Cell wall

By
Prof Ichha Purak
Department of Botany
Ranchi Women’s College,Ranchi
CELL WALL
PART-1
PART-2
Other Types of Cell walls

Part- 1 Part-2
Plant Cell Wall : Structure and Function Other types of cell walls
Definition Algal cell wall
Chemical Composition of cell wall Fungal cell wall
Plant cell wall :Morphology of Plant cell wall/Structure Bacterial cell wall
Middle lamella Archaeal cell wall
Primary wall
Secondary wall - simple and bordered pits
Plasmodesmata
Tertiary wall
Ultrastructure of Plant cell wall
Origin/Biogenesis/Formation of Plant Cell wall
Functions of cell wall
Properties of cell wall
Rigidity , Permeability
CELL WALL
CONTENTS

The outermost structure of most plant cells is a dynamic and rigid layer called cell
wall (Buchanan et al 2000 ) .
It is mainly composed of carbohydrates, such as cellulose, pectin, hemicelluloses
and lignin, proteins and certain fatty substances such as waxes.
Ultrastructurally cell wall is found to consist of microfibrilar network lying in a gel
like matrix. The microfibrils are mostly made up of cellulose.
There is a pectin rich cementing substance between the walls of adjacent cells
which is called middle lamella.
The cell wall which is formed immediately after the division of cell, constitutes the
primary cell wall.
Many kind of plant cells have only primary wall around them. Primary cell wall is
composed of loose network of cellulose microfibrils, pectin and hemicellulose.
DEFINITION

In certain type of cells such as phloem and xylem, an additional layer is added to
the inner surface of primary cell wall at a later stage. The layer is called
secondary cell wall and it consists mainly of cellulose, hemicellulose and lignin.
In many plant cells, there are tunnels running through the cell wall called
plasmodesmata which allow communication with other cells in a tissue.
The cell wall constitutes a kind of exoskeleton that provides protection and
mechanical support to the plant cell. It determines the shape of plant cell and
prevents it from desiccation.
The cell wall is rigid, transparent ,permeable (having pits and pores) and flexible
outer most layer surrounding plasma membrane in some cells as plants.
1/3/2019 4Plant cell wall

Chemical Composition of cell wall
In plants cell wall is made up of cellulose, hemicelluloses and pectins.
Cellulose microfibrils are embedded in matrix.
Matrix is the gel like ground substance which consists of water , hemicellulose,
pectin, glycoproteins and lipids.
Hemicellulose contains arabinose,mannose,xylose and galactose.
Pectin contains Galactose, arabinose ,galactouronic and glucuronic acid.
The cell wall may have lignin for hardness, silica for stillness and protection, cutin to
prevent water loss and suberin for impermeability
The cell walls of higher plants and some algae are composed principally of cellulose,
which is the single most abundant polymer on earth.
In bacteria, cell wall is composed of peptidoglycan which consists of polymers of
NAG (N-acetyl glucosamine) and NAM (N-acetyl muramic acid) cross-linked by short
peptides.

The cell wall polysaccharides can be of three classes
The microfibrillar polysaccharides
The pectins
The hemicelluloses
In fungi, cell wall is made up of chitin or fungal cellulose (Polymer of N –acetyl
glucosamine - NAG). Chitin is also microfibrilar polysaccharide as cellulose.
The most common microfibrilar polysaccharide component found in all plant cell
walls is cellulose. It consists of a collection of β-1,4-linked glucan chains that
interact with each other via hydrogen bonds to form a crystalline microfibril
(Somerville, 2006).

Cellulose is a homopolysaccharide, its repeating unit is cellubiose .
(Figure-1) It was first isolated by the French chemist Anselme Payen(1838)
Cellulose is a linear polymer of glucose residues, often containing more than
10,000 D-glucose monomers. (Figure- 1)
Several dozen such chains then associate in parallel with one another to
form cellulose microfibrils , which can extend for many micrometers in length.
Within the cell wall, cellulose microfibrils are embedded in a matrix consisting of
proteins and two other types of polysaccharides: hemicelluloses and pectins.

Figure-1: Structure of cellobiose
and cellulose
The chemical formula of the cellulose is (C6H10O5)n.
Cellulose is a polysaccharide made from linear chains
of D-glucose units
Cellulose is the most abundant organic polymer found on earth.

