cytoskeleton -Eukaryotic cell 123456.ppt

cytoskeleton

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

2. 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

3. •Introduction
•The cytoskeletonis a network of fibers
extending throughout the cytoplasm.
•The cytoskeleton
organizes the
structures and
activities of
the cell.

4. •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

5. •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

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

8. •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

9. •In many cells, microtubules grow out from a
centrosomenear the nucleus.
–These microtubules resist compression to the
cell.

10. Fig. 7.22
•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.

11. •Microtubules are the central structural
supports in ciliaand 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

12. •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.

13. •A flagellum has an undulatory movement.
–Force is generated parallel to the flagellum’s
axis.
Fig. 7.23a

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

15. •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.

16. Fig. 7.24

17. •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

18. •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.

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

20. •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

21. •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

22. •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

23. •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