FINE STRUCTURE OF PRIMARY AND SECONDARY CELL WALL AND CELL WALL THICKENNING

Gajendra C VResearch scholar

Department of Tree Breeding Forest College and Research Institute, Mettuapalayam

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

Cell wall

Layers of secondary wall

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

Primary cell wall

• Consists of cellulose (45%), 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

Structure of primary call

Cellulose

• It has a very high molecular weight

• It is a linear polymers of glucose molecules

• The cellulose fibrils are about 0.16 μm2 wide and upto 1 μm 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

Structure of cellulose

Secondary cell wall

• The 20 wall is very thick (lignin), rigid and inelastic and consists of three

layers known as S1 (outer), S2 (middle) and S3 (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 S3 and are at an angle of

10 -200 to the longitudinal axis in S2 and are at the angle of 500 in S1

• 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.

Structure of secondary cell wall

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

Pictorial representation of cell wall thickening

A & B Annular, C&D. Spiral, E&F. Scaliform. G. Reticulate. H. Pitted (Simple). I. Pitted (Bordered)

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

Difference between the primary and secondary cell wall

SL. NO. FEATURES PRIMARY CELL WALL SECONDARY CELL WALL

1 Occurrence In all the plant cellsIn 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%)

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

Thank you….