I. The Cytoskeleton is a network of fibers that organizes structures and activities in the cellII. Extracellular components and connections between cells help coordinate cellular activitiesI. Cytoplasmic determinants and inductive signals contribute to cell fate specificationChapter 6 (part 2) – A Tour of the Cell – Lecture 6I. The Cytoskeleton is a network of fibers that organizes structures and activities in the cell 1. Cytoskeleton – a network of fibers extending throughout the cytoplasm that plays a major role in organizing the structures and activities of the cell composed of three types of molecular structures: microtubules, microfilaments, and intermediate filaments. 2. Most obvious function of the cytoskeleton is to give mechanical support to the cell and maintain its shape. 3. Cell motility (movement) encompasses both changes in cell location and more limited movements of parts of the cell. 4. Cell motility generally requires the interaction of the cytoskeleton with motor proteins.5. Microtubules (the thickets of the 3 types) – all eukaryotic cells have these hollow rods measuring 25nm in diameter and from 200nm to 25 μm in length. Compression-resisting role 6. The wall of hollow tube is constructed from a globular protein called tubulin. Each tubulin is a dimer, a molecule made up of two subunits. 7. Tubulin dimer consists of two slightly different polypeptides α-tubulin and β-tubulin 8. Microtubules shape and support the cell and also serve as tracks along which organelles equipped with motor proteins can move. They guide secretory vesicles from the Golgi apparatus to the plasma membrane. They also are involved in the separation of chromosomes during cell division. 9. In animals, microtubules grow out from a centrosome, a region that is often located near the nucleus and is considered a “microtubule-organizing center”. 10. Within the centrosome is a pair of centrioles, each composed of nine sets of triplet microtubules arranged in a ring. 11. In eukaryotes, a specialized arrangement of microtubules is responsible for the beating of flagella and cilia, microtubule-containing extensions that project from some cells. 12. A flagellum has an undulating motion that generates force in the same direction as the flagellum’s axis13. Cilia work more like oars, with alternating power and recovery strokes generating force in a direction perpendicular to the cilium’s axis.14. The microtubule assembly of a cilium or flagellum is anchored in the cell by abasal body, which is structurally very similar to a centriole, with microtubule triplets in a “9+0” pattern. 15. Dyneins- large motor proteins each composed of several polypeptides. They are responsible for the bending movements of the organelle. 16. Microfilaments are solid rods about 7nm in diameter. They are also called actin filaments because they are built from molecules of actin, a globularprotein. it is a twisted double chain of actin subunits besides occurring as linear filaments they can form structural networks when certain proteins bind along the side of an actin filament and allow new filaments to extend as a branch. 17. Structural role of microfilaments in the cytoskeleton is to bear tension (pulling forces). 18. A three-dimensional network formed by microfilaments just inside the plasma membrane helps support the cell’s shape. This network give the cytoplasmic layer of a cell, called the cortex, the semisolid consistency of a gel. 19. Microfilaments are well known for their role in cell motility, particularly as part of the contractile apparatus of muscle cells. Thousands of actin filaments are arranged parallel to one another along the length of a muscle cell, interdigitated with thicker filaments made of protein called myosin. 20. Pseudopodia – cellular extensions that extend by assembly of actin subunits into microfilament networks that convert cytoplasm from a sol to a gel insidethese cell projections. 21. Cytoplasmic streaming – a circular flow of cytoplasm within cells. It speeds the distribution of materials within a cell. 22. Intermediate filaments – fibrous proteins (keratins, lamins, etc.), coiled cables (8-12nm) physical support, anchorage of organelles and component of nuclear lamina resist stretching II. Extracellular components and connections between cells help coordinate cellular activities1. Cell wall- an extracellular structure of plant cells that distinguishes them from animal cells. Protects the plant cell, maintains its shape, and preventsexcessive uptake of water. 2. A young plant cell first secretes a relatively thin and flexible wall called the primary cell wall.3. Between primary walls of adjacent cells is the middle lamella, a thin layer rich in sticky polysaccharides called pectins. Glues adjacent cells together and when the cell matures and stops growing it strengthens its wall. 4. Some plant cells do this simply by secreting hardening substances into the primary wall other cells add a secondary cell wall between the plasma membrane and the primary wall. Often deposited in several laminated layers, has a strong and durable matrix that affords the cell protection and support. 5. Animal cells lack walls akin to those of plant cells, but they do have an elaborate extracellular matrix (ECM), which is made by glycoproteins and other carbohydrate-containing molecules secreted by the cells. 6. Most abundant glycoprotein in the ECM of most animal cells is collagen, which forms strong fibers outside the cells.7. They are embedded in a network woven out of proteoglycans, that consists of a small core protein with many carbohydrate chains covalently attached, secreted by cells 8. Some cells are attached to the ECM by ECM glycoproteins such as fibronectin, like other ECM proteins that bind to cell-surface receptor proteins celled integrins that are built into the plasma membrane. 9. Cells in an animal or plant are organized into tissues, organs, and organ systems. 10. Plant cell walls are perforated with plasmodesmata, membrane-lined channels filled with cytoplasm. Cytosol passes through and joins the internal chemical environments of adjacent cells, which unifies most of the plant into one living continuum. 11. Cell junctions connect adjacent cells, allowing direct transfer of molecules and ions between cytosols.12. In animals there are three main types of cell junctions: tight junctions, desmosomes, and gap
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