microtubules and microfilaments

helpful in understanding cytoskeleton of the cell

2. The eukaryotic cells possess a skeletal system
called cytoskeleton that has got analogous
function.
The cytoskeleton is composed of 3 well defined
filamentous structurs – microtubules,
microfilaments and intermediate filaments with
distinct functions .
Each filaments are made of protein subunits
held together by weak non covalent bonds.
 This type of construction allows rapid assembly
and disasembly contolled by cell regulation

5. MICROTUBULES
They are components of a diverse array of substances including the mitotic
spindles of dividing cells and core of flagella and cilia
STRUCTURE AND COMPOSITION
Have an outer diameter of 25nm and a wall thickness of 4nm and may extend
across the length and breadth of the cell
The wall of microtubule is composed of globular proteins arranged in
longitudinal rows called protofilaments that are allinged parallel to the long axis
of the tubule .

6. When veiwed in cross section they are seen
to have 13 protofilaments allinged side by
side in a circular pattern
Each protofilament is assembled from
dimeric building blocks consisting of one
alpha and one beta subunits
The protofilament is asymmetric with alpha
subunit on one end and beta on the other
One end of the protofilament is known as
the plus end is terminated by a row of beta
tubulin units and the mnus end is
terminated by the alpha tubulin units

8. 1. ACT AS STRUCTURAL SUPPORT AND
ORGANIZERS
They are stiff enough to resist the forces that can
bend or compress the fibre
The distribution of microtubules through the
cytoplasm of a cell determines the shape of a cell
eg: in coloumnar epithelial cells the microtubules
are alligned along the axis of the cell
Maintain a key role in the internal organization of
a cell

9. 2.ACT AS AGENTS OF
INTRACELLULAR MOTILITY

Involved in the movement of
vesicles,proteins,organellsetc across
the cytoplasm throught the cell
Eg: AXONAL TRANSPORT: proteins
such as neurotransmittors are
secreated and packed in membranous
vesicles by golgi body and
endoplasmic reticulum of the cell
body are transported through the
axon which consists of a number of of
microtubules and motor proteins
which takes it down the axon

10. MOTOR PROTEINS :
 they convert chemical energy into mechanical energy that
is used to generate force or move the Types of cargo include
vesicles,chromosomes , mitochondria, proteins etc
They can be classified into 3 types mainly : kinesins and
dyneins that move along the microtubules and myosin that
move along microfilaments
The binding of ATP and its hydrolysis provides energy to
them to travel
cargo attached to the motor

11. KINESINS: is a tetramer constructed by 2 identical
heavy chains and 2 identical light chains, has a
globular head that binds ATP ,a neck a stalk and a fan
shaped tail that binds to the cargo to be transported

12. DYENEINS: it is a huge protein composed of two identical
heavy chains and a variety of intermediate and light
chains. They are responsible for the movement of cilia and
flagella .move towards the minus end of the microtubule.
They act as:
As a force-generating agent in positioning the spindle and
moving chromosomes during mitosis
As a minus end–directed microtubular motor with a role
in positioning the centrosome and Golgi complex and
moving organelles,vesicles,and particles through the
cytoplasm

14. MICROTUBULE ORGANIZING
CENTRE
1. CENTROSOME: In animal cells, the
centrosome has a pair of centrioles,
each with nine triplets of microtubules
arranged in a ring.
•the centroles are surrounded by an
electron rich pericentriolar matrix

16. Microtubules are major
components of spindle
fibre used to pull apart
chromosomes during cell
division

18. 2. BASAL BODIES
the outer microtubules in a cillia or flagella arise from
basal bodies attached to the base of cillia or flagella

19. 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.

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

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

23. 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.

25. 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

26. 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.

27. 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

28. 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

29. 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