Primary and secondary cell wall

cell wall means rigid layer of polysaccharides lying outside the plasma membrane of the cells of plants, fungi, and bacteria. In the algae and higher plants it consists mainly of cellulose.

1. FINE STRUCTURE OF PRIMARY AND SECONDARY CELL WALL AND CELL WALL THICKENNING Gajendra C V Research scholar Department of Tree Breeding Forest College and Research Institute, Mettuapalayam

2. Introduction The cell wall is the most characteristic feature of a plant cell The cell wall is always non-living but is formed and maintained by the living organism Its primary function is to provide protection to the contents of cell Due to semi-rigid nature, the cell walls are responsible for giving shapes to different kinds of cells during cell differentiation of tissues In multicellular and woody plants of cell wall is differentiated into three parts i.e., the middle lamella, the primary wall and secondary wall

3. Cell wall Layers of secondary wall

4. The middle lamella It is a common structure between adjacent cells and therefore, binds them with each other It is an amorphous layer and is composed of calcium and magnesium pectate The middle lamella remains unlignified in case of softer living tissues namely Parenchyma, collenchyma and arenchyma , but in woody tissues Sclerenchyma it becomes highly lignified

5. Primary cell wall Consists of cellulose ( 4 5% ), hemicellulose ( 25 % ), pectins ( 35% ) and structural proteins ( upto 8% ) on the basis of dry weight The primary wall is thin and elastic It is capable of growth and expansion The backbone of the primary wall is formed by the cellulose fibrils. The matrix is composed of hemicellulose, pectin, gums, tannins, resins, silica, waxes etc. and small structured proteins

6. Structure of primary call

7. Cellulose It has a very high molecular weight It is a linear polymers of glucose molecules The cellulose fibrils are about 0.16 μ m 2 wide and upto 1 μ m long Each fibril is made up of 250 microfibrils . Each microfibril composed of about 20 micelles Each micelle is made up of 2000 to 25000 individual cellulose molecules The microfibrills arranged in the form of loose mat These give maximum tensile strength to the wall

8. Structure of cellulose

9. Secondary cell wall The 2 wall is very thick (lignin), rigid and inelastic and consists of three layers known as S 1 (outer), S 2 (middle) and S 3 (inner) The microfibrils in these layers run parallel to each other but the directions are different in three layers The microfibrils are transversely arranged in the S 3 and are at an angle of 10 -20 to the longitudinal axis in S 2 and are at the angle of 50 in S 1 The lignin is formed from three different phenyl propanoid alcohols: coniferyl , coumaryl and sinapyl alcohols Lignin is covalently bonded to cellulose and other polysaccharides of cell wall.

10. Structure of secondary cell wall

11. Nature of thickening of secondary cell wall Annular or ringlike : thickening is noticed in the protoxylem elements where secondary matters are placed centripetally in form of rings at regular intervals Spiral : thickening is also found in protoxylem elements, secondary wall being deposited in form of spiral Scalariform : Secondary matters took like the rungs of a ladder here Reticulate: The secondary matters here assume the form of a network Pitted: In this case secondary cell wall materials are deposited practically all over the primary wall , only leaving some small thin areas here and there. These unthickened areas are the pits

12. Pictorial representation of cell wall thickening A & B Annular, C&D. Spiral, E&F. Scaliform . G. Reticulate. H. Pitted (Simple). I. Pitted (Bordered)

13. Diagrammatic representation of pit with torus The pit membrane usually has a thickening called torus. A. A vessel with bordered bits in front view. B. Same in sectional view C. Perspective diagram of the same D. sectional view of bordered with changed position of torus

14. Difference between the primary and secondary cell wall SL. NO. FEATURES PRIMARY CELL WALL SECONDARY CELL WALL 1 Occurrence In all the plant cells In only mature and non-dividing cells 2 Position Inner to middle lamella Inner to primary cell wall 3 Nature Elastic and thinner Inelastic, rigid and thicker 4 Nature of growth Intussusceptional Accretional 5 Pits Absent Present 6 Additional materials Absent Present lignin, suberin or cutin 7 Amount of cellulose Low High 8 Extensibility Present Generally absent 9 Arrangement of fibrils Wavy and loosely arrangement Closely, straight and parallel arranged 10 Hydration More (60%) Less (30 -40%)

15. Functions of cell wall They determine the morphology, growth, and development of plant cells They protect the protoplasm from invasion by viral, bacterial and fungal pathogens They are rigid structures and thus help the plant in withstanding the gravitational forces They are involved in the transport of materials and metabolites into and out of cell They withstand the turgor pressure which develops within the cells due to high osmotic pressure

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