Cellular Structures & Functions Lecture 8 PDF

CELLULAR STRUCTURAL SUPPORT The Cytoskeleton, cilia & flagella, cell junctions, etc.. Lesson 8 (Chapter 4 (4.16-4.22) BIO 131 What is the Cytoskeleton? The Cytoskeleton is a network of fibers throughout the cell's cytoplasm. Functions of Cytokeleton Eucaryotic cells contain...

CELLULAR STRUCTURAL SUPPORT The Cytoskeleton, cilia & flagella, cell junctions, etc.. Lesson 8 (Chapter 4 (4.16-4.22) BIO 131 What is the Cytoskeleton? The Cytoskeleton is a network of fibers throughout the cell's cytoplasm. Functions of Cytokeleton Eucaryotic cells contain protein fibers that are involved in ○ provides cell structure and shape ○ providing mechanical strength ○ cell movement ○ chromosome separation ○ intracellular transport of organelles  Intercellular Transport Associated with dyneins and kinesins. Transports organelles, such as mitochondria or vesicles. Is responsible for the movement of vesicles and organelles. Kinesin protein walking on microtubule Role as Mitotic Spindle The role as a mitotic spindle is the help align and separate chromosomes. Overall Function to Cell & Animal as a Whole Function to Cell Function to Animal It keeps the cells together, which Cell Shape keeps the cells alive, which in turn keeps the animal alive. Organelle Movement Cell Movement Cell Division  THE SELF-ASSEMBLY AND DYNAMIC STRUCTURE OF CYTOSKELETAL FILAMENTS Three types of macromolecular fibers 1. Actin Filaments (also called microfilaments) 2. Intermediate Filaments 3. Microtubules In addition, a large number of accessory proteins, including the motor proteins, are required for the properties associated with each of these filaments Each type of filament has distinct mechanical properties and dynamics, but certain fundamental principles are common to all. 8 1. Microfilaments/Actin Filaments Microfilaments are little strands made of proteins called actin. The microfilament within the cytoskeleton is in the shape of a double helix. Mostly found in the cortex, just beneath the plasma membrane. 2. Intermediate Filaments Intermediate Filaments are insoluble proteins that help form the a cell's cytoskeleton system, the internal framework that gives a cell its shape. The intermediate filaments structure is of two antiparallel helices forming “ropelike” fibers Stabilizes organelles.  Intermediate filaments provide mechanical strength and resistance to shear stress. There are several types of intermediate filaments, each constructed from one or more proteins characteristic of it. ○ Keratins are found in epithelial cells, hair and nails ○ Nuclear lamins form a meshwork that stabilizes the inner nuclear membrane ○ Neurofilaments strengthen the long axons of neurons ○ Vimentins provide mechanical strength to muscle and other cells 3. Microtubules Microtubules are intracellular structures that are shaped like long cylinders or tubes. Made of protein, tubulin  Microtubules participate in a wide variety of cell activities. ○ Most involve motion that is provided by protein “motors” that use ATP. ○ They determine the positions of membrane-enclosed organelles and direct intracellular transport. ○ The migration of chromosomes during mitosis and meiosis takes place on microtubules that make up the spindle fibers. MICROTUBULAR ARRAYS: CENTRIOLES Short, hollow cylinders Composed of 27 microtubules Microtubules arranged into 9 overlapping triplets One pair per animal cell Located in centrosome of animal cells Oriented at right angles to each other Separate during mitosis to determine plane of division May give rise to basal bodies of cilia and flagella 14 CYTOSKELETON: CENTRIOLES Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. empty center of centriole one microtubule triplet one centrosome: one pair of centrioles two centrosomes: two pairs of 200 nm centrioles (Middle): Courtesy Kent McDonald, University of Colorado Boulder; (Bottom): Journal of Structural Biology, Online by Manley McGill et al. Copyright 1976 by Elsevier 15 Science & Technology Journals. Reproduced with permission of Elsevier Science & Technology Journals in the format Textbook via Copyright Clearance Center MICROTUBULAR ARRAYS: CILIA AND FLAGELLA Hair-like projections from cell surface that aid in cell movement Very different from prokaryotic flagella Outer covering of plasma membrane Inside this is a cylinder of 18 microtubules arranged in 9 pairs In center are two single microtubules This 9 + 2 pattern used by all cilia & flagella In eukaryotes, cilia are much shorter than flagella Cilia move in coordinated waves like oars Flagella move like a propeller or corkscrew 16 STRUCTURE OF A FLAGELLUM Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. outer Flagellum microtubule doublet radial spoke central shaft The shaft of the microtubules flagellum has a ring of nine microtubule doublets anchored dynein to a central pair of side arm microtubules. 25 nm Flagellum cross section The side arms dynein Sperm of each doublet side arms plasma are composed triplets membrane of dynein, a motor molecule. Basal body ATP In the presence of ATP, the dynein side arms reach out to their neighbors, The basal body of a flagellum has and bending occurs. 100 nm a ring of nine microtubule triplets Basal body cross section with no central microtubules. (Flagellum, Basal body): © William L. Dentler/Biological Photo Service 17 CELL CONNECTIONS & JUNCTIONS  CELL JUNCTION ★ Eukaryotic cells contain protein filaments that are collectively called as cytoskeleton. ★ These cytoskeleton filaments plays an important role in the establishment of Cellular Junctions  DIRECT CONTACT: CELL JUNCTION Cell junction is the connection between the neighbouring cells or the contact between the cell and extracellular matrix. It is also called membrane junction. Cell junction are classified into three types a-Occluding/Tight Junction b-Communicating junction c-Anchoring junction. ★ Cell Junctions ○ aid in cellular communication ○ help cells stick together to make tissues ○ prevent the passage of materials 1. OCCLUDING JUNCTION A cell-cell junction that produces an impermeable or semi permeable barriers between cells, especially in epithelial membranes. Tight Junction - occluding junctions / zona occludens - zonula occludens), are the closely associated areas of two cells whose membranes join together forming a virtually impermeable barrier to fluid. ✔ A type of junctional complex present only in vertebrates.  2. ADHERENS JUNCTIONS (Anchoring junction) ❑Desmosome ❑Connects intermediate filament of one cell with other cells. ❑cell structure specialized for cell-to-cell adhesion. ❑Prevent lateral tearing of tissues. ❑Hemidesmosome ❑Hemidesmosomes look like half-desmosomes that attach cells to the underlying basal lamina. DESMOSOMES AND HEMIDESMOSOMES BOTH LINK TO INTERMEDIATE FILAMENTS  3. COMMUNICATING JUNCTION (Gap Junction) Cell junction which permit the intercellular exchange of substance(e.g. ions and molecules from one cell to another cell. ) Cell Junctions Gap junction: communication junctions connect plasma membrane channels connecting cytoplasm of adjacent cells connexin form channel pass through: ions, regulatory molecules rapid chemical and electrical communication In body: pancreas, some nerve cells, cardiac muscle (synchronize contractions) IN PLANT CELLS PLASMODESMATA PERFORM MANY OF THE SAME FUNCTIONS AS GAP JUNCTIONS VARIOUS CELL JUNCTIONS FOUND IN A VERTEBRATE EPITHELIAL CELL, CLASSIFIED ACCORDING TO THEIR PRIMARY FUNCTIONS THE END