Chitin
Chitin constitutes the microfibrilar component of cell walls of most fungi and is
principal component of hard exoskeleton of many invertebrates. Chitin molecules
are long unbranched chains of N-acetyl D-Glucosamine residues linked by β 1→4
glycosidic linkages. Chitobiose is the repeating unit. (Figure-2)
Hydrogen bonding between different chitin molecules are established between C-3
hydroxyl group of N-acetyl D glucosamine residue and glucoside oxygen of next.
The chitin residues in microfibrils run in antiparallel direction.
The key difference between cellulose and chitin is that cellulose is the significant
structural polymer in the primary cell walls of the plant cells while chitin is
the main structural polymer found in the fungal cell wall.

Figure : -2 Chemical structure of Chitin
The chemical formula of chitin is (C8H13O5N)n.
Albert Hofmann determined the structure of chitin in 1929
Repeating unit of chitin (chitobiose)
Chitin is an un-branched structural polysaccharide which contributes to
strengthening and protecting organisms
Chitin is also an organic compound
composed of modified glucose
monomer which are derivatives of
glucose known as
N-acetylglucosamine
Chitin is second only to cellulose from its abundance on earth.

MATRIX POLYSACCHARIDES
Plant cell walls contain several matrix polysaccharides that are grouped into two
general categories upon solubility difference
1. The hemicellulosic polysaccharides include xyloglucans, glucomannans, xylans,
and mixed-linkage glucans (Scheller and Ulvskov, 2010) and
2. The pectic polysaccharides include homogalacturonan, and rhamnogalacturonan I
and II (Harholt et al., 2010)
Hemicelluloses are not related to cellulose in structure and also not precursors of
cellulose.
Hemicelluloses are highly branched polysaccharides that are hydrogen-bonded to
the surface of cellulose microfibrils. This crosslinks the cellulose microfibrils into a
network of tough, fibrous molecules, which is responsible for the mechanical
strength of plant cell walls.
Each subgroups contains polysaccharides with considerable structural diversity and
amount and is named after the predominant monosaccharides

Hemicellulose structure
Hemicelluloses are heterogenous , consisting of different sugar residues
(sub-groups Xylans,mannans and galactans ) and uronic acids.
Figure -3 : Monomer sugars found in hemicellulose
Hemicellulosic polysaccharides are known to
bind tightly to cellulose microfibrils via hydrogen
bonds (Keegstra,2010)

a) The xylans are rich in D-xylopyranose residue whether in main chain or as
frequent branches attached to it. Xylopyranose resiues are linked together by β
1→4 Glycosidic linkage. 7 out of every ten residues are acetylated at C-3 or C-2.
Some of D-xylopyranose residues are also linked by α 1 →2 glycosidic linkage.
b) The Mannans – present in hemicellulose of coniferous wood
The mannans yield D-mannose, D-Glucose and D-galactose in ratio 3:1:1. The
main chain has β D-mannopyranose and β D glucopyranose in 3:1 ratio and are
linked together by β 1→4 glycosidic linkage. β D Galactopyranose residue is
linked by β 1→6 linkage as branching to some of the mannose residues of the
main chain.
c) Galactans mainly composed of arabinogalalctoses which consist of a main
axis of β galactopyranose units linked to each other by β 1→3 glycosidic linkage
attached to which are branches composed of D-galactopyranose or L-
arabofuranose containing disaccharide.

Pectins
Pectins are not only important cell wall matrix polysaccharide, but also occur in
some plant juices.
Pectins are branched polysaccharides containing a large number of negatively
charged galacturonic acid residues. Because of these multiple negative charges,
pectins bind positively charged ions (such as Ca2+) and trap water molecules to
form gels.
In higher plants Pectin is polymer having linear chain of alpha (1-4) linked
D- galacturonic acid residues
However, in the marine brown algae the polyuronic acid is structurally different and
is usually called alginic acid.
Chemically, pectin consists of the partial methyl esters of polygalacturonic acid and
their salts (sodium, potassium, calcium, and ammonia), with a molecular weight of
up to 150,000 Daltons.
.

PECTIN
Figure -4 : Chemical structure of Pectin
Glucuronic acid
In the cell wall, the pectins
form a gel-like network that
is interlocked with the
crosslinked cellulose
microfibrils

Lignins
Lignins are high-molecular-weight, insoluble plant polymers, which have complex
and variable structures. They are composed essentially of many methoxylated
derivatives of benzene (phenylpropanoid alcohols, also called monolignols),
especially coniferyl, sinapyl and coumaryl alcohols .
The proportions of these three differ between angiosperms and gymnosperms
and between different plants.
Lignifications of the cell wall occurs after the laying down of the polysaccharide
components of the wall and towards the end of growing period of the cell.
The purpose of lignifications is to strengthen the cell wall by forming a ramified
network throughout the matrix, thus anchoring the cellulose microfibrils more firmly.
Lignin protects the microfibrils of the wall from chemical, physical and biological
attack.

