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 WALL Gross Structure Detailed Structure Chemistry Features

3. GROSS STRUCTURE

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

5. Primary wall First wall the develops on new cell Cellulose, pecticcpds., non-cellulosic polysaccharides and hemicellulose May be lignified Assoc. with living protoplasts --eg. meristematic cells, parenchyma, collenchyma

6. 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

7. Formation of wall

8. 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.

10. Fine structure of the wall Cellulose fibrils Matrix (non-cellulosic): - with lignin, cutin, suberin, hemicelluloses etc.

13. Ml- middle lamella Pm- plasma membrance

14. 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

16. CHEMISTRY OF WALLS Cellulose Pectic substances Gums and mucilages Lignin Fatty substances

17. Cellulose Hydrophilic crystalline compound Repeating monomers of glucose

18. Pectic substances Amorphous colloidal substances Plastic and hydrophilic

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

20. Lignin Phenolic compounds May be found in middle lamella, primary wall, and secondary wall hydrophobic filler that replaces the wall’s water compressive strength and bending stiffness Microbial attack resistance

21. 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

22. Cellulose Tensile strength (bend under compressive stress) Incrustation– eg. Lignification Cell wall growth A. intussusception B. apposition C. mosaic growth D. multinet growth

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

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

25. 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

26. Multinet growth separation of crossed microfibrils and alteration in their orientation transverse  longitudinal

27. Special structures of the cell wall Primary pit fields Pits Crassulae Trabeculae Wart structures Cystoliths

28. Primary pit fields Primordial pits/ primary pit fields Certain areas of primary wall of young cells remain thin May appear beaded in xs

31. 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

35. 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?

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

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

39. 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.)

43. Angiosperm Gymnosperm

45. Pit-pair Pit cavity – break in S-wall Pit membrane/ closing membrane –primary wall + middle lamella Pit aperture

46. Types of pit-pair Simple pit pair Bordered pit pair Half bordered pit pair Blind pit Unilateral compound pitting

47. Torus pit membrane thickening; disc shaped flexible; can go median or lateral Aspirate condition (lateral)– latewood and all heartwood Coniferales, Gnetales

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

49. 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

51. Bordered pits arrangement Scalariform Opposite Alternate

52. crassulae Linear or crescent-shaped thickenings of the primary wall and middle lamella gymnosperms

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

54. 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.