Cytoskeleton
Eukaryotic cells can take on an amazing array of shapes, from
neurons with projections running from your spinal cord to your
toes to skin cells that appear as compact, elongated cubes. This
flexibility of shape is made possible by a complex internal skeleton, a structure made even more impressive by the cell’s ability to
reorganize portions of it, depending on the cell’s needs.
"Cytoskeleton is composed of three types of proten fibers"
The cytoplasm of all eukaryotic cells is crisscrossed by a network of three kinds of protein fibers that support the shape of the cell and anchor organelles to fixed locations. This network,
called the cytoskeleton, is a dynamic system, constantly assembling and disassembling. Individual fibers consist of polymers of
identical protein subunits that attract one another and spontaneously assemble into long chains. Fibers disassemble in the same
way, as one subunit after another breaks away from one end of
the chain.
Actin Filaments (microfilaments)
Actin filaments are long fibers about 7 nm in diameter. Each
filament is composed of two protein chains loosely twined
together like two strands of pearls. Each “pearl,” or subunit, on
the chain is the globular protein actin. Actin filaments exhibit
polarity; that is, they have plus (+) and minus (-) ends. These
designate the direction of growth of the filaments. Actin molecules spontaneously form these filaments, even in a test tube.
Cells regulate the rate of actin polymerization through other
proteins that act as switches, turning on polymerization
when appropriate.
Microfilaments are
thin, linear structures, first observed distinctly in
muscle cells, where they are responsible for the
ability of a cell to contract. They are made of a
protein called actin. Several dozen other proteins are known that bind with actin and determine its configuration and behavior in particular
cells. One of these is myosin, whose interaction with actin causes contraction in muscle and
other cells. Actin microfilaments provide
a means for movement of molecules and organelles through the cytoplasm, as well as movement
of messenger RNA from the nucleus to
particular positions within the cytoplasm. Actin and actin-binding
proteins are also important in movement of vesicles between the
ER, Golgi complex, and plasma membrane or lysosomes.see figure
Microtubules
Microtubules, the largest of the cytoskeletal elements, are hollow tubes about 25 nm in diameter, each composed of a ring of
13 protein protofilaments. Globular proteins consisting of
dimers of α- and β-tubulin subunits polymerize to form the
13 protofilaments. The protofilaments are arrayed side by side
around a central core, giving the microtubule its characteristic
tube shape.
Microtubules are in a constant state of flux, continually
polymerizing and depolymerizing. The average half-life of a
microtubule ranges from as long as 10 minutes in a nondividing
animal cell to as short as 20 seconds in a dividing animal cell.
The ends of the microtubule are designated as plus (+; away
from the nucleation center) or minus (−; toward the nucleation
center).
Microtubules play a vital role in moving chromosomes toward daughter cells during cell division, and they are important in
intracellular architecture, organization, and transport. In addition,
microtubules form essential parts of the structures of cilia and flagella.
Centrosomes are microtubule -organizing centers
Centrioles are barrel-shaped organelles found in the cells of
animals and most protists. They occur in pairs, usually located
at right angles to each other near the nuclear membranes
(figure).
Centrioles: Each centriole is composed of nine triplets of microtubules. Centrioles are usually not found in plant
cells. In animal cells they help to organize microtubules.
The region surrounding the pair in almost all animal
cells is referred to as a centrosome. Surrounding the centrioles in
the centrosome is the pericentriolar material, which contains ring-shaped structures composed of tubulin. The
pericentriolar material can nucleate the assembly
of microtubules in animal cells. Structures
with this function are called microtubule-organizing centers. The centrosome is also
responsible for the reorganization of microtubules that occurs during cell division.
The centrosomes of plants and fungi lack
centrioles but still contain microtubule-organizing centers.
Intermediate Filaments
The most durable element of the cytoskeleton is a system of tough,
fibrous protein molecules twined together in an overlapping
arrangement. These intermediate filaments are characteristically
8 to 10 nm in diameter—between the sizes of actin filaments and
microtubules. Once formed, intermediate filaments are stable and
only rarely break down.
Intermediate filaments constitute a mixed group of
cytoskeletal fibers. The most common type, composed of
protein subunits called vimentin, provides structural stability
for many kinds of cells.keratin, another class of intermediate
filament, is found in epithelial cells (cells that line organs
and body cavities) and associated structures such as hair
and fingernails.