Chemical structures of the phenylpropanoid alcohols used to construct the lignin polymer.
These are also called monolignols.
Figure-5 : Schematic formula of the polymer structure of angiosperm lignin.

Proteins
Plant cell walls particularly of growing cells also contain many proteins which
represent 5-10% of the cell wall and glycoproteins, including various enzymes and
structural proteins (Rose and Lee, 2010).
i) Enzymes – A considerable range of hydrolases have been found in cell walls,
including invertase, various glucanases, pectin methylesterase, ATPase, DNAase,
RNase and various phosphotases, including ascorbic acid oxidase involved in
lignin formation.
ii) structural proteins – A glycoprotein, exceptionally rich in the amino acid 4-trans-
hydroxy-L proline has been found in cell walls, particularly primary cell walls.
Glycoproteins are thought to be concerned with cell-wall extension.

Water
Water is an important structural component of the cell wall. It forms part of the
gel structure of the pectins.
Because of the changes in the water content of the wall cause reversible
changes in the texture of the cell wall matrix as the pectin gel changes to a
viscous solution.
Changes in the water content of the wall cause changes in the degree of
adherence between the microfibrils and the matrix.
As a solvent water affects the permeability of the wall to other molecules and
ions; the more water in the wall the more permeable it is.
As growth of the cell ceases the space occupied by water in the wall becomes
progressively filled with lignin, thus making the matrix, and therefore the whole
wall, much more rigid.

Incrustating substances
i) Cuticular substances ( Cutin and Suberin)
The outermost surface of the cell walls of epidermal cells are covered with a
hydrophobic cuticle.
The principle function of the cuticle is to reduce the excessive loss and gain of water by
the underlying tissue.
It also protects the tissue from chemical,physical and biological attack to some degree.
The cutin is the principle polymer of the cuticle. It consists of a complex mixture of
hydroxyl fatty acids which are linked together by ester bonds to give a three
dimentional network.
The majority of the fatty acids that are the cutin building units contain 16 to 18 carbon
atoms.
The epicuticular wax is a complex mixture containing mixtures of esters of fatty acids
and long chain alcohols.

The cuticle is generally composed of three layers. The outermost is a surface
layer of wax, often called the epicuticullar wax.
Beneath the epicuticular wax is a layer of cutin embedded in wax.
The innermost layer is a mixture of cutin,wax,cell wall polysaccharides
and possibly traces of proteins.
This layer adheres to the middle lamella of the epidermal cells.
Figure - 6 : Schematic reprentation of a cross section through the cuticle of a leaf

Suberin is complex mixture of fatty acids deposited on cork cells.
Cork cells are found in a secondary protective layer called the periderm and in the
bark of trees.
Cork tissue, consisting of dead cells surrounded by alternating layers of suberin and
wax, has a particularly high suberin content.
Cork layers containing suberin protect plants against loss of water, infection
by microorganisms, and heat exposure.
Suberin is present in many C4 plants as an impermeable layer between the bundle
sheath and mesophyll cells.
It is a waxy substance that is found in the cell wall of higher plants.
It helps in the control and regulation of movement of solutes through to the xylem.
Suberin is a polymeric compound formed from phenylpropanoids, long chain fatty
acids and fatty alcohols (C18–C30), as well as hydroxyfatty acids and dicarboxylic
acids (C14–C20) (Figure-7)

Figure-7 : Chemical structure of suberin
In suberin, the monolignols are connected similarly as in lignin, but the 9′-OH
groups usually remain free. Instead they form esters with long-chain fatty acids
and hydroxyfatty acids. Carboxylic acid esters provide a link between two
monolignols

PLANT CELL WALL
In plants cell wall is located outside the cell membrane and provides these cells
with structural support and protection, and also acts as a filtering mechanism.
A major function of the cell wall is to act as a pressure vessel, preventing over-
expansion when water enters the cell.
The structure of cell wall determines the architecture and function of plant cell.
The cell wall is found in plants, algae ,fungi, bacteria and some archaea.
Animals and protozoa do not have cell walls

Figure : 8a General organisation of plant cell wall. All
layers are not present in all cellsFigure-8 Structure of plant cell wall
A typical cell wall is composed of 3-4 layers that are formed sequentially from outside
to inwards as follows :
Middle lamella
Primary wall
Secondary wall and occasionally
Tertiary wall

However, the different layers may differ from one another in several respects, the
most important of which are :-
i) their thickness
ii) the ratio of the microfibrillar component to matrix component
iii) the orientation of the microfibrils within the matrix relative to long axis of
the cell.
iv) the nature of matrix polysaccharides.
v) the degree of lignifications and
vi) the water content
These layers are being composed of microfibrils embedded in polysaccharide matrix

