Structure of cellwall

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

3. 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

4. Cell wall
Layers of secondary wall

5. 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

6. 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

7. Structure of primary call

8. 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

9. Structure of cellulose

10. 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.

11. Structure of secondary cell wall

12. 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

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

14. 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

15. 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%)

16. 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