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Cytoskeleton

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Cytoskeleton Cytoskeleton Presentation Transcript

  • Cytoskeleton Providing structural support to the cell, the cytoskeleton also functions in cell motility and regulation
  • Structural Support Mechanical support Maintains shape Fibers act like a geodesic dome to stabilize and balance opposing forces Provides anchorage for organelles Dynamic Dismantles in one spot and reassembles in another to change cell shape
  • Introduction The cytoskeleton is a network of fibers extending throughout the cytoplasm. The cytoskeleton organizes the structures and activities of the cell.
  • The cytoskeleton also plays a major role in cell motility. This involves both changes in cell location and limited movements of parts of the cell. The cytoskeleton interacts with motor proteins. In cilia and flagella motor proteins pull components of the cytoskeleton past each other. This is also true in muscle cells. Fig. 7.21a
  • Motor molecules also carry vesicles or organelles to various destinations along “monorails’ provided by the cytoskeleton. Interactions of motor proteins and the cytoskeleton circulates materials within a cell via streaming. Recently, evidence is accumulating that the cytoskeleton may transmit mechanical signals that rearrange the nucleoli and other structures. Fig. 7.21b
  • There are three main types of fibers in the cytoskeleton: microtubules, microfilaments, and intermediate filaments.
  • Microtubules, the thickest fibers, are hollow rods about 25 microns in diameter. Microtubule fibers are constructed of the globular protein, tubulin, and they grow or shrink as more tubulin molecules are added or removed. They move chromosomes during cell division. Another function is as tracks that guide motor proteins carrying organelles to their destination. Fig. 7.21b
  • In many cells, microtubules grow out from a centrosome near the nucleus. These microtubules resist compression to the cell.
  • • In animal cells, the centrosome has a pair of centrioles, each with nine triplets of microtubules arranged in a ring. • During cell division the centrioles replicate. Fig. 7.22
  • Microtubules are the central structural supports in cilia and flagella. Both can move unicellular and small multicellular organisms by propelling water past the organism. If these structures are anchored in a large structure, they move fluid over a surface.  For example, cilia sweep mucus carrying trapped debris from the lungs. Fig. 7.2
  • Cilia usually occur in large numbers on the cell surface. They are about 0.25 microns in diameter and 2-20 microns long. There are usually just one or a few flagella per cell. Flagella are the same width as cilia, but 10-200 microns long.
  • A flagellum has an undulatory movement. Force is generated parallel to the flagellum’s axis. Fig. 7.23a
  • Cilia move more like oars with alternating power and recovery strokes. They generate force perpendicular to the cilia’s axis. Fig. 7.23b
  • In spite of their differences, both cilia and flagella have the same ultrastructure. Both have a core of microtubules sheathed by the plasma membrane. Nine doublets of microtubules arranged around a pair at the center, the “9 + 2” pattern. Flexible “wheels” of proteins connect outer doublets to each other and to the core. The outer doublets are also connected by motor proteins. The cilium or flagellum is anchored in the cell by a basal body, whose structure is identical to a centriole.
  • Fig. 7.24
  • The bending of cilia and flagella is driven by the arms of a motor protein, dynein. Addition to dynein of a phosphate group from ATP and its removal causes conformation changes in the protein. Dynein arms alternately grab, move, and release the outer microtubules. Protein cross-links limit sliding and the force is expressed as bending. Fig. 7.25
  • Microfilaments, the thinnest class of the cytoskeletal fibers, are solid rods of the globular protein actin. An actin microfilament consists of a twisted double chain of actin subunits. Microfilaments are designed to resist tension. With other proteins, they form a three-dimensional network just inside the plasma membrane.
  • Fig. 7.26 The shape of the microvilli in this intestinal cell are supported by microfilaments, anchored to a network of intermediate filaments.
  • In muscle cells, thousands of actin filaments are arranged parallel to one another. Thicker filaments, composed of a motor protein, myosin, interdigitate with the thinner actin fibers. Myosin molecules walk along the actin filament, pulling stacks of actin fibers together and shortening the cell. Fig. 7.21a
  •  In other cells, these actin-myosin aggregates are less organized but still cause localized contraction. A contracting belt of microfilaments divides the cytoplasm of animals cells during cell division. Localized contraction also drives amoeboid movement.  Pseudopodia, cellular extensions, extend and contract through the reversible assembly and contraction of actin subunits into microfilaments. Fig. 7.21b
  • In plant cells (and others), actin-myosin interactions and sol-gel transformations drive cytoplasmic streaming. This creates a circular flow of cytoplasm in the cell. This speeds the distribution of materials within the cell. Fig. 7.21c
  • Intermediate filaments, intermediate in size at 8 - 12 nanometers, are specialized for bearing tension. Intermediate filaments are built from a diverse class of subunits from a family of proteins called keratins. Intermediate filaments are more permanent fixtures of the cytoskeleton than are the other two classes. They reinforce cell shape and fix organelle location. Fig. 7.26