cell wall
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CELL WALL
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CELL WALL
Gross Structure Detailed Structure Chemistry Features
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GROSS STRUCTURE
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Middle lamella
Cement; amorphous subs. Bet. P-walls of neighboring cells Pectic substances (Ca, Mg pectate)
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Primary wall
First wall the develops on new cell Cellulose, pectic cpds., non-cellulosic
polysaccharides and hemicellulose May be lignified Assoc. with living protoplasts
--eg. meristematic cells, parenchyma, collenchyma
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Secondary wall
Formed in the inner surface of P-wall Same content as Pwall ( > cellulose) + lignin In cells that ceased to grow; devoid of protoplast at
maturity* xylem ray, xylem parenchyma – still living
Mechanical support
Compound middle lamella *= 3-layered or 5-layered= middle lamella + 2 P-walls (+ 2 S-walls)
*if middle lamella is obscured
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Formation of wall
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Cell plate- precursor of cell wall; rich in pectins
Phragmoplast- a complex of microtubules and ER that forms
during late anaphase or early telophase from dissociated spindle subunits.
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Fine structure of the wall
Cellulose fibrils Matrix (non-cellulosic):
- with lignin, cutin, suberin, hemicelluloses etc.
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Ml- middle lamellaPm- plasma membrance
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Levels of organization of cell wall
Long chains of linked glucose residues
Micellae – bundles of cellulose molecules or ELEMENTARY FIBRIL = ~40 cellulose molecules
Microfibril Bundles of microfibril
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CHEMISTRY OF WALLS
Cellulose Pectic substances Gums and mucilages Lignin Fatty substances
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Cellulose
Hydrophilic crystalline compound Repeating monomers of glucose
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Pectic substances
Amorphous colloidal substances Plastic and hydrophilic
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Gums and mucilages
Appear as a result of physiological or pathological disturbances that induce breakdown of walls and cell contents
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Lignin
Phenolic compounds May be found in middle lamella,
primary wall, and secondary wall hydrophobic fi ller that replaces the
wall’s water compressive strength and bending stiffness Microbial attack resistance
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Fatty substances
Cutin, suberin, waxes Waxes- glaucous condition; assoc. with cutin
and suberin Suberin- cork cells of periderm; endodermis
and exodermis; prevents apoplastic transport Cutin- cuticle layer; epidermis of aerial parts
Cutinization, suberization- impregnation in cell wall
Cuticularization- formation of layer
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Cellulose
Tensile strength (bend under compressive stress)
Incrustation– eg. Lignification
Cell wall growthA. intussusceptionB. appositionC. mosaic growthD. multinet growth
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Intussusception
Material of new wall is laid down bet. Particles of the existing substance of the expanding wall
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Apposition
Growth is due to the centripetal addition of new layers one upon the other
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Mosaic growth
Fibrillar texture in certain wall areas become loosened as a result of turgor pressure and afterwards mended by deposition of new microfibrils in the gaps caused by the strain
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Multinet growth
separation of crossed microfibrils and alteration in their orientation
transverse longitudinal
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Special structures of the cell wall
Primary pit fieldsPitsCrassulaeTrabeculaeWart structuresCystoliths
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Primary pit fields
Primordial pits/ primary pit fields Certain areas of primary wall of
young cells remain thin May appear beaded in xs
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Plasmodesmata
Plasmodesmata- connnect protoplasts of neighboring cells- transport; relay of stimuli* symplast- 2 or more interconnected protopolast* apoplast – cell walls, intercellular spaces and lumen
desmotubule
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Pits
Portions of the cell wall that remained thin even as secondary wall is formed
Primary wall only Can develop over primary pit fields
Function?
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Pits
TYPES:a. simple pitb. bordered pit—S-wall develops over the pit cavity to form an overarching roof
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Simple pits
Branched simple pits (ramiform) Found in parenchyma cells with
thickened walls, libriform fibers, sclereids, phloem fibers
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Bordered pits
Structure: Pit cavity / pit chamber Pit aperture Pit border Pit canal, inner and outer aperture (very
thick S-wall) water-conducting and mechanical
xylem cells (vessel elements, tracheids, etc.)
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Angiosperm Gymnosperm
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Pit-pair
Pit cavity – break in S-wall Pit membrane/ closing membrane –
primary wall + middle lamella Pit aperture
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Types of pit-pair
Simple pit pair Bordered pit pair Half bordered pit pair Blind pit Unilateral compound pitting
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Torus
pit membrane thickening; disc shaped
flexible; can go median or lateral Aspirate condition (lateral)–
latewood and all heartwood Coniferales, Gnetales
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Margo
– porous pit membrane around the torus--conifer tracheids-- occurs through matrix dissolution
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Shape of pit aperture
Round, elliptic, linear In thick cell walls:
*inner aperture becomes long and narrow*outer aperture remains circular round* pit canal is funnel-shaped*fiber-tracheid feature
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Bordered pits arrangement
Scalariform Opposite Alternate
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crassulae
Linear or crescent-shaped thickenings of the primary wall and middle lamella
gymnosperms
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trabeculae
Rod shape thickenings of the wall which traverse the cell lumen radially
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References
Fahn, A. 1990. Plant Anatomy, 4th ed.. Pergamon Press
Esau, K. 1958. Plant Anatomy. John Wiley and Sons, Inc.
Evert, R. 2006. Esau’s Plant Anatomy. John Wiley and Sons, Inc.