Middle lamella is the first layer to be formed when a cell divides .
It is an amorphous intercellular layer between primary walls of adjacent cells .
It is cementing layer made up of Ca and Mg pectinate/pectate which joins or
glues two neighbouring plant cells.
It is absent on the free surface of plant cells and in plasmadesmata region.
Middle lamella dissolves in ripe fruits which results in softening.
Middle lamella can be dissolved artificially by means of acid treatment
( during root tip cytological preparation )
The cell walls and the middle lamella of plants never occur in the form of continuous
layers but have many minute apertures through which the cells of a tissue maintain
cytoplasmic relations with each other.

Primary cell wall
The primary cell wall, generally a thin, flexible and extensible layer formed while
the cell is growing.
It is the first deposition product of protoplasm outside plasma membrane and is
present inner to middle lamella.
Primary cell wall is delicate,flexible and thin (0.1-3.0µm) and capable of
extension.
Its thickness increases with the growth of the plant cell. It grows by further
deposition of wall material into the existing primary wall.
Parenchymatous,meristematic cells and cell involved in photosynthesis,
respiration and secretion and unicellular plants have only primary cell wall.

Both the structure and function of cell walls change as plant cells develop.
The thickness, as well as the composition and organization of cell walls can vary
significantly.
The walls of growing plant cells (called primary cell walls) are relatively thin and
flexible, allowing the cell to expand in size.
The microfibrils of the cellulose are deposited on both sides of the middle lamella
and form the primary cell wall
Once cells have ceased growth, they frequently lay down secondary cell walls
between the plasma membrane and the primary cell wall
Such secondary cell walls, which are both thicker and more rigid than primary
walls, are particularly important in cell types responsible for conducting water
and providing mechanical strength to the plant.

Secondary wall
Secondary cell wall – it is often deposited on inner side of primary walls after the
growth of cell stops ( cell matures).
It is 4-10 µm thick, rigid, non-elastic, permeable and made up of cellulose and
lignin deposits.
Secondary wall deposition is not uniform. At some places secondary wall is not
laid down.
Such unthickened areas are called pits .Pits of two neighbouring cell form pit pair.
Pits can be of two types – simple or bordered , pit chamber becomes flask
shaped due to deposition in the form of border. (Figure-9)
During the development of pits, the secondary cell wall may over arch the pit
cavity forming a border, leaving an inner opening called pit-aperture. Such pits
with borders are called bordered pits. Two opposite bordered pits are called
bordered pit pair.

A transverse section through simple pit shows only one circle and transverse section
through border pit shows two circles , one of pit and other of border.
Bordered pits are present in tracheids of Gymnosperms and vessels of angiosperms.
Pits which lack the borders are called simple pits. Two opposite simple pits on
adjacent cells are called simple pit pair.
Plasmodesmata (Singular Plasmodesma ) A number of fine cytoplasmic strands
(20-40 nm in diameter) pass through pits from one cell to other and make connection
between the cytoplasm of two cells.
Endoplasmic reticulum plays a significant role in origin of plasmodesmata .
Plasmodesmata were studied in details by Strasburger (1901).
Plasmodesmata help in transfer of nutrients, stimuli and other material between
adjacent cells and thus Produce a protoplasmic continuum called symplast.

Pit pairs. (a) Simple pit pair, (b) Bordered pit pair,
(c) Half bordered pit pair. [Source: Siau,1995
L S and Surface view of Simple pit(a) and
Bordered pit (b)
Figure-9 :structure of simple and bordered pits

Figure – 10 Primary and secondary walls
Secondary cell walls are laid down between the primary cell wall and the plasma
membrane.It lies near the plasma membrane or the tertiary cell wall.
Secondary walls frequently consist of three concentric layers (S1,S2 and S3) which
occur one after the other and differ in the orientation of their cellulose microfibrils.
(Figure-10)
(Primary wall, courtesy of F. C. Steward; secondary wall, Biophoto
Associates/Photo Researchers, Inc.)
The secondary plant cell wall, which is
often deposited inside the primary cell
wall as a cell matures, sometimes has
a composition nearly identical to that of
the earlier-developed wall.

