of cross-linking proteins 1 3 2
(c) Wavelike motion

19. (a) Effect of unrestrained dynein movement
Fig. 6-25a
Microtubule doublets
ATP
Figure 6.25a How dynein “walking” moves flagella and cilia
Dynein protein
(a) Effect of unrestrained dynein movement

20. Cross-linking proteins inside outer doublets
Fig. 6-25b
ATP
Cross-linking proteins inside outer doublets
Anchorage in cell
(b) Effect of cross-linking proteins
Figure 6.25b, c How dynein “walking” moves flagella and cilia 1 3 2
(c) Wavelike motion

21. Microfilaments (Actin Filaments)
Microfilaments are solid rods about 7 nm in diameter, built as a twisted double chain of actin subunits
The structural role of microfilaments is to bear tension, resisting pulling forces within the cell
They form a 3-D network called the cortex just inside the plasma membrane to help support the cell’s shape
Bundles of microfilaments make up the core of microvilli of intestinal cells
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22. Microfilaments (actin filaments)
Fig. 6-26
Microvillus
Plasma membrane
Microfilaments (actin filaments)
Figure 6.26 A structural role of microfilaments
Intermediate filaments
0.25 µm

23. Microfilaments that function in cellular motility contain the protein myosin in addition to actin
In muscle cells, thousands of actin filaments are arranged parallel to one another
Thicker filaments composed of myosin interdigitate with the thinner actin fibers
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24. Figure 6.27 Microfilaments and motility
Muscle cell
Actin filament
Myosin filament
Myosin arm
(a) Myosin motors in muscle cell contraction
Cortex (outer cytoplasm):
gel with actin network
Inner cytoplasm: sol
with actin subunits
Extending
pseudopodium
(b) Amoeboid movement
Figure 6.27 Microfilaments and motility
Nonmoving cortical
cytoplasm (gel)
Chloroplast
Streaming
cytoplasm
(sol)
Vacuole
Parallel actin
filaments
Cell wall
(c) Cytoplasmic streaming in plant cells

25. Muscle cell Actin filament Myosin filament Myosin arm
Fig, 6-27a
Muscle cell
Actin filament
Myosin filament
Myosin arm
Figure 6.27a Microfilaments and motility
(a) Myosin motors in muscle cell contraction

26. Cortex (outer cytoplasm): gel with actin network
Fig. 6-27bc
Cortex (outer cytoplasm): gel with actin network
Inner cytoplasm: sol with actin subunits
Extending pseudopodium
(b) Amoeboid movement
Nonmoving cortical cytoplasm (gel)
Chloroplast
Streaming cytoplasm (sol)
Figure 6.27b,c Microfilaments and motility
Vacuole
Parallel actin filaments
Cell wall
(c) Cytoplasmic streaming in plant cells

27. Localized contraction brought about by actin and myosin also drives amoeboid movement
Pseudopodia (cellular extensions) extend and contract through the reversible assembly and contraction of actin subunits into microfilaments
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28. Video: Cytoplasmic Streaming
Cytoplasmic streaming is a circular flow of cytoplasm within cells
This streaming speeds distribution of materials within the cell
In plant cells, actin-myosin interactions and sol-gel transformations drive cytoplasmic streaming
Video: Cytoplasmic Streaming
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29. Intermediate Filaments
Intermediate filaments range in diameter from 8–12 nanometers, larger than microfilaments but smaller than microtubules
They support cell shape and fix organelles in place
Intermediate filaments are more permanent cytoskeleton fixtures than the other two classes
For the Cell Biology Video Interphase Microtubule Dynamics, go to Animation and Video Files.
For the Cell Biology Video Microtubule Sliding in Flagellum Movement, go to Animation and Video Files.
For the Cell Biology Video Microtubule Dynamics, go to Animation and Video Files.
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30. These extracellular structures include:
Concept 6.7: Extracellular components and connections between cells help coordinate cellular activities
Most cells synthesize and secrete materials that are external to the plasma membrane
These extracellular structures include:
Cell walls of plants
The extracellular matrix (ECM) of animal cells
Intercellular junctions
For the Cell Biology Video Ciliary Motion, go to Animation and Video Files.
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31. Prokaryotes, fungi, and some protists also have cell walls
Cell Walls of Plants
The cell wall is an extracellular structure that distinguishes plant cells from animal cells
Prokaryotes, fungi, and some protists also have cell walls
The cell wall protects the plant cell, maintains its shape, and prevents excessive uptake of water
Plant cell walls are made of cellulose fibers embedded in other polysaccharides and protein
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32. Plant cell walls may have multiple layers:
Primary cell wall: relatively thin and flexible
Middle lamella: thin layer between primary walls of adjacent cells
Secondary cell wall (in some cells): added between the plasma membrane and the primary cell wall
Plasmodesmata are channels between adjacent plant cells
For the Cell Biology Video E-cadherin Expression, go to Animation and Video Files.
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33. Secondary cell wall Primary cell wall Middle lamella Central vacuole
Fig. 6-28
Secondary cell wall
Primary cell wall
Middle lamella
1 µm
Central vacuole
Cytosol
Figure 6.28 Plant cell walls
Plasma membrane
Plant cell walls
Plasmodesmata

34. RESULTS 10 µm Distribution of cellulose synthase over time
Fig. 6-29
RESULTS
10 µm
Figure 6.29 What role do microtubules play in orienting deposition of cellulose in cell walls?
Distribution of cellulose synthase over time
Distribution of microtubules over time