CELL WALL

CELL WALL

Gross Structure Detailed Structure Chemistry Features

GROSS STRUCTURE

Middle lamella

Cement; amorphous subs. Bet. P-walls of neighboring cells Pectic substances (Ca, Mg pectate)

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

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

Formation of wall

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.

Fine structure of the wall

Cellulose fibrils Matrix (non-cellulosic):

- with lignin, cutin, suberin, hemicelluloses etc.

Ml- middle lamellaPm- plasma membrance

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

CHEMISTRY OF WALLS

Cellulose Pectic substances Gums and mucilages Lignin Fatty substances

Cellulose

Hydrophilic crystalline compound Repeating monomers of glucose

Pectic substances

Amorphous colloidal substances Plastic and hydrophilic

Gums and mucilages

Appear as a result of physiological or pathological disturbances that induce breakdown of walls and cell contents

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

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

Cellulose

Tensile strength (bend under compressive stress)

Incrustation– eg. Lignification

Cell wall growthA. intussusceptionB. appositionC. mosaic growthD. multinet growth

Intussusception

Material of new wall is laid down bet. Particles of the existing substance of the expanding wall

Apposition

Growth is due to the centripetal addition of new layers one upon the other

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

Multinet growth

separation of crossed microfibrils and alteration in their orientation

transverse longitudinal

Special structures of the cell wall

Primary pit fieldsPitsCrassulaeTrabeculaeWart structuresCystoliths

Primary pit fields

Primordial pits/ primary pit fields Certain areas of primary wall of

young cells remain thin May appear beaded in xs

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

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?

Pits

TYPES:a. simple pitb. bordered pit—S-wall develops over the pit cavity to form an overarching roof

Simple pits

Branched simple pits (ramiform) Found in parenchyma cells with

thickened walls, libriform fibers, sclereids, phloem fibers

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.)

Angiosperm Gymnosperm

Pit-pair

Pit cavity – break in S-wall Pit membrane/ closing membrane –

primary wall + middle lamella Pit aperture

Types of pit-pair

Simple pit pair Bordered pit pair Half bordered pit pair Blind pit Unilateral compound pitting

Torus

pit membrane thickening; disc shaped

flexible; can go median or lateral Aspirate condition (lateral)–

latewood and all heartwood Coniferales, Gnetales

Margo

– porous pit membrane around the torus--conifer tracheids-- occurs through matrix dissolution

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

Bordered pits arrangement

Scalariform Opposite Alternate

crassulae

Linear or crescent-shaped thickenings of the primary wall and middle lamella

gymnosperms

trabeculae

Rod shape thickenings of the wall which traverse the cell lumen radially

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.