Primary and secondary cell walls differ in composition as well as in thickness.
Primary cell walls contain approximately equal amounts of cellulose, hemicelluloses,
and pectins.
In contrast, the more rigid secondary walls generally lack pectin and contain 50 to
80% cellulose.
Many secondary walls are further strengthened by lignin, a complex polymer of
phenolic residues(aromatic alcohols ) that is responsible for much of the strength
and density of wood. (Figure-11)
The orientation of cellulose microfibrils also differs in primary and secondary cell
walls. In primary wall microfibrils are short, wavy and loosely scattered.
In secondary wall microfibrils are long, straight, close and parallely arranged
The cellulose fibers of primary walls appear to be randomly arranged, whereas
those of secondary walls are highly ordered (Figure -12 a )

Figure -11 : Diversity of plant cell wall structure. (A) primary, and (B)-(C) secondary
cell walls (source: Taiz L., Zeiger E., 2010)
cell walls commonly are classified into two major types: primary walls and
secondary walls
Primary walls are formed by growing cells and are usually considered to be
relatively unspecialized and similar in molecular architecture in all cell types
Secondary walls are the cell walls that form after cell growth (enlargement) has
ceased
Secondary walls may become highly specialized in structure and composition,
reflecting the differentiated state of the cell.

Secondary walls are frequently laid down in layers in which the cellulose fibers
differ in orientation, forming a laminated structure that greatly increases cell wall
strength.
In certain plant cells, there occurs another cell wall beneath the secondary cell
wall which is known as tertiary cell wall. It differs from the primary and
secondary cell wall in terms of its morphology, chemistry and staining
properties.
Tertiary wall –In some tissue a tertiary cell wall is formed on the inner surface of
the secondary cell wall. This layer is very thin and is found in the xylem
tracheids of gymnosperms. Taxus – Tertiary spiral thickening makes the wood
elastic and strong, so is used for making bows. It is composed mainly of xylan,
instead of cellulose. It is not found in all the cells. Typically, it does not contain
any cellulose micro-fibrils.

Ultra-structure of plant cell wall
The primary wall and the secondary wall have the same basic structure.
In both cases cellulose micro-fibrils (chief constituent) are found embedded in an
amorphous gel-like matrix (consisting of proteins and two polysaccharides :
hemicelluloses and pectins) (Figure-12)
Each cellulose chain (1 -5 µm long) consists of about 2000-25000 glucose units.
Nearly 100 cellulose chains arranged parallel to form minute bundle called
crystalline domain or micelle (1.0 nm thick).
Micelle is the smallest structural unit of cell wall. About 20-40 micelles assemble
in the matrix to form a microfibril (2.6 nm thick).
Nearly 250 microfibrils aggregate in bigger bundles called macrofibrils
(~ 0.5 µm in diameter, may reach, 4µm in length). (Figure- 13)
A cotton fibre has 1500 macro fibrils.

Ultra structure of primary cell wall showing
interconnections between the two major
components of the primary cell wall , the
cellulose microfibrils and the matrix
Cellulose microfibrils (green) – provide
strength
Hemicellulose (dark green)- At regular
intervels along with cellulose microfibrils
Pectin (red and yellow) – gelling matrix
Glycoproteins (purple) – weave throughout
the matrix
Figure-12 Ultrastructure of primary cell wall

Figure: 12 a A diagrammatic representation of the arrangement of cellulose
Fibrils in the cell wall of higher plants. A. Primary wall with loosely organised cellulose
fibrils. B. Secondary wall with densely packed cellulose bundles with parallel
orientation
Glucose
molecules
Cellulose
chain
MicelleMicrofibrilMacrofibril
Figure – 13 Stepwise formation of Macrofibrils from glucose

Origin/Biogenesis/Formation of Plant Cell wall/Expansion
Origin of cell wall takes place from cell plate during cytokinesis.
Many cell wall vesicles provided by Golgi bodies and Endoplasmic reticulum
combine to form a cell plate .
After some physical and chemical changes , the cell plate (rich in pectin) grows on
both sides to form a middle lamella, which glues neighbouring plant cells (Figure-
14)
After which primary wall and secondary walls are laid down on the middle lamella
to form cell wall.
As per research work of (McCann and Rose, 2010), It has been estimated that more
than 2000 genes are required for the synthesis and metabolism of cell wall
components
Plant cell wall biosynthesis involves multiple cellular compartments.
As cells expand, new components of the cell wall are deposited outside the plasma
membrane

Figure : 14 Telophase and cytokinesis showing formation of cell plate (rich in pectin),
extends as middle lamella.
New wall material is laid down on either side of the middle lamella to strengthen the cell
wall
Matrix contains a glycoprotein called expansin which causes the loosening and
expansion of cell wall by the addition of cellulose molecules to the
microfibrils.

