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Transcript
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.