Cytoskeleton components,structure and functioning. Cell organelle.

1. CYTOSKELETON
CONTENTS
• Introduction
• Importance of cytoskeleton
• Resemblance with skeleton
• Different from skeleton
• Structure and function of cytoskeleton
• Components
• Microtubules
• Structure of microtubules
• Function of microtubules
• Microfilaments
• Structure of microtubule
• Function of microtubule
• Intermediate Filaments
• Structure
• Classification
• Function

2. INTRODUCTION
Within the cytoplasm, a network of protein fibers is present that help maintain the shape of the
cell, secure some organelles in specific positions, allow cytoplasm and vesicles to move within
the cell, and enable cells within multicellular organisms to move. Collectively, this network of
protein fibers is known as the cytoskeleton.
It is the network of protein filament in cytoplasm that provide framework to cell.The
cytoskeleton of a cell provides structure, strength, and motility. It provides a cellular scaffolding
upon which the cellular organization is arranged. The figure shows a portion of a cell's
cytoskeleton. Cytoskeleton is very extensive. Many ribosomes appear to be attached to the
cytoskeleton. Polysome refers to two or more ribosomes. The ribosomes attached to the
cytoskeleton are often referred to as "free" ribosomes to distinguish them from those ribosomes
attached to the nuclear or ER membranes.
IMPORTANCE OF CYTOSKELETON
The cytoskeleton extends throughout the cell’s cytoplasm and provides a number of important
functions.
• It helps the cell maintain its shape and gives support to the cell.
• A variety of cellular organelles are held in place by the cytoskeleton.
• It assists in the formation of vacuoles.
• The cytoskeleton is not a static structure, but is able to disassemble and reassemble its
parts in order to enable internal and overall cell mobility. Types of intracellular
movement supported by the cytoskeleton include transportation of vesicles into and out
of a cell, chromosome manipulation during mitosis and meiosis, and organelle migration.
The cytoskeleton makes cell migration possible as cell motility is needed for tissue

3. construction and repair, cytokinesis in the formation of daughter cells, and in immune
cell responses to germs.
• The cytoskeleton assists in the transportation of communication signals between cells.
• It forms cellular appendage-like protrusions, such as cilia and flagella, in some cells.
• Moves substances within the cell-cellular trafficking.
• Anchors cellular structures - organelles, proteins, flagella, cilia.
Resemblance with skeleton
• Like skeleton, it provides support to cell shape.
• It also has structural and transport function like skeleton.
Different from skeleton
• Unlike skeleton , it is dynamic and motile.
• It shift and rearrange in response to the needs of cell.
STRUCTURE AND FUNCTION OF CYTOSKELETON
Cytoskeleton is a network of fibers that forms the “infrastructure” of eukaryotic cells,
prokaryotic cells, and archaeans. In eukaryotic cells, these fibers consist of a complex mesh of
protein filaments and motor proteins that aid in cell movement and stabilize the cell.
COMPONENTS
There are three types of fibers within the cytoskeleton:
• Microtubules

4. • Microfilaments
• Intermediate filaments
Microtubules are found in the interior of the cell where they maintain cell shape by resisting
compressive forces.
Microfilaments thicken the cortex around the inner edge of a cell; like rubber bands, they
resist tension.
Intermediate filaments are found throughout the cell and hold organelles in place.
PROPERTY MICROTUBULES MICROFILAMENTS INTERMEDIATE
FILAMENTS
Structure Tube like, 13
coloumns of tubulin
molecule
2 strands of intermixed
actin
Fibrous protein ,
coil into thicker
cables
Diameter 35 nm(outer),
14nm (inner)
7mm 8-12nm
Protein
subunits
Tubulin Actin Fibrous(caroteins)
Function Maintain cell shape,
help in cell
motility(flagella,cilia),
help in chromosomal
movement
Maintain cell shape,
motion in pseudopodia,
help in cleavage furrow
formation and
cytoplasmic streaming
Maintain cell shape,
help in formation of
nuclear lamina

5. MICROTUBULES
Microtubules are microscopic hollow tubes made of the proteins alpha and beta tubulin that are
part of a cell’s cytoskeleton, a network of protein filaments that extends throughout the cell,
gives the cell shape, and keeps its organelles in place. Microtubules are the largest structures in
the cytoskeleton at about 24 nanometers thick. They have roles in cell movement, cell division,
and transporting materials within cells.
STRUCTURE
Microtubules are hollow cylinders made up of repeating protein structures, specifically dimers of
alpha and beta tubulin. Dimers are complexes of two proteins. ɑ-tubulin and β-tubulin bind to
each other to form a dimer, and then multiple units of these dimers bind together, always
alternating alpha and beta, to form a chain called a protofilament. Then, thirteen protofilaments
arrange into a cylindrical pattern to form a microtubule. Microtubules are constantly assembling
and disassembling via the addition and removal of dimers. They are said to be in a state
of dynamic equilibrium because their structure is maintained even though the individual
molecules themselves are constantly changing.

