Download Plant cell wall Composition

CHAPTER – II
CELL AND CELL WALL
The cell is the basic unit of life. Plant cells (unlike animal cells) are surrounded by a
thick, rigid cell wall.
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Animal Cell
Shape:
Round
Glyoxysomes:
No
Centrioles:
Always present
Lysosomes:
Plasma Membrane:
Cell wall:
Plant Cell
Rectangular