Cellulose is synthesized by a plasma membrane multiunit enzyme complex
(cellulose synthase) having at least three different enzymes and probably some
other proteins (Figure-15)
From cellulose, microfibrils are synthesized and deposited directly into the
extracellular matrix.
Microfibrils are synthesized on the plasma membrane by protein complexes called
particle rosettes.
In expanding cells, the newly synthesized cellulose microfibrils are deposited at right
angles to the direction of cell elongation. (Figure-16)
Matrix components, including hemicelluloses , pectins and glycoproteins, are
synthesized in the Golgi apparatus (endomembranous system) and delivered to the
wall via secretory vesicles.
Components synthesized in different locations are assembled into a functional wall
matrix with the help of different kind of proteins.

Figure -15 : Schematic representation of the key events in cell wall biosynthesis.
Cellulose biosynthesis occurs at the plasma membrane in large complexes visualized as
rosettes. The synthesis of matrix polysaccharides and glycoproteins occurs in the Golgi where
the products accumulate in the lumen before transport to the cell wall via vesicles.
Abbreviations used in the figure:
CesA, cellulose synthase proteins that
form the rosette
NDP- sugar,nucleotide sugars
that act as donors for the sugars that go
into polysaccharides
Csl, cellulose synthase-like proteins that
are known to be involved in
hemicellulose biosynthesis.

Figure-16 : Cellulose synthesis during cell elongation. New cellulose microfibrils,
synthesized by a plasma membrane enzyme complex (cellulose synthase), are laid down
at right angles to the direction of cell elongation. The direction of cellulose synthesis is
parallel
Source :The Cell: A Molecular Approach. 2nd edition.
Cooper GM.Sunderland (MA): Sinauer Associated , 2000.

The cell walls are the products of the cytoplasm. The cytoplasmic organelles such
as endoplasmic reticulum, Golgi apparatus, etc. play a very important role in the
formation of cell wall.
The cell wall formation is started by formation of phragmoplast cell plate. The cell
plate is formed by the small vesicles of endoplasmic reticulum which cut off from
the endoplasmic reticulum and migrate from the periphery to the equator of the cell.
In the equator region of the dividing cells, the vesicles of endoplasmic reticulum get
arranged on the equator and thus, separate the two daughter parts of the
cytoplasm.
Later on, all the vesicles, except a few which form the plasmodesmata, fuse with
one another to form a discontinuous cell plate.

The cell plate in later stages develops pectin and changes into middle lamella.
The formation of the middle lamella is also accompanied by some large vesicles
known as phragmosomes and the vesicles of golgi apparatus which provide non-
cellulose material.
Then, the fibrils of the cellulose deposit on both sides of the middle lamella and
form the primary cell wall.
The secondary cell wall develops later on by the deposition of cellulose,
hemicellulose and pectin beneath the primary cell wall.
The plasma membrane is formed by the Golgi apparatus beneath the primary cell
wall.

Plant cell walls encase the plant cells and provide many structural and
functional roles.
A major role of the cell wall is to form a framework for the cell to prevent over
expansion.
Cellulose fibers, structural proteins, and other polysaccharides help to maintain
the shape and form of the cell.
The cell wall provides mechanical strength and support. It also controls the
direction of cell growth.
Cell wall protects the cell (protoplast) against loss of water, excessive heat and
foreign attacks(plant virus and other pathgens ).
It bestows definite shape ,supporting frame work and rigidity to cell.
Provides mechanical strength in higher plants having vascular system
It protects the cell from mechanical injury.
Functions of cell wall

Cells communicate with one another via plasmodesmata ( fine cytoplasmic strands)
which pass through pores or channels present between plant cell walls and form a
system of interconnected protoplasts
(two adjacent cells )called the symplast.
It provides a porous medium for circulation (movement) of water, minerals and other
nutrients from between adjacent cells
Cuticle present on outer surface of epidermal cells (leaves) and suberin present in
periderm ( bark) prevents water loss.
The cell wall has an important function in regulating how plant cells achieve their final
size and shape and consequently have an essential role in regulating plant growth.
Some of the proteins of cell wall possess catalytic activity by acting as enzymes to
polymerize wall monomers, enzymes that cross-link polymers and enzymes that
cleave polymers.

The cell wall plays a very important role by performing certain functions such as
giving mechanical strength to cells and plants as a whole and maintaining the shape
of the cell.
It also prevents the osmotic bursting of the cells by inhibiting excessive endosmosis
The walls of xylem vessels, tracheids and sieve tube allow movement of materials
to a long distance. Besides that, cutin and suberin deposits check loss of water from
the cell surface by evaporation.
Also, the orientation of cellulose microfibrils help to control cell growth and shape.
The pits present in the wall help produce a protoplasmic continuum or symplast
amongst cells.