6. Microtubules are polar molecules, with a positively charged end that grows relatively fast and a
negatively charged end that grows relatively slow. Protofilaments arrange themselves parallel to
each other in a microtubule, so the positive end of the microtubule always has beta subunits
exposed, while the negative end has alpha subunits exposed.In animal cells, microtubules radiate
outwards from an organelle in the center of the cell called a centrosome, which is a microtubule
organizing center (MTOC). The cells of plants and fungi do not have centrosomes, and instead
the nuclear envelope the membrane surrounding the cell’s nucleus is an MTOC.
TYPES OF MICROTUBULES
There are 2 types of microtubules : cytoplasmic microtubule and axonemal microtubule.
Cyoplasmic microtubule
It is present in the position of ER and golgi complex.It forms mitotic apparatus.Also provide
track for molecular motor to transport organelles and other cargo.
Axonemal microtubule
It is stable and static microtubule in basal bodies of cilia and flagella. It forms duplet and triplet
structure to associated proteins.It is a control unit of cilia and flagella with a bundle of
microtubule.
FUNCTIONS OF MICROTUBULES
Cell Movement
Microtubules give structures like cilia and flagella their structure. Cilia are small protuberances
of a cell. In humans, they are found on cells lining the trachea, where they prevent materials like
mucus and dirt from entering the lungs. They are also found in the fallopian tubes of the female
reproductive system, where they help move the egg that is released from the ovary to the uterus.

7. Flagella are tail-like appendages that allow cells to move. They are found in some bacteria, and
human sperm also move via flagella. Microtubules also allow whole cells to “crawl” or migrate
from one place to another by contracting at one end of the cell and expanding at another.
Cell Division
Microtubules play a key role in forming the mitotic spindle, also called the spindle apparatus.
This is a structure that is formed during mitosis (cell division) in eukaryotic cells. The mitotic
spindle organizes and separates chromosomes during cell division so that the chromosomes can
be partitioned into two separate daughter cells. Its components include microtubules, the MTOC,
and microtubule-associated proteins (MAPs).
Three subgroups of microtubules aid in the process of mitosis: astral, polar, and kinetochore
microtubules. Astral microtubules radiate from the MTOCs of a cell to the cell membrane,
keeping the mitotic spindle in place. Polar microtubules intertwine between two MTOCs and
help separate chromosomes. (All microtubules are polar; these ones are just specifically called
polar microtubules.) Kinetochore microtubules attach to chromosomes to help pull them apart;
the chromosomes are attached to the microtubules by a complex of proteins called a kinetochore.
Cell Transport
As part of the cytoskeleton, microtubules help move organelles inside a cell’s cytoplasm, which
is all of the cell’s contents except for its nucleus. They also help various areas of the cell
communicate with each other. However, even though microtubules help components of the cell
to move, they also provide the cell with shape and structure.
Microfilaments
Microfilaments, also called actin filaments, are polymers of the protein actin that are part of a
cell’s cytoskeleton. The cytoskeleton is the network of protein filaments that extends throughout
the cell, giving the cell structure and keeping organelles in place. Microfilaments are the smallest
filaments of the cytoskeleton. They have roles in cell movement, muscle contraction, and cell
division.
Microfilament Structure
Microfilaments are composed of two strands of subunits of the protein actin (hence the name
actin filaments) wound in a spiral. Specifically, the actin subunits that come together to form a
microfilament are called globular actin (G-actin), and once they are joined together they are
called filamentous actin (F-actin). Like microtubules, microfilaments are polar. Their positively
charged, or plus end, is barbed and their negatively charged minus end is pointed. Polarization

8. occurs due to the molecular binding pattern of the molecules that make up the microfilament.
Also like microtubules, the plus end grows faster than the minus end.
Microfilaments are the thinnest filaments of the cytoskeleton, with a diameter of about 6 to 7
nanometers. A microfilament begins to form when three G-actin proteins come together by
themselves to form a trimer. Then, more actin binds to the barbed end. The process of self-
assembly is aided by autoclampin proteins, which act as motors to help assemble the long strands
that make up microfilaments. Two long strands of actin arrange in a spiral in order to form
microfilament.
Stress Fibers
Stress fibers lie immediately underneath the cell membrane. The stress fibers are composed of
two twisted microfilaments. The actin is marked with an orange fluorescence on the left and a
green flourescence on the right.

