The document summarizes the composition and architecture of plant cell walls. It discusses the primary components of plant cell walls including cellulose, hemicellulose, pectin and lignin. It describes how these components interact through different proposed models and compares the structure of primary and secondary cell walls. Secondary cell walls are thicker and stronger and contain more lignin, which provides rigidity important for conducting tissues. The recalcitrance of secondary cell walls is important for applications in biofuels and pulp/paper industries.

1. Plant Cell Wall: Composition & Architecture
• What is Cell Wall?
• General Functions of Cell Wall
• General composition of Cell Wall
• How Cell Wall originates?
• Primary & Secondary Cell Wall
• Composition & Architecture of Cell Wall of
different plant types
Bacterial Cell Wall
Algal Cell Wall
Fungal Cell Wall
Higher Plant Cell Wall

4. The structure of cell walls could well be compared to that of reinforced
concrete: the scaffolding substance, cellulose in plants, iron in concrete
is embedded in an amorphous ground substance, the matrix.

5. The cell wall has a number of functions:
• Lends the cell stability
• Determines cell shape
• Influences cell development
• Protects the cell against pathogens (viruses, bacteria,
fungi, etc.)
• Counterbalances the osmotic pressure
The cell wall of elongating cells is still elastic, a property
that is lost in fully differentiated cells. It is therefore
distinguished between a primary and a secondary wall.

6. How Cell Wall originates?
The primary cell wall is laid out during the first division
of the cell. It develops normally between the two
daughter cells during early telophase.
The early stage of the new cell wall is the cell plate, a
lamella-like structure in the former equatorial plane of
the mitotic apparatus.
The cell plate develops by fusion of numerous vesicles
and grows centrifugally until it reaches the longitudinal
lateral walls of the mother cell. Electron dense material
is deposited at both its sides. The thus developing
structure is called the phragmoplast. It is the immediate
precursor of the primary wall.

7. What is a primary cell wall
Growing plant cells are surrounded by a polysaccharide-rich primary wall.
This wall is part of the apoplast which itself is largely self-contiguous and
contains everything that is located between the plasma membrane and the
cuticle. The primary wall and middle lamella account for most of the
apoplast in growing tissue. The symplast is another unique feature of plant
tissues. This self-contiguous phase exists because tube-like structrues
known as plasmodesmata connect the cytoplasm of different cells.

9. Composition of primary cell wall:
• Cellulose (35-50%)
• Hemicellulose (20-35%)
• Lignin (10-25)
• Pectin
The cellulose microfibrils linked via hemicellulose tethers to form the
cellulose-hemicellulose network which is embeded in the pectin matrix.
The most common hemicellulose in primary cell wall is xyloglucan.
Lignin fills the spaces in the cell wall between cellulose, hemicellulose &
pectin components.
Plant Cell wall also incorporated a number of proteins:
• Hydroxyproline-rich glyco protein (extensins)
• Arabinogalactan proteins
• Glycine-rich proteins
• Proline-rich proteins
All proteins are cross linked to the cell wall & have structural functions

10. Functions of the primary wall:
• Structural and mechanical support
• Maintain and determine cell shape
• Resist internal turgor pressure of cell
• Control rate and direction of growth
• Ultimately responsible for plant architecture & form
• Regulate diffusion of material through the apoplast
• Carbohydrate storage - walls of seeds may be
metabolized
• Protect against pathogens, dehydration, & other
environmental factors
• Source of biologically active signalling molecules
• Cell-cell interactions.

11. The actual structure of cell wall is not clearly
defined & several models exists:
• Covalently linked cross model
• Tether model
• Diffuse layer model
• Stratified layer model

12. The covalently cross-linked model
Peter Albersheim and colleagues in 1973 proposed that the wall matrix
polymers (xyloglucan, pectin, and glycoprotein) are covalently linked to
one another. The binding of xyloglucan to cellulose microfibrils results in
a non-covalently cross-linked cellulose-hemicellulose network that gives
the wall tensile strength. This model has been questioned because of the
lack of evidence for the existence of covalent linkages between
xyloglucan, pectin and glycoprotein.

13. The tether model: Xyloglucan molecules are
hydrogen bonded to and cross-link cellulose
microfibrils. The cellulose-xyloglucan network is
emeshed in a non-covalently cross-linked pectic
network.
The diffuse layer model: Xyloglucan molecules are
hydrogen bonded to the surface of cellulose
microfibrils but do not directly cross link them. The
tightly-bound xyloglucan is surrounded by a layer
of less-tightly bound polysaccharides. The
cellulose and xyloglucan are embedded in a pectic
matrix.
The stratified layer model: Xyloglucan molecules
are hydrogen bonded to and cross-link cellulose
microfibrils. The cellulose-xyloglucan lamellae are
separated by strata of pectic polysaccharides.

14. Algal Cell Wall:
• Algae are plants with the simplest organization
• Many of them are singled celled & some are multicellular
• Algal cell wall differs from higher plant cell wall
• Algal cell walls contain cellulose and a variety of glycoproteins. The
inclusion of additional polysaccharides in algal cell walls is used as a
feature for algal taxonomy:
Manosyl – Codium, Acetabularia (Green Algae), Porphyra (Red Algae)
Xylanes
Alginic acid – Brown Algae
Sulfonated polysaccharides (like agarose, porphyran, funoran etc. –
common in red algae
Other compounds:
• Sporopollenin
• Calcium ions
• Silicic acid – in Diatom

