Download Cytoskeleton 14

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The cytoskeleton is a network of fibers that organizes
structures and activities in the cell. It is cellular
skeleton contained within a cell's cytoplasm.
The cytoskeleton is present in prokaryotic and eukaryotic
cells.
It forms structures such as flagella and cilia and plays
important roles in both intracellular transport (the
movement of vesicles and organelles) and cell division.
In 1903 Nikolai K Koltsov proposed that the shape of cells
was determined by a network of tubules that he termed
the cytoskeleton.
Cytoskeleton is composed of microtubules, microfilaments
and intermediate filaments.
COMPONENTS OF CYTOSKELETON
Microtubules are thickest and their role is
compression resistance.
 Hollow rods about 25nm in diameter and 200nm
to 25 μm in length.
 The wall of hollow tube is made of globular
protein called tubulin
 Tubulin protein is a dimer consists of α-tubulin
and β-tubulin.
 They grow by the addition of tubulin dimer.
 Due to the orientation of tubulin dimer there are
two ends of microtubles; one end can
accumulate and release tubulin dimer with much
higher rate thus grows and shrinks faster with
the celluler activities. It is plus end.
 Other end has slow rate of on and off.
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 Microtubules
shape and support the cell
 It serves as track along which organelles
equipped with motor proteins can move.
 Guide secretory vesicles from golgi apparatus
to plasma membrane.
 Involved in separation of chromosomes
during cell division.
Centrosomes: Microtubules grow out from
centrosome located near the nucleus and known
as microtubule organizing center.
 Microtubules
are known as girders of
cytoskeleton.
 Centrioles: Within the centrosome is a pair of
centrioles. Each composed of nine sets of triplet
microtubules arranged in a ring. Before the
division of animal cell centriole replicates. In
plant cell centrosomes
lack centrioles.
Centrioles are not present in the cells of higher
plants. One of the important functions that
centrioles perform is the generation of cilia and
flagella for cells. These are surface features that
cells use for movement.
 The cells of higher plants have no centrioles
because they do not make cilia or flagella. A cell
can make a spindle without centrioles, but it
can't make cilia or flagella without centrioles.
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 Cilia
and Flagella: In eukaryotes special
arrangement of microtubule is responsible
for the beating of flagella and cilia.
 Unicellular organisms and sperms of algae
move via flagella or cilia. They can move in a
wave to propelled the surrounding liquid thus
cause movement. i.e. Cilia lining of windpipe
sweeps mucus out of lungs. The cilia lining of
oviducts help move an egg towards uterus.
 Cilia are large in number, 0.25 μm in ф and
2-20 μm in length.
 Flagella are 0.25 μm in ф and 10-200 μm in
length, usually one or few per cell.
Flagella and cilia differ in beating pattern.
 Flagellum
has an undulating motion that
generates force in the same direction of its axis.
Cilia work with alternating power strokes
generating force perpendicular to the axis of
cilium.
 Cilia act as antenna to receive signals for the cell.
Usually one/cell. Membrane proteins on cilia
transmit molecular signals from the environment
of the cell to the interior.
 Flagella and cilia are similar in ultrastructure.
Nine doublets of microtubules make a ring having
two single microtubles in the center. Cross linking
motor proteins along the length of flagellum and
cilium connect the outer neighboring doublet.
Centriole like basal body anchored the flagellum
or cilium in the cell. Basal body of sperm in
animals/humans enter the egg and become
centriole.
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 The
motor protein extends from one doublet
to another is know dynein. Dynein arms
perform movements cause by conformational
changes in proteins at the expense of ATP.
Like a cat climbing on tree the arms of one
doublet attach to the arms of the other and
pull to slide the doublet. Then arms release
and attach further. Walking of doublets.
 For bending flagellum/cilium, the two
doublets can not slide but held in, thus cause
the bending i.e. bending of knee (bone and
microtuble)
Microfilaments: (Actin filaments): These are
solid rods about 7nm in ф in all eukaryotic cells.
Made up of actin a globular protein. They are
twisted double chain of actin subunits. They
form structural network due to the proteins that
bind along the side of an actin filament and
extend a new filament as branch.
 Microfilament in cytoskeleton bears tension
(pulling force).
 They for 3-D network in cytoplasm, thus supports
the cell’s shape. This network gives the cortex ,
a semisolid like a gel.
 In animal cell they help in transport of material
across the plasma membrane i.e. In intestinal
cells they form the core of microvilli.
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Microfilament are involved in cell motility i.e.
contratile apparatus of muscle cells.
 Thousands of actin filament are arranged
parallel along the muscle cell interdigitating
with myosin protein filament. Myosin act as
motor protein by means of projections that walk
along the actin filaments. Contraction of the
muscle cell results from the sliding of actin and
myosin filaments.
 Actin-myosin
aggregate
cause
localized
contractions of the cells i.e. furrow formation in
the dividing cell to form two daughter cells.
 Actin-myosin contractions results in ameoboid
movements by extending pseudopodia (toothpast
release from tube). Another example is white
blood cells.
 Cytoplasmic streaming in large plant cells is
caused by microfilaments for even distribution
materials within the cell.
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Intermediate Filaments: They have intermediate
ф of 8-12 nm. Specialized for bearing tension.
 They are diverse class of cytoskeleton elements,
constructing from a different protein subunits of
keratins.
 They are more permanent elements.
 They persists after cell death. Outer layer of skin
is made up of keratin. They do not dissolve in
chemicals that remove microtubules and
microfilaments.
 They give shape to the cell and fix the positions
of cell organelles. i.e. nucleus sits within the
cage of IF.
 They make the nuclear lamina.
 Long
extensions of nerve cell contain
intermediate filaments.
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 Cytoskeleton
gives mechanical support to the
cell and maintain its shape. Specially in
animal cells which lack cell walls.
 Architecture of cytoskeleton is like a dome
tent and stabilized by the balance between
the opposing forces exerted by its elements.
 It provides anchorage for many organelles
and cytosolic enzyme molecules.
 It is very dynamic and may dismantled in one
part of the cell and reassembled in new
location, changing the shape of the cell
 It
plays a role in cell motility and movements
of cell organelles.
 Cell motility requires the interaction of
cytoskeleton with motor proteins work
together with plasma membrane molecules
to allow whole cell to move along fibers
outside the cell.
 Motor proteins cause the bending of the cilia
and flagella by gripping microtubules and
sliding them against each other.
 Same mechanism involves the muscle cell
contraction.
 Inside
cell vesicles and other organelles use
motor protein as “feet to walk” along a track
provided by cytoskeleton
 This
is how the vesicles containing
neurotransmitter molecules migrate to the
tips of axons, the long extensions of nerve
cells that release these molecules as
chemical signals to adjacent nerve cells.
 Cytoskeleton manipulates plasma membrane
to form phagocytic vesicles.
 Cytoplasmic
streaming
that
circulate
materials within the plant cells is due to
cytoskeleton.