2. Cytoskeleton
Providing structural support to the cell, the
cytoskeleton also functions in cell motility and
regulation

3. Structural Support
Mechanical support
Maintains shape
Fibers act like a geodesic dome to stabilize and balance
opposing forces
Provides anchorage for organelles
Dynamic
Dismantles in one spot and reassembles in another to change
cell shape

4. Introduction
The cytoskeleton is a network of fibers extending
throughout the cytoplasm.
The cytoskeleton
organizes the
structures and
activities of
the cell.

5. The cytoskeleton also plays a major role in cell motility.
This involves both changes in cell location and limited movements of
parts of the cell.
The cytoskeleton interacts with motor proteins.
In cilia and flagella motor proteins pull components
of the cytoskeleton past each other.
This is also true
in muscle cells.
Fig. 7.21a

6. Motor molecules also carry vesicles or organelles to various
destinations along “monorails’ provided by the cytoskeleton.
Interactions of motor proteins and the cytoskeleton circulates
materials within a cell via streaming.
Recently, evidence is accumulating that the cytoskeleton may
transmit mechanical
signals that rearrange
the nucleoli and
other structures.
Fig. 7.21b

7. There are three main types of fibers in the cytoskeleton:
microtubules, microfilaments, and intermediate
filaments.

9. Microtubules, the thickest fibers, are hollow rods about 25
microns in diameter.
Microtubule fibers are constructed of the globular protein, tubulin,
and they grow or shrink as more tubulin molecules are added or
removed.
They move chromosomes during cell division.
Another function is
as tracks that guide
motor proteins
carrying organelles
to their destination.
Fig. 7.21b

10. In many cells, microtubules grow out from a centrosome near
the nucleus.
These microtubules resist compression to the cell.

11. • In animal cells, the centrosome has a pair of
centrioles, each with nine triplets of microtubules
arranged in a ring.
• During cell division the
centrioles replicate.
Fig. 7.22

12. Microtubules are the central structural supports in cilia and
flagella.
Both can move unicellular and small multicellular organisms by
propelling water past the organism.
If these structures are anchored in a large structure, they move fluid
over a surface.
 For example, cilia sweep mucus carrying trapped debris from the lungs.
Fig. 7.2

13. Cilia usually occur in large numbers on the cell surface.
They are about 0.25 microns in diameter and 2-20 microns long.
There are usually just one or a few flagella per cell.
Flagella are the same width as cilia, but 10-200 microns long.

14. A flagellum has an undulatory movement.
Force is generated parallel to the flagellum’s axis.
Fig. 7.23a

15. Cilia move more like oars with alternating power and recovery
strokes.
They generate force perpendicular to the cilia’s axis.
Fig. 7.23b

16. In spite of their differences, both cilia and flagella have the same
ultrastructure.
Both have a core of microtubules sheathed by the plasma membrane.
Nine doublets of microtubules arranged around a pair at the center,
the “9 + 2” pattern.
Flexible “wheels” of proteins connect outer doublets to each other
and to the core.
The outer doublets are also connected by motor proteins.
The cilium or flagellum is anchored in the cell by a basal body,
whose structure is identical to a centriole.

17. Fig. 7.24

18. The bending of cilia and flagella is driven by the arms of a motor
protein, dynein.
Addition to dynein of a phosphate group from ATP and its removal
causes conformation changes in the protein.
Dynein arms alternately
grab, move, and release
the outer microtubules.
Protein cross-links limit
sliding and the force is
expressed as bending.
Fig. 7.25

19. Microfilaments, the thinnest class of the cytoskeletal fibers, are
solid rods of the globular protein actin.
An actin microfilament consists of a twisted double chain of actin
subunits.
Microfilaments are designed to resist tension.
With other proteins, they form a three-dimensional network just
inside the plasma membrane.

20. Fig. 7.26 The shape of the
microvilli in this intestinal cell
are supported by microfilaments,
anchored to a network of
intermediate filaments.

21. In muscle cells, thousands of actin filaments are arranged
parallel to one another.
Thicker filaments, composed of a motor protein, myosin,
interdigitate with the thinner actin fibers.
Myosin molecules walk along the actin filament, pulling stacks of actin
fibers together and shortening
the cell.
Fig. 7.21a

22.  In other cells, these actin-myosin aggregates are less organized but still
cause localized contraction.
A contracting belt of microfilaments divides the cytoplasm of animals
cells during cell division.
Localized contraction also drives amoeboid movement.
 Pseudopodia, cellular extensions, extend and contract through the
reversible assembly and contraction of actin subunits into microfilaments.
Fig. 7.21b

23. In plant cells (and others), actin-myosin interactions and sol-gel
transformations drive cytoplasmic streaming.
This creates a circular flow of cytoplasm in the cell.
This speeds the distribution of materials within the cell.
Fig. 7.21c

24. Intermediate filaments,
intermediate in size at 8 - 12
nanometers, are specialized for
bearing tension.
Intermediate filaments are built from
a diverse class of subunits from a
family of proteins called keratins.
Intermediate filaments are more
permanent fixtures of the
cytoskeleton than are the other
two classes.
They reinforce cell shape and fix
organelle location.
Fig. 7.26