9. FUNCTIONS OF MICROFILAMENTS
Muscle Contraction
One of the most important roles of microfilaments is to contract muscles. There is a high
concentration of microfilaments in muscle cells, where they form myofibrils, the basic unit of the
muscle cell. Actin is an indispensable protein for muscle movement, and microfilaments are
often called actin filaments because actin is so prominent in the muscular system of the body. In
muscle cells, actin works together with the protein myosin to allow the muscles to contract and
relax. Here, neither actin nor myosin can work properly without the other, and they form a
complex called actomyosin. Groups of actomyosin are found in sarcomeres, the basic unit
of muscle tissue.
Cell Movement
Microfilaments play a role in causing cells to move. This occurs throughout the body and it is
also very important for organisms whose entire body consists of one cell, such as amoebae;
without microfilaments, they would not be motile. Actomyosin plays a role here just as it does in
muscle cells. In order for cells to move, one end of a microfilament must elongate while the
other end must shorten, and myosin acts as a motor to make this happen.
Microfilaments also have a role in cytoplasmic streaming. Cytoplasmic streaming is the flow of
cytoplasm throughout the cell. It allows nutrients, waste products, and cell organelles to travel
from one part of the cell to another. Microfilaments can attach to a cell organelle and then
contract, pulling the organelle to a different area of the cell.
Cell Division

10. Another important function of microfilaments is to help divide the cell during mitosis.
Microfilaments aid the process of cytokinesis, which is when the cell “pinches off” and
physically separates into two daughter cells. During cytokinesis, a ring of actin forms around the
cell that is separating, and then myosin proteins pull on the actin and cause it to contract. The
ring gets narrower and narrower around the cell, dragging the cell membrane with it, until it
splits into two cells. Afterward, the microfilaments depolymerize, or break down, into actin
molecules, causing the ring to dissemble when it is no longer needed.
INTERMEDIATE FILAMENTS
They are a type of cytoskeletal element made of multiple strands of fibrous proteins wound
together. As their name suggests, intermediate filaments have an average diameter of 8 to 10 nm,
in between that of microfilaments and microtubules.
Intermediate filaments come in a number of different varieties, each one made up of a different
type of protein. One protein that forms intermediate filaments is keratin, a fibrous protein found
in hair , nail and skin.
Intermediate filaments have no role in cell movement. Their function is purely structural. They
bear tension, thus maintaining the shape of the cell, and anchor the nucleus and other organelles
in place.
The intermediate filaments are the most diverse group of cytoskeletal elements. Several types of
fibrous proteins are found in the intermediate filaments.
STRUCTURE
Intermediate filaments vary much in structure, in humans there are 70 different human cell type-
specific intermediate filament proteins which are divided into five classes. However, all IF
proteins are share a common three domain structure that consists of an amino-terminal head
domain, a carboxy-terminal tail domain and a central a-helical rod domain. From this there is
substantial variability in the amino acid sequence of the globular head and tail domains, whereas
the rods a more highly conserved.
CLASSES OF INTERMEDIATE FILAMENTS

11. FUNCTIONS OF INTERMEDIATE FILAMENTS
SUPPORT AND STRUCTURE
In the cytoplasm
The function of the intermediate filaments is mainly structural and they provide support for cells
which come under physical stress such as muscle cells, neurons and some epithelial cells. They
can be describes as ‘internal guy ropes’ which support the entire cytoskeleton.
They are often most developed in areas of the cell which are subject to regular mechanical stress
by the extracellular environment.
They can also provide support for the microfilaments and microtubules which are more fragile
and are connected to both of the other types of cytoskeletal fibres by plakin proteins.
The IF's are necessary for stretch in epithelial cells.
In the nucleus
IF’s form the nuclear lamina, (made up of proteins called lamins) which strengthens the nuclear
envelope and protects the genome inside.
In nerve cells
There are three types of neurofilaments, (NF-L ,NF-H and NF-M)which are long bundles of
Intermediate filaments which run parallel to a neve axon. They have sidearms which project
outward from the filament which maintain the correct spacing between the parallel
neurofilaments of the axon. They also provide support in development when the axon
dramatically increases it’s diameter.

12. In muscle cells
Desmin forms a lattice around the sarcomere. The desmin filments form bridges between Z discs
and form myofibrils in muscle cells. As they are found outside the sarcomere, they are not
involved in muscle contraction but do provide structural support.
INTRACELLULAR ORDER AND TRANSPORT
Vimentin and Keratin Intermediate filaments have a key role in maintaining the position of the
nucleus within cells. They often form a ring-like network around the nucleus to hold it in place.
These possibly extend downward to connect the nuclear envelope to the nuclear cortex. They
also extend outwards through the cytoplasm to the outer edge of the cell by desmosomes and
hemidesmosome connections.
CELL DIVISION
During mitosis, the IF’s which make up the nuclear lamina disassemble as the lamin proteins are
phosphorylated by specific protein kinases. The nuclear lamina also provides anchor points for
chromatin fibres at the outside of the cell.
CELLULAR MOVEMENT
Unlike the microtubules and microfilaments, the intermediate filaments have no direct
involvement in cell movement. They do however appear to support this mechanism by providing
mechanical strength to cells and tissues involved.