The cytoskeleton of the cell consists of three main filamentous structures: microtubules, microfilaments, and intermediate filaments, which provide structural support, organelle positioning, and intracellular motility. Microtubules are hollow structures made of tubulin, microfilaments are flexible actin filaments, and intermediate filaments are robust polypeptides providing mechanical strength. Motor proteins, including kinesins and dyneins, facilitate the movement of cellular cargo along these cytoskeletal elements.

1. Cytoskeleton of the cell
Asma Saleh Alilesh

2. Cytoskeleton
 The cytoskeleton is composed of three
well-defined filamentous structures:
1. Microtubules,
2. microfilaments
3. intermediate filaments
 Each of the three types of cytoskeletal
filaments is a polymer of protein subunits
held together by weak, noncovalent bonds.
 They together form an elaborate interactive
network.

4. Cytoskeletal Functions
1- structural support that can determine the shape of
the cell and resist forces that tend to deform it.
2- responsible for positioning the various organelles
within the interior of the cell.
3- direct the movement of materials and organelles
within cells.
4- The force-generating apparatus that moves cells
from one
place to another
5- An essential component of the cell’s division
machinery. Cytoskeletal elements make up the
apparatus responsible for separating the

5. Microtubules
 Hollow, relatively rigid, tubular structures.
 They found in nearly every eukaryotic cell.
 Microtubules have an outer diameter of 25 nm and
a wall thickness of approximately 4 nm.
 The wall of a microtubule is composed of globular
proteins (tubulin) arranged in rows, termed
protofilaments, that are aligned parallel to the
long axis of the tubule.
 Each protofilament is assembled from dimeric
building blocks consisting of one α-tubulin and one
β-tubulin subunit.

7.  Has polarity.One end of a microtubule is known as
the plus end and is terminated by a row of β -
tubulin subunits The opposite end is the minus end
and is terminated by a row of α -tubulin subunits
 Microtubules function as structural supports and
organizers: (structure of cilia and flagella and the
axons of nerve cells.)
 Microtubules as agents of intracellular motility.

8. Microfilaments
 Flexible helical filament composed of globular
subunits of the protein actin In the presence of ATP,
actin monomers polymerize to form.
 They also called actin filament, F-actin.
 8 nm diameters
 Actin filaments have polarity, which is denoted as a
plus and minus end.
 Actin is an ATPase.
 They found in nearly every eukaryotic cell.
 functioning in cytokinesis, amoeboid movement,
and changes in cell shape and involved in
intracellular motile processes.

9. .

10. Intermediate filaments (IFs)
 Intermediate filaments are strong, flexible ropelike
filaments.
 Have only been identified in animal cells.
 10 nm diameter
 provide mechanical strength to cells that are
subjected to physical stress, including neurons,
muscle cells, and the epithelial cells that line the
body’s cavities.
 IFs are a chemically heterogeneous polypeptide
(Keratin and Keratin- like proteins), in humans, are
encoded by approximately 70 different genes.
 Have no enzymatic activity or polarity

12. Motor proteins
 The motor proteins of a cell are proteins convert
chemical energy (stored in ATP) into mechanical energy,
which is used to generate force to move cellular cargo
attached to the motor.
 Types of cellular cargo transported by these proteins
include vesicles, mitochondria, lysosomes,
chromosomes.
 A single cell may contain a hundred different motor
proteins, specialized for a different activity.
 motor proteins can be grouped into three broad
superfamilies: kinesins, dyneins, and myosins.
 Kinesins and dyneins move along microtubules.
 Myosins move along microfilaments.

14. Kinesins :
 Kinesin is a tetramer constructed from two
identical heavy chains and two identical light
chains.
 Heads that bind a microtubule and act as ATP-
hydrolyzing unit (engines).
 Move along microtubules toward their plus end.
Kinesin is therefore said to be a plus end-directed
microtubular motor.
 kinesins tend to move vesicles and organelles
(e.g., peroxisomes and mitochondria) in an
outward direction toward the cell’s plasma
membrane.

16. Dynein:
 Cytoplasmic dynein is a huge protein (molecular
mass of approximately 1.5 million daltons) composed
of two identical heavy chains and a variety of
intermediate and light chains.
 Dynein moves along a microtubule toward the minus
end—opposite that of kinesin.
- Function as:
1. As a force-generating agent in positioning the
spindle and moving chromosomes during mitosis
2. 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.