15. Fungal Cell Wall:
• In fungi the plasma membrane is followed by three layers of cell wall
materials. From inside out these are:
A Chitin layer (polymer consisting mainly of unbranched chains of
N-acetyl-D-glucosamine)
A layer of -1,3-glucan
A layer of mannoprotein (mannose-containing glycoprotein) which
are heavily glycosylated at the outside of the cell.
The composition, properties and form of the fungal cell wall change
during cell cycle and depend on growth conditions.
The group Oomycetes (water mold) anomalously possess cellulose cell
walls

17. Gram –ve bacterium:
Thin Cell wall sandwiched between the yellow outer membrane & thin
red plasmamembrane. Thin Cell Wall consists of few layers of peptidoglycan
surrounded by a second lipid membrane Containing lipopolysaccharides and
lipoproteins.
Peptydoglycan is also called murein is made from polysaccharide chains
cross-linked by unusual peptides containing D-amino acids
• Most bacteria are
Gram –ve
• Vancomycin cann’t
kill Gram –ve bacteria

19. Gram +ve bacterium:
• Possess a thick Cell wall containing many layers of peptydoglycan &
teichoic acids.
• Vancomycin can kill only Gram +ve bacteria
• Cell wall is essential for survival of bacterium & the antibiotic like
penicillin is able to kill bacteria by inhibiting a step in the synthesis of
peptidoglycan

22. Plants differ in shape and size. These differences result from the different
morphologies of the various cells that make up the vegetative and
reproductive organs of the plant body. Changes in tissue and organ
morphology that occur during plant growth and development result from
controlled cell division and growth together with modification and
structural reorganiztion of the wall, and the synthesis and insertion of new
material into the existing wall.
Primary walls are the major textural component of plant-derived foods. The
ripening of fruits and vegetables is associated with changes in wall
structrue and composition. Plant-derived beverages often contain
significant amounts of wall polysaccharides. Some wall polysaccharides
bind heavy metals, stimulate the immune system or regulate serum
cholesterol. Wall polysaccharides are used commercially as gums. gels,
and stabilizers. Thus, cell wall structure and organization is of interest to
the plant scientist, the food processing industry and the nutritionist.

23. Primary wall composition and architecture
Primary walls isolated form higher plant tissues and cells are composed
predominantly of polysaccharides together with lesser amounts of structural
glycoproteins (hydroxyproline-rich extensins) , phenolic esters (ferulic and
coumaric acids), ionically and covalently bound minerals (e.g. calcium and
boron), and enzymes. In addition walls contain proteins (expansins) that are
believed to have a role in regulating wall expansion. Lignin, a macromolecule
composed of highly cross-linked phenolic molecules, is a major component of
secondary walls.The major polysaccharides in the primary wall are:
Cellulose - a polysaccharide composed of 1,4-linked β-D-glucose residues
Hemicellulose - branched polysaccharides that are structurally homolgous to
cellulose because they have a backbone composed of 1,4-linked β-D-hexosyl
residues. The predominant hemicellulose in many primary walls is xyloglucan.
Other hemicelluloses found in primary and secondary walls include
glucuronoxylan, arabinoxylan, glucomannan, and galactomannan.
Pectin - a family of complex polysaccharides that all contain 1,4-linked α-D-
galacturonic acid. To date three classes of pectic polysaccharides have been
characterized: Homogalacturonans, rhamnogalacturonans, and substituted
galacturonans.
The organization and interactions of wall components is not known with
certainty and there is still considerable debate about how wall organization is
modified to allow cells to expand and grow. Several models have been
proposed to account for the mechanical properties of the wall:

24. Secondary cell walls
Plants form two types of cell wall that differ in function and in
composition. Primary walls surround growing and dividing plant cells.
These walls provide mechanical strength but must also expand to allow
the cell to grow and divide. The much thicker and stronger secondary
wall (see figure on right), which accounts for most of the carbohydrate in
biomass, is deposited once the cell has ceased to grow. The secondary
walls of xylem fibers, tracheids, and sclereids are further strengthened
by the incorporation of lignin.
The evolution of conducting tissues with rigid secondary cell walls was a
critical adaptive event in the history of land plants, as it facilitated the
transport of water and nutrients and allowed extensive upright growth.
Secondary walls also have a major impact on human life, as they are a
major component of wood and are a source of nutrition for livestock. In
addition, secondary walls may help to reduce our dependence on
petroleum, as they account for the bulk of renewable biomass that can
be converted to fuel. Nevertheless, numerous technical challenges must
be overcome to enable the efficient utilization of secondary walls for
energy production and for agriculture.

25. Primary and secondary walls contain cellulose, hemicellulose and pectin,
albeit in different proportions. Approximately equal amounts of pectin and
hemicellulose are present in dicot primary walls whereas hemicellulose is
more abundant in grasses (e.g., switchgrass). The secondary walls of
woody tissue and grasses are composed predominantly of cellulose,
lignin, and hemicellulose (xylan, glucuronoxylan, arabinoxylan, or
glucomannan). The cellulose fibrils are embedded in a network of
hemicellulose and lignin. Cross-linking of this network is believed to result
in the elimination of water from the wall and the formation of a hydrophobic
composite that limits accessibility of hydrolytic enzymes and is a major
contributor to the structural characterisitics of secondary walls.
Xylan, which accounts for up to 30% of the mass of the secondary walls in
wood and grasses contributes to the recalcitrance of these walls to
enzymic degradation. A high xylan content in wood pulp increases the
economic and environmental costs of bleaching in paper manufacturing.
Thus, reducing the xylan content of secondary walls and altering xylan
structure, molecular weight, ease of extractability, and susceptibility to
enzymic fragmentation are key targets for the genetic improvement of
plants. However, progress in these areas is limited by our incomplete
understanding of the mechanisms of xylan biosynthesis.