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Unformatted text preview: MCDB321 Plant Physiology
Jan 11, 2011
Lecture 2 Plant Cell Wall & Plasmodesmata I. II. Plant Cell Walls
2. Formation and cellulose fibrils
3. Biosynthesis of matrix olgosaccharides
4. Secondary cell wall and lignification
5. Lignocellulose & Biofuel
2. Rehulation of SEL The Importance of plant cell walls
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%
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
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
glucose units in
the primary cell wall Hydrogen-bonding between
adjacent glucan chains Cellulose Microfibrils are Synthesized
at the Plasma Membrane
is synthesized in the Golgi body
and secreted in vesicles Each having many
of cellulose synthesis Steam Engine Model for Cellulose Deposition
One way to visualize this process is to imagine an old Steam Engine (Rosette).
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
of the cellulose
2. Plant often contain many
genes encoding similar
cellulose synthase Matrix oligosaccharides are synthesized in the Golgi body and
secreted to the cell wall Hemicellulose, cellulose-bind component
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
1. Contain acidic sugars (galacturonic acid) and neutral sugars
2. Most soluble components of the cell wall (75% water)
3. Galactoronic acids are often esterified Arabinogalactan proteins might function in
cell adhesion and cell signaling New primary cell walls are assembled during cytokinesis
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
breakage and rejoining of hemicelluloses
2. Orientation of celulose microfibrils
determines the direction of cell expansion Endo-glycosylase
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
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
are responsible for the transportation of molecules
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)
(SEL) Plasmodesmata are also a weakness for plants,
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
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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