Download The cytoskeleton The cell surface and junctions

The cytoskeleton
The cell surface and junctions (Lecture 5)
The cytoskeleton:
gives the cell shape,
anchors some organelles and
directs the movement of others,
and may enable the entire cell to change shape or move.
It may play a regulatory role, by mechanically transmitting signals from the cell's surface
to its interior.
Motor molecules and the cytoskeleton (Fig.6.21)
The microtubules and microfilaments interact with proteins called motor molecules.
Motor molecules change their shapes, moving back and forth something like microscopic
legs.
ATP powers these conformational changes.
(a) In some types of cell motility, motor molecules attached to one element of the
cytoskeleton cause it to slide past another cytoskeletal element.
For example, a sliding of neighboring microtubules moves cilia and flagella.
In muscle cell contraction, motor molecules slide microfilaments rather than
microtubules.
(b) Motor molecules can also attach to receptors on organelles such as vesicles and
enable the organelles to "walk" along microtubules of the cytoskeleton.
For example, vesicles containing neurotransmitters migrate to the tips of axons,
the long extensions of nerve cells that release transmitter molecules as chemical
signals to adjacent nerve cells.
Flagella and Cilia (Fig.6.23)
Locomotive appendages that protrude from some cells.
A specialized arrangement of mictotubules responsible for their beating
(a) A flagellum usually undulates, its snakelike motion driving a cell in the same
direction as the axis of the flagellum.
Propulsion of a sperm cell is an example of flagellate locomotion (SEM).
(b) A dense nap of beating cilia covers this Paramecium, a motile protist (SEM).
The cilia beat at a rate of about 40 to 60 strokes per second.
Cilia have a back-and-forth motion, alternating active strokes with recovery strokes.
This moves the cell, or moves a fluid over the surface of a stationary cell, in a direction
perpendicular to the axis of the cilium.
Ultrastructure of microtubules (Fig.6.24)
The basal body anchoring the cilium or flagellum to the cell has a ring of nine
microtubule triplets (structurally identical to a centriole).
The nine doublets of the cilium extend into the basal body, where each doublet joins
another microtubule to form the ring of nine triplets.
The two central microtubules of the cilium terminate above the basal body (TEM).
Dynein – motor protein (Fig.6.25)
Responsible for the bending movements of cilia and flagella
The dynein arms of one microtubule doublet grip the adjacent doublet, pull, release, and
then grip again.
This cycle of the dynein motors is powered by ATP.
The doublets cannot slide far because they are physically restrained within the cilium.
Instead, the action of the dynein arms causes the doublets to bend.
Actin and tubulin components of the cytoskeleton.
Microfilaments – actin filaments.
They are built from molecules of a globular protein – actin.
A mictofilament is a twisted double chain of actin subunits (7 nm in diameter)
Role of microfilaments
Maintenance of cell shape (as a tension-bearing elements)
Changes in cell shape
Muscle contruction
Cytoplasmic streaming
Cell motility
Cell division – cleavage furrow formation
A structural role of microfilaments
The surface area of this nutrient-absorbing intestinal cell is increased by its many
microvilli, cellular extensions reinforced by bundles of microfilaments.
The actin filaments are anchored to a network of intermediate filaments
Microfilaments and motility (Fig.6.27)
(a) In muscle cells, actin filaments (orange) lie parallel to thick myosin filaments (purple).
Myosin acts as a motor molecule by means of arms that "walk" the two types of
filaments past each other.
The teamwork of many such sliding filaments enables the entire muscle cell to
shorten.
(b) In a crawling cell (ameboid movement), actin is organized into a network in the gellike cortex (outer layer).
It also exists as subunits and linear filaments in the more fluid (sol) interior.
According to one hypothesis, filaments at the cell's trailing end interact with
myosin, causing contraction.
This contraction forces the interior fluid into the pseudopod, where the actin
network has been weakened.
The pseudopod extends until the actin reassembles into a network.
(c) In cytoplasmic streaming, a layer of cytoplasm cycles around the cell, moving over a
carpet of parallel actin filaments.
Myosin motors attached to organelles in the fluid cytosol may drive the streaming
by interacting with the actin.
Also, rapid changes between the gel and sol states may occur locally.
Role of microtubules
Hollow tubes with wall that consists of 13 columns of tubulin molecules (25 nm in
diameter)
Involved in cell shape maintenance (compression-resisting “girders”)
Cell motility (as in cilia or flagella)
Chromosome movement in cell division
Fibrous proteins supercoiled into thicker cables (8-12 nm)
Depending on the cell type, it is presented by one of the several different proteins
of the keratin family
Responsible for:
maintenance of cell shape (tension-bearing elements)
anchorage of nucleus and certain other organelles
formation of nuclear lamina
Cell surface and junctions
Plant cell walls
Protects the plant cell,
Maintains its shape,
Prevents excessive uptake of water
Holds the plant up against the force of gravity
Young cells first construct thin primary walls.
Stronger secondary walls are added to the inside of the primary wall when growth
ceases.
A sticky middle lamella cements adjacent cells together.
The walls do not isolate the cells:
the cytoplasm of one cell is continuous with the cytoplasm of its neighbors via
lasmodesmata, channels through the walls (TEM).
Extracellular matrix (ECM) of an animal cell (Fig.6.29)
The molecular composition and structure of the ECM varies from one cell type to another.
In this example, three different types of glycoproteins are present:
fibers made of the glycoprotein collagen are embedded in a web of proteoglycans,
which can be as much as 95% carbohydrate;
the proteoglycan molecules have formed complexes by noncovalently attaching to
long polysaccharide molecules;
the third glycoprotein is fibronectin, the adhesive that attaches the ECM to the
plasma membrane of the cell;
membrane proteins called integrins are bound to the ECM on one side and to the
microfilaments of the cytoskeleton on the other. This linkage can transmit stimuli
between the cell's external environment and its interior.
Intracellular junctions in animals (Figure 6.30)
(a) Tight junctions - continuous belts around the cell.
Fusion of neighboring cell membranes forms a seal that prevents leakage of
extracellular fluid across a layer of epithelial cells.
For example, the tight junctions of the intestinal epithelium keep the contents of
the intestine separate from the body fluid on the opposite side of the epithelium
(TEM).
(b) Desmosomes (anchoring junctions) function like rivets, fastening cells together into
strong epithelial sheets.
Intermediate filaments made of the sturdy protein keratin reinforce desmosomes
(TEM).
(c) Gap junctions (communicating junctions) provide cytoplasmic channels between
adjacent cells.
Special membrane proteins surround each pore.
The pore is wide enough for salts, sugars, amino acids, and other small molecules
to pass (TEM).
In the muscle tissue of the heart, the flow of ions through gap junctions
coordinates the contractions of the cells.
Gap junctions are especially common in animal embryos, in which chemical
communication between cells is essential for development.
Reading
Campbell et al. Biology. Ch. 6 A tour of the cell, 112-122.