Properties of cell wall
The cell wall provides shape and support to the cell and appears to be rigid having
considerable tensile strength, but the cell wall is flexible up to some extent.
The rigidity of healthy plant cells results from a combination of the wall
construction and turgor pressure of protoplast (inflation ) by passive uptake
of water.
In some plant cells , a secondary wall is developed after cell ceases growth.
These additional layers are added in between primary wall and plasma membrane.
Additional layers may be formed containing lignin in xylem cell walls or suberin in
cork cell walls.
The cell walls of such cells become thicker which increases rigidity.
Rigidity of cell wall

Secondary cell walls contain a wide range of additional compounds as cellulose
(35-50%), hemicelluose (20 to 35%) and lignin (10 to 25%).
Lignin penetrates the space in the cell between cellulose, hemicelluloses and
pectin driving out water ,that modify mechanical properties and permeability of
secondary cell wall.
The outer part of the primary cell wall of the plant epidermis is usually
impregnated with cutin and wax, forming a permeability barrier known as the
plant cuticle
The walls of cork cells in the bark of trees are impregnated with suberin, and
suberin also forms the permeability barrier
Permeability of cell wall
1/3/2019 51Plant cell wall

Part-2
Other types of cell walls
Algal cell wall
Fungal cell wall
Bacterial cell wall
Archaeal cell wall

Algal cell walls
Algal cell walls contain either polysaccharides such as cellulose (a glucan)) or a
variety of glycoproteins (Volvocales) or both.
Besides cellulose presence of other polysaccharides in algal cell walls, is used
as a feature for algal taxonomy.
The β-1,4 mannans form microfibrils in the cell walls in green algal families,
Codiaceae, Dasycladaceae and Derbesiaceae and Acetabularia
β-1-3 xylans occur in green algal families
Bryopsidaceae,Caulerpaceae,Udotaceae and Dichotomosiphonaceae.
The algae Porphyra and Bangia (Red algae) possess both β-1,4 mannans and
β-1-3 xylans.
Alginic acid: It is a common polysaccharide found in the cell walls of brown
algae.

Figure – 17: Scanning electron micrographs of
diatoms showing the external appearance of the
cell wall
The cell walls (Frustules/ valves ) of diatoms are composed of silicic acid
The acid is polymerized intra-cellularly, then the wall is extruded to protect the
cell.
.
Sulfonated polysaccharides: They occur in the cell walls of most algae; those
familiar in red algae include agarose, carrageenan, porphyrin, furcelleran, and
funoran.
Other compounds that may accumulate in algal cell walls include sporopollenin and
calcium ions

Fungal cell walls
Most true fungi have a cell wall consisting largely of chitin and other
polysaccharides but no cellulose.
The fungal cell wall is a matrix of three main components chitin: polymers
consisting mainly of unbranched chains of β-(1,4)-linked-N-
Acetylglucosamine.
Chitin are cross linked by glucans( glucose polymers linked by β-(1,3)- or β-
(1,6)- bonds and provide rigidity to the cell wall and proteins (enzymes and
structural proteins) .
Structural proteins are glycosylated contain mannose known as
mannoproteins.

Bacterial cell wall
Bacterial cell walls are different from the cell walls of plants and fungi which
are made of cellulose and chitin, respectively.
Outside cell membrane bacterial cell walls composed of peptidoglycan
(murein) having polysaccharides cross linked by peptides is present . There
are two different type of bacterial cell based on response to the gram stain :
Gram positive and gram negative .
Gram-positive bacteria possess a thick cell wall containing many layers of
peptidoglycan and teichoic acids.
Gram-negative bacteria have a relatively thin cell wall consisting of a few
layers of peptidoglycan surrounded by a second lipid membrane containing
lipopolysaccharides and lipoproteins.

Figure- 18 Bacterial cell walls The plasma
membrane of Gram-negative bacteria is
surrounded by a thin cell wall beneath the
outer membrane. Gram-positive bacteria
lack outer membranes and have thick cell
walls.
Figure -19 : The peptidoglycan of E.coli
Polysaccharide chains consist of alternating
N-acetylglucosamine(NAG)and
N-acetylmuramic acid (NAM) residues joined by
β(1→4) glycosidic bonds. Parallel chains are
crosslinked by tetrapeptides attached to the NAM
residues. The amino acids forming the tetrapeptides
vary in different species of bacteria.
From cell walls and the extracellular matrix
The Cell: A Molecular Approach. 2nd edition.
Cooper GM. Sunderland (MA): Sinauer Associates ; 2000

Archaeal cell wall
All archaeal cell walls lack peptidoglycan, with the exception of one group of
methanogens
There are four types of cell wall currently known among the Archaea.
In some methanogens, such as Methanobacterium and Methanothermus
cell wall is composed of pseudopeptidoglycan (pseudomurein).
overall structure of archaeal pseudopeptidoglycan superficially resembles that of
bacterial peptidoglycan, but there are a number of significant chemical differences.
Pseudopeptidoglycan consists of polymer chains of cross-linked by short peptide
connections
However, unlike peptidoglycan, the sugar N-acetylmuramic acid is replaced by N-
acetyltalosaminuronic acid, and the two sugars are bonded with a β,1-3 glycosidic
linkage instead of β,1-4. Additionally, the cross-linking peptides are L-amino acids
rather than D-amino acids as they are in bacteria.