The Cytoskeleton Helps Move Materials Within the Cell
Actin filaments and microtubules often
orchestrate their activities to affect cellular processes. For example, during cell
reproduction, newly
replicated chromosomes move to opposite sides of a dividing cell because
they are attached to shortening microtubules. Then, in animal cells, a belt of
actin pinches the cell in two by contracting like a purse string.
Muscle cells also use actin filaments,
which slide along filaments of the motor protein
myosin when a muscle contracts. The fluttering of an eyelash, the
flight of an eagle, and the awkward crawling of a baby all depend
on these cytoskeletal movements within muscle cells.
Not only is the cytoskeleton responsible for the cell’s shape
and movement, but it also provides a scaffold that holds certain enzymes and other macromolecules in defined areas of the
cytoplasm. For example, many of the enzymes involved in cell
metabolism bind to actin filaments, as do ribosomes. By moving
and anchoring particular enzymes near one another, the
cytoskeleton, like the endoplasmic reticulum, helps organize the
cell’s activities.
Molecular Motors
All eukaryotic cells must move materials from one place to another
in the cytoplasm. One way cells do this is by using the channels of
the endoplasmic reticulum as an intracellular highway. Material can
also be moved using vesicles loaded with cargo that can move along
the cytoskeleton like a railroad track. For example, nerve cells have
long projections that extend away from the cell body. Vesicles can
move along tracks from the cell body to the end of the cell.
Four components are required to move material along
microtubules: (1) a vesicle or an organelle that is to be transported,
(2) a motor protein that provides the energy-driven motion, (3) a
connector molecule that connects the vesicle to the motor molecule, and (4) microtubules on which the vesicle will ride like a
train on a rail (figure).
Molecular motors:Vesicles can be
transported along microtubules using motor proteins that use
ATP to generate force. The vesicles are attached to motor
proteins by connector molecules, such as the dynactin complex
shown here. The motor protein dynein moves the connected
vesicle along microtubules.
The direction a vesicle is moved depends on the type of
motor protein involved and the fact that microtubules are organized with their plus ends toward the periphery of the cell. In one case, a protein called kinectin binds vesicles to the motor protein
kinesin. Kinesin uses ATP to power its movement toward the cell
periphery, dragging the vesicle with it as it travels along the
microtubule toward the plus end. As nature’s tiniest motors, these
proteins pull the transport vesicles along the microtubular tracks.
Another set of vesicle proteins, called the dynactin comples,
binds vesicles to the motor protein dynein , which directs movement in the opposite direction
along microtubules toward the minus end, inward toward the
cell’s center. (Dynein is also involved in the movement of
eukaryotic flagella. The
destination of a particular transport vesicle and its content is
thus determined by the nature of the linking protein embedded
within the vesicle’s membrane.
Eukaryotic Cells Use Cytoskeletal Elements to Crawl or Swim
Essentially all cell motion is tied to the movement of actin filaments, microtubules, or both. Intermediate filaments act as intracellular tendons, preventing excessive stretching of cells. Actin
filaments play a major role in determining the shape of cells.
Because actin filaments can form and dissolve so readily, they
enable some cells to change shape quickly.
The arrangement of actin filaments within the cell cytoplasm allows cells to crawl, literally! Crawling is important within
your own body, essential to such diverse processes as inflammation, clotting, wound healing, and the spread of cancer. White
blood cells, in particular, exhibit this ability. Produced in the bone
marrow, these cells are released into the circulatory system and
then eventually crawl out of venules and into the tissues to destroy
potential pathogens.
At the leading edge of a crawling cell, actin filaments rapidly polymerize, and their extension forces the edge of the cell
forward. This extended region is stabilized when microtubules
polymerize into the newly formed region. Overall forward movement of the cell is then achieved through the action of the protein
myosin, which is best known for its role in muscle contraction.
Myosin motors along the actin filaments contract, pulling the contents of the cell toward the newly extended front edge.
Cells crawl when these steps occur continuously, with a
leading edge extending and stabilizing, and then motors contracting to pull the remaining cell contents along. Receptors on the cell
surface can detect molecules outside the cell and stimulate extension in specific directions, allowing cells like white blood cells to
move toward particular targets.
Flagella and Cilia Aid Movement
Many protists use flagella to swim. Eukaryotic cells have a completely different kind of flagellum, consisting of a circle of nine
microtubule pairs surrounding two central microtubules. This
arrangement is referred to as the 9 + 2 structure (figure).
