The cell wall provides structure and protection for plant cells. It has three main layers - the middle lamella, primary wall, and secondary wall. The middle lamella binds adjacent cells, the primary wall is the first layer deposited and allows cell growth, and the secondary wall provides support and is thicker with lignin. The cell wall is composed of cellulose microfibrils in a matrix of pectin and hemicellulose. It also contains structural components like lignin, cutin and suberin. Pit pairs and plasmodesmata allow communication between cells and transport through the cell wall. Bordered and simple pits are thin areas in the secondary wall that facilitate water and nutrient transport.

1. CELL WALL

2. CELL WALLGross StructureDetailed StructureChemistryFeatures

3. GROSS STRUCTURE

4. Middle lamellaCement; amorphous subs.Bet. P-walls of neighboring cellsPectic substances (Ca, Mg pectate)

5. Primary wallFirst wall the develops on new cellCellulose, pecticcpds., non-cellulosic polysaccharides and hemicelluloseMay be lignifiedAssoc. with living protoplasts --eg. meristematic cells, parenchyma, collenchyma

6. Secondary wallFormed in the inner surface of P-wallSame content as Pwall ( > cellulose) + ligninIn cells that ceased to grow; devoid of protoplast at maturity * xylem ray, xylem parenchyma – still livingMechanical supportCompound middle lamella *= 3-layered or 5-layered= middle lamella + 2 P-walls (+ 2 S-walls)*if middle lamella is obscured

7. Formation of wall

8. Cell plate- precursor of cell wall; rich in pectinsPhragmoplast- a complexof microtubules and ER that forms during late anaphase or early telophase from dissociated spindle subunits.

10. Fine structure of the wallCellulose fibrilsMatrix (non-cellulosic): - with lignin, cutin, suberin, hemicelluloses etc.

13. Ml- middle lamellaPm- plasma membrance

14. Levels of organization of cell wall Long chains of linked glucose residuesMicellae – bundles of cellulose molecules or ELEMENTARY FIBRIL = ~40 cellulose moleculesMicrofibrilBundles of microfibril

16. CHEMISTRY OF WALLSCellulosePectic substancesGums and mucilagesLigninFatty substances

17. CelluloseHydrophilic crystalline compoundRepeating monomers of glucose

18. Pectic substancesAmorphous colloidal substancesPlastic and hydrophilic

19. Gums and mucilagesAppear as a result of physiological or pathological disturbances that induce breakdown of walls and cell contents

20. LigninPhenolic compoundsMay be found in middle lamella, primary wall, and secondary wallhydrophobic filler that replaces the wall’s watercompressivestrength and bending stiffnessMicrobial attack resistance

21. Fatty substancesCutin, suberin, waxesWaxes- glaucous condition; assoc. with cutin and suberinSuberin- cork cells of periderm; endodermis and exodermis; prevents apoplastic transportCutin- cuticle layer; epidermis of aerial partsCutinization, suberization- impregnation in cell wallCuticularization- formation of layer

22. CelluloseTensile strength (bend under compressive stress)Incrustation– eg. LignificationCell wall growth A. intussusception B. apposition C. mosaic growth D. multinet growth

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

24. AppositionGrowth is due to the centripetal addition of new layers one upon the other

25. Mosaic growthFibrillar 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

26. Multinet growthseparation of crossed microfibrils and alteration in their orientationtransverse  longitudinal

27. Special structures of the cell wallPrimary pit fieldsPitsCrassulaeTrabeculaeWart structuresCystoliths

28. Primary pit fieldsPrimordial pits/ primary pit fieldsCertain areas of primary wall of young cells remain thinMay appear beaded in xs

31. PlasmodesmataPlasmodesmata- connnect protoplasts of neighboring cells - transport; relay of stimuli * symplast- 2 or more interconnected protopolast * apoplast – cell walls, intercellular spaces and lumendesmotubule

35. PitsPortions of the cell wall that remained thin even as secondary wall is formedPrimary wall onlyCan develop over primary pit fieldsFunction?

37. PitsTYPES: a. simple pit b. bordered pit—S-wall develops over the pit cavity to form an overarching roof

38. Simple pitsBranched simple pits (ramiform)Found in parenchyma cells with thickened walls, libriform fibers, sclereids, phloem fibers

39. Bordered pitsStructure:Pit cavity / pit chamberPit aperturePit borderPit canal, inner and outer aperture (very thick S-wall)water-conducting and mechanical xylem cells (vessel elements, tracheids, etc.)

43. AngiospermGymnosperm

45. Pit-pairPit cavity – break in S-wallPit membrane/ closing membrane –primary wall + middle lamellaPit aperture

46. Types of pit-pairSimple pit pairBordered pit pairHalf bordered pit pairBlind pitUnilateral compound pitting

47. Toruspit membrane thickening; disc shapedflexible; can go median or lateralAspirate condition (lateral)– latewood and all heartwoodConiferales, Gnetales

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

49. Shape of pit apertureRound, elliptic, linearIn thick cell walls: *inner aperture becomes long and narrow *outer aperture remains circular round * pit canal is funnel-shaped *fiber-tracheid feature

51. Bordered pits arrangementScalariformOppositeAlternate

52. crassulaeLinear or crescent-shaped thickenings of the primary wall and middle lamellagymnosperms

53. trabeculaeRod shape thickenings of the wall which traverse the cell lumen radially

54. ReferencesFahn, A. 1990. Plant Anatomy, 4th ed.. Pergamon PressEsau, K. 1958. Plant Anatomy. John Wiley and Sons, Inc.Evert, R. 2006. Esau’s Plant Anatomy. John Wiley and Sons, Inc.