Bacteria Archaea
Peptidoglycan Pseudopeptidoglycan
Sugar- N-acetylmuramic acid (MurNAc) Sugar- N-Acetyltalosaminuronic acid
β,1-4. glycosidic linkage β,1-3 glycosidic linkage
cross-linking peptides are D-amino acids cross-linking peptides are L-amino acids
Cell wall of Bacteria Cell wall of Methanobacterium and
Methanothermus
Formula -C11H19NO8 Formula- C8H13NO7
Table – 1: Showing chemical differences between Peptidoglycan and Pseudopeptidoglycan

A second type of archaeal cell wall is found in Methanosarcina and Halococcus.
This type of cell wall is composed entirely of a thick layer of polysaccharides, which
may be sulfated in the case of Halococcus
A third type of wall among the Archaea consists of glycoprotein, and occurs in the
hyperthermophiles, Halobacterium, and some methanogens.
In Halobacterium, the proteins in the wall have a high content of acidic amino acids,
giving the wall an overall negative charge.
Halobacterium thrives only under conditions with high salinity.
In other Archaea, such as Methanomicrobium and Desulfurococcus, the wall may be
composed only of surface-layer (S-layer) proteins and polysaccharides
The other three types of Archaeal cell walls are composed of polysaccharides,
glycoproteins, or pure protein.

Most archaea stain Gram-negative, independent of their basic cell envelope
structure or chemical composition. An interesting exception
is Methanobacterium formicicum that stains Gram-positive, since its cell wall
contains pseudomurein, a type of peptidoglycan that lacks muramic acid.

REFERENCES
Buchanan B B et. al.( 2000) Biochemistry and Molecular Biology of Plants
Orca Book Services and American Society of Plant Physiologists.
Cooper G M. (2000) The Cell: A Molecular Approach. 2nd edition.Sunderland (MA): Sinauer
Associated
Harholt J, Suttangkakul A and Scheller HV (2010) Biosynthesis of pectins. Plant
Physiol 153: 384–395
Hofmann A(1929) On the enzymatic degradation of chitin and chitosan. Ph.D. thesis, University
of Zurich (Switzerland)
Kenneth Keegstra (2010) Plant Cell Walls :Plant physiology: Future perspectives in plant
physiology: 483-486
McCann M and Rose J (2010) Blueprint for building plant cell walls. Plant Physiol 153: 365

Ordog Vince and Molnar Zoltan (2011)Plant Physiology Chapter 4. Physiology of plant growth
and development:Cell wall biogenesis and expansion
Payen A (1838) Memoir on the composition of the tissue of plants and of woody material,
Comptes rendus, vol. 7: 1052–1056.
Rose JKC and Lee SJ (2010) Straying off the highway: trafficking of secreted plant proteins
and complexity in the plant cell wall roteome. Plant Physiol 153: 433–436
Roberts A. G. and. Oparka K. J (2003) Plasmodesmata and the control of symplastic transport
Plant, Cell and Environment .26, 103–124 (Citation Tangle 1879 )
Scheller HV and Ulvskov P (2010) Hemicelluloses. Annu Rev Plant Biol 61: 263–289
Somerville C (2006) Cellulose synthesis in higher plants.
Annu Rev Cell Dev Biol 22: 53–78
Strasburger E (1901) Uber plasmaverbindungen pflanzlicher Zellen. Jb Wiss Bot: 36: 493-610

THANKS
Source :Koning, Ross E. 1994. Basic Plant Cytology 1. Plant
Physiology Information Website.
http://plantphys.info/plant_physiology/basiccytology1.shtml.
(12-28-2018)
In addition to cellulose, walls have a range of various polymers of sugars and
sugar-derivatives. Hemicellulose rhamnogalacturonan, and pectins are shown
here.
Hemicellulose provides cross-linking of
the cellulose microfibrils. Pectins (present
in middle lamella )are the glue that holds
adjacent cells together. In secondary
xylem cells, another polymer class
condensed lignins is present

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