Lecture02 - MCDB321 Plant Physiology MCDB321 Jan 11, 2011...

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Unformatted text preview: MCDB321 Plant Physiology MCDB321 Jan 11, 2011 Lecture 2 Plant Cell Wall & Plasmodesmata I. II. Plant Cell Walls 1. Functions/components/structure 2. Formation and cellulose fibrils 3. Biosynthesis of matrix olgosaccharides 4. Secondary cell wall and lignification 5. Lignocellulose & Biofuel Plasmodesmata 1. Formation/structure 2. Rehulation of SEL The Importance of plant cell walls The 1. Determine the mechanical strength of plant structures 2. Glue cells together 3. A cellular exoskeleton that control cell shape 4. Determine the plant morphogenesis 5. Required for normal water relations of plants 6. Water-conducting xylem 7. Acts a diffusion barrier 8. Function as food reserves during seed germination 9. Might function in plant pathogen recognition and symbiosis Major Components of the primary cell wall Matrix polysaccharides 35% 30% 30% 30% H2O Ca2+ 1-5% A Reinforced Concrete Model of the Plant Primary Cell Wall Reinforced The Structural model of A cellulose microfibril Cellulose Fibrils have high tensile strength. They are insoluble, chemically inert and resist enzymatic attack! Because of the orientation of the bonds linking the glucose residues, the rings of glucose are arranged in a flip-flop manner. This produces a long, rigid molecule. The absence of side chains allows these linear molecules to lie close together. Because of the many -OH groups, as well as the oxygen atom in the ring, there are many opportunities for hydrogen bonds to form between adjacent chains. ~36 cellulose ~36 molecules, each having ~2000-6000 glucose units in the primary cell wall Hydrogen-bonding between Hydrogen-bonding adjacent glucan chains Cellulose Microfibrils are Synthesized Cellulose at the Plasma Membrane is synthesized in the Golgi body and secreted in vesicles Each having many multisubunit complexes of cellulose synthesis Steam Engine Model for Cellulose Deposition Steam One way to visualize this process is to imagine an old Steam Engine (Rosette). One The Tracks (Microtubules) provide the direction of travel for the Engine. The Smoke (Cellulose) is positively aligned with the Tracks (Microtubules). Cellulose microfibrils, microtubules, and cellulose synthases Cellulose 1. Glucose is the building block 1. of the cellulose 2. Plant often contain many genes encoding similar cellulose synthase Matrix oligosaccharides are synthesized in the Golgi body and Matrix secreted to the cell wall Hemicellulose, cellulose-bind component Hemicellulose, 1. Same β-1.4 glycosidic linkage 2. Short chain length, with lots of short side-chains 3. Prevents the assembly of cellulose into a crystalline microfibril 4. Link several cellulose microfibrils together Pectins are gel-forming components of the matrix Pectins 1. Contain acidic sugars (galacturonic acid) and neutral sugars acidic 2. Most soluble components of the cell wall (75% water) 3. Galactoronic acids are often esterified Arabinogalactan proteins might function in Arabinogalactan might cell adhesion and cell signaling New primary cell walls are assembled during cytokinesis New Microtubules ER membrane Golgi vesicles Cellulose fibril deposition during Interphase occurs in the Plasmalemma and is directed by Microtubules in the Cytoplasm. Cell Wall synthesis after Telophase involves Golgi Vesicles which are directed by Microtubules. This forms the Cell Plate, Cell 1. Plant cell growth requires 1. breakage and rejoining of hemicelluloses 2. Orientation of celulose microfibrils determines the direction of cell expansion Endo-glycosylase Endo- Expansin Expansin Lignification of Cellulose Microfibrils Lignification 1. All bonds leading to the formation of a three-dimensional molecular network are covalent. As a consequence lignins form a network that fulfils all requirements of stability (flexible and tension-proof). 2. The lignins of the single plant groups differ in the percentages of these starting compounds and in the way they are linked. they Plant Cell Wall & Biofuel Production (lignocellulosic ethanol production) Plant During cytokinesis of plant cells, the endoplasmic reticulum has During been associated with the dictyosomal vesicles of the Golgi bodies forming phragmoplast of the dividing cell. Sometimes, portions of the endoplasmic reticulum are trapped within these vesicles as they fuse, these tubular ER elements manage to traverse the cell plate, and forming a new subcellular structure called plasmodesmata. 1. Plasmodesmata are cylinder type things that act like bridges connecting one cell wall to another. 2. They consist of two tubes, one inside the other. The outer tube is called the plasmalemma. It protects the inner tube, the desmotubule. 3. The desmotubule is made up of membrane and tightly wound cytoplasmic strands, and the back bone of the plasmodesmata. 4. The cytoplasmic sleeve is the gap between the plasmalemma and the desmotubule, and is thought to be where molecules pass through to get from one cell wall to another. Plasmodesmata is important in cell communication and the transportation of molecules Recent studies have shown that Plasmodesmata Recent that Plasmodesmata are responsible for the transportation of molecules are from one cell wall to another cell wall, and the signaling and communication between cells. Plasmodesmata transports different kinds of molecules such as RNAs, proteins, nutrients, and hormones. They can regulate which kind and how much of molecules going through them. A lot of communication between molecules occur in plasmodesmata. Such communications initiate several reactions in the plasmodesmata. The Size Exclusion Limit (SEL) is approximately 700 daltons (1.6 nm) Size (SEL) Plasmodesmata are also a weakness for plants, Plasmodesmata as they permit free movement of viruses (and other pathogens) from cell to cell. Plant viruses have evolved specific movement functions, mediated by one or Plant more virus-specified proteins, which interact with the plasmodesmatal machinery so as to increase the "pore" size, and allow specific transport of viral nucleoprotein complexes. All plant-infecting viruses possess one or more movement-related protein (MP) genes: these are very varied, although there are distinct groups of them, and they appear to derive from host plant genes for chaperonins and plasmodesmata-associated proteins. ...
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