Flagella and cilia: A eukaryotic flagellum
originates directly from a basal body. The flagellum has two
microtubules in its core connected by radial spokes to an outer
ring of nine paired microtubules with dynein arms (9 + 2
structure). The basal body consists of nine microtubule triplets
connected by short protein segments. The structure of cilia is
similar to that of flagella, but cilia are usually shorter.
As pairs of microtubules move past each other using arms
composed of the motor protein dynein, the eukaryotic flagellum
undulates, rather than rotates. When examined carefully, each
flagellum proves to be an outward projection of the cell’s interior,
containing cytoplasm and enclosed by the plasma membrane. The
microtubules of the flagellum are derived from a basal body,
situated just below the point where the flagellum protrudes from
the surface of the cell.
The flagellum’s microtubular structure evolved early in the
history of eukaryotes. Today the cells of many multicellular and
some unicellular eukaryotes no longer possess flagella and are nonmotile. Other structures, called cilia (singular,cilium), with an organization similar to the 9 + 2 arrangement of microtubules, can still be
found within them. Cilia are short cellular projections that are often
organized in rows. They are more numerous than flagella on the cell
surface but have the same internal structure. Often the beating of
rows of cilia moves water over the tissue surface (see figures).
Flagella and Cilia: a. A green alga with
numerous flagella that allow it to move through the water.
b. Paramecia are covered with many cilia, which beat in unison
to move the cell. The cilia can also be used to move fluid into the
paramecium’s mouth to ingest material.
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Eukaryotic Cells Use Cytoskeletal Elements to Crawl or Swim
Essentially all cell motion is tied to the movement of actin filaments, microtubules, or both. Intermediate filaments act as intracellular tendons, preventing excessive stretching of cells. Actin
filaments play a major role in determining the shape of cells.
Because actin filaments can form and dissolve so readily, they
enable some cells to change shape quickly.
The arrangement of actin filaments within the cell cytoplasm allows cells to crawl, literally! Crawling is important within
your own body, essential to such diverse processes as inflammation, clotting, wound healing, and the spread of cancer. White
blood cells, in particular, exhibit this ability. Produced in the bone
marrow, these cells are released into the circulatory system and
then eventually crawl out of venules and into the tissues to destroy
potential pathogens.
At the leading edge of a crawling cell, actin filaments rapidly polymerize, and their extension forces the edge of the cell
forward. This extended region is stabilized when microtubules
polymerize into the newly formed region. Overall forward movement of the cell is then achieved through the action of the protein
myosin, which is best known for its role in muscle contraction.
Myosin motors along the actin filaments contract, pulling the contents of the cell toward the newly extended front edge.
Cells crawl when these steps occur continuously, with a
leading edge extending and stabilizing, and then motors contracting to pull the remaining cell contents along. Receptors on the cell
surface can detect molecules outside the cell and stimulate extension in specific directions, allowing cells like white blood cells to
move toward particular targets.
Flagella and Cilia Aid Movement
Many protists use flagella to swim. Eukaryotic cells have a completely different kind of flagellum, consisting of a circle of nine
microtubule pairs surrounding two central microtubules. This
arrangement is referred to as the 9 + 2 structure (figure).
originates directly from a basal body. The flagellum has two
microtubules in its core connected by radial spokes to an outer
ring of nine paired microtubules with dynein arms (9 + 2
structure). The basal body consists of nine microtubule triplets
connected by short protein segments. The structure of cilia is
similar to that of flagella, but cilia are usually shorter.
As pairs of microtubules move past each other using arms
composed of the motor protein dynein, the eukaryotic flagellum
undulates, rather than rotates. When examined carefully, each
flagellum proves to be an outward projection of the cell’s interior,
containing cytoplasm and enclosed by the plasma membrane. The
microtubules of the flagellum are derived from a basal body,
situated just below the point where the flagellum protrudes from
the surface of the cell.
The flagellum’s microtubular structure evolved early in the
history of eukaryotes. Today the cells of many multicellular and
some unicellular eukaryotes no longer possess flagella and are nonmotile. Other structures, called cilia (singular,cilium), with an organization similar to the 9 + 2 arrangement of microtubules, can still be
found within them. Cilia are short cellular projections that are often
organized in rows. They are more numerous than flagella on the cell
surface but have the same internal structure. Often the beating of
rows of cilia moves water over the tissue surface (see figures).
Flagella and Cilia: a. A green alga with
numerous flagella that allow it to move through the water.
b. Paramecia are covered with many cilia, which beat in unison
to move the cell. The cilia can also be used to move fluid into the
paramecium’s mouth to ingest material.
Press the Bell-icon and Subscribe
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