a basic level of introduction of nature of cell wall and its functions

1. PLANT CELL
ENVELOPES
P . H A R I T H A A N D S H A R A T H D E E P I K A
D E G R E E L E C T U R E R S I N B O T A N Y
T E L A N G A N A T R I B A L W E L F A R E
R E S I D E N T I A L D E G R E E C O L L E G E S O F
S A N G A R E D D Y A N D M E D A K ,
T E L A N G A N A

2. INTRODUCTION
• Important feature of the plant cell is the
presence of cell wall.
• It provides support and rigidity to the
plant body.
• It also protects protoplasm from
external injury.
• It prevents the movement of water in to
the cell (endosmosis).

4. STRUCTURE
• The cell wall is formed during cell division, it is
thin in young cells than in fully matured cells.
• This wall differentiates into three layers
1. Primary wall
2. Secondary wall
3. Tertiary wall

5. PRIMARY CELL WALL
• It is the first formed cell wall
• Composed of cellulose, hemicellulose,
polysaccharides and pectin substances
• The primary walls of two cells are present on
either side of the middle lamella.
• It is thin and elastic in a young cell and becomes
thick and rigid when the cell enlargement has
ceased.

7. SECONDARY CELL WALL
• It is present inner to the primary wall after the cell
stops growth and differentiates.
• Composed of cellulose, hemicellulose and
polysaccharides at young stage.
• Lignin, suberin, waxes, tannins, calcium carbonate
at maturity.
• Thus it possess high mechanical strength
• It shows three layers outer layer (S1), middle layer
(S2) and inner layer (S3). Amongst these middle
layer is the thickest.

9. TERTIARY CELL WALL
• It develops inner to the secondary wall only in
very few species.
• Composed of hemicellulose and xylon.

10. THICKENING OF CELL WALL
• The wall materials (lignin) get deposited
uniformly throughout the cell but in
some cases they show special patterns.
• These thickenings provide mechanical
support to the cell, usually develop when
the cell attains full size.

11. SECONDARY WALL THICKENINGS
i. Annual: in the form of rings, inner to the
primary cell wall.
ii. Spiral: in the form of spring / spiral manner
iii. Scalariform: in the form of ladder
iv. Reticulate: in the form of irregular network
v. Pitted: All along the wall but except in some
areas forming pits.

13. PITS
Pits are of two types
1. Primary pit fields:
2. Secondary pit fields:

14. PRIMARY PIT FIELDS
• Primary wall- under microscope-beaded
structure-presence of depressions
• Primary cell wall-small openings called Pores in
this region
• Protoplasmic connections-adjoining cells
through the pit fields-Plasmodesmata
• Ex: Phoenix – seeds, Diospyros – seeds.

15. SECONDARY PIT FIELDS
• Secondary wall also have cavities called
Pits that facilitate the cytoplasmic
connections.
• Each pit has a complementary pit
opposite to it in the neighboring cell – pit
pair, membrane separating them – Pit
membrane or closing membrane.
• Opening of such pit – Pit aperture.

17. MORPHOLOGY OF PITS
1. Simple pits: Pit cavity is uniform and pit membrane
is not swollen
2. Bordered pits: secondary wall develops over the
pit cavity to form an over arching roof with narrow
pore – this causes a chamber called pit chamber –
opening pit aperture.
Ex: Trachery elements of Pteridophytes and
Gymnosperms.

19. TERMS
i. Simple pit pair – if both the adjacent
pits are simple
ii. Bordered pit pair – if both the adjacent
pits are bordered.
iii. Half bordered pit pair - if one among
the both is simple and another is
bordered.
iv. Blind pit – If there is no neighboring pit.

20. ORIGIN AND GROWTH OF THE CELL WALL
• New cell wall formation takes place
during cell division through a barrel
shaped Phragmoblast at equatorial plate
of the dividing mother cell.
• Small vesicles from ER migrate to the
equatorial plane, fuse with each other to
form cell plate enlarging from the centre
to the Periphery.

22. CONTD….
• This wall formed is primary wall, slowly
secondary walls are also formed.
• The cell wall increases both in length by
Intussusception method and in
thickness by Apposition method.

23. CHEMICAL NATURE OF THE CELL WALL
The plant cell wall is composed of
1. Cellulose
2. Hemicellulose
3. Pectins
4. Lignins
5. Suberin, cutin and waxes

24. 1. CELLULOSE
Main constituent
• Provides structural frame work with in which
other components are embedded
• Simplest polysaccharide, makes upto 50% of
total plant weight
• Polymer of D-glucose inter linked by beta 1,4-
glycosidic bonds
• Formula - (C6H1005)n

25. 2. HEMICELLULOSE
• Comprised of non cellulosic
polysacccharides
• Include mono saccharide units such as
arabinose, xylose, mannose and
galactose
• Do not form microfibrils but form
hydrogen bonds with cellulose
• Formula : (C5H804)m

26. 3. PECTINS
• Located in middle lamella and outer layer
of secondery wall
• Derivatives of polygalactouronic acid
• These are plastic, highly hydrophilic and
determine cell wall porosity
• Formula : C6H10O7

27. 4. LIGNINS
• 25% of dry weight characterized by aromatic
alcohols of high molecular weight-
hydroxyphenyl propane
• Strengthening agent in the wall
• Resists fungal/pathogen attack
• Lignin depositions usually begins in the middle
lamella and then proceeds to primary and
secondary cell walls
• Formula : C81H92O28

28. 5. SUBERIN, CUTIN AND WAXES
• These are variety of lipids associated with the
cell wall for strengthening
• They also check the evaporation of water from
the plant body
• Cutin usually forms a continuous layer on the
epidermis
• Suberin occurs in association with cellulose in
cork cells of the periderm

29. ULTRA STRUCTURE OF CELL WALL
• Electron microscope studies reveal the
cellulose structure – in the form of fine strands
– macrofibrils 0.5 ս in thickness and 1 ս long.
• Each macrofibril – 250 microfibrils of 25 nm
thick.
• Each microfibril – 1050 micelles.
• Each micelle – 100 parallel cellulose chains
• Each cellulose unit – polymer (several hundreds
to ten thousands) of D-Glucose units by beta
1,4 glycosidic bonds.

30. • In plants, macrofibrils are placed compactly – with
no space or loosely packed with spaces.
• These spaces are filled with water or chemical
substances like lignin, pectin, cutin, hemicellulose.
• In primary walls the microfibrils are arranged 90
degree to the wall and in secondary along the axis.
• This arrangement determined the direction of cell
growth.

32. FUNCTIONS OF THE CELL WALL
• Provides definite shape to the cell
• Structural skeleton
• Cellulose microfibrils orientation limits
and controls cell growth and shape.
• Physical barrier protecting cells from
invading pathogens like viruses, fungal
spores, bacteria.
• Acts as defense mechanism

33. ACKNOWLEDGEMENTS:
• We would like to acknowledge all the
authors and owners for the pictures
used in our presentation
• We would like to emphasize that all
pictures were taken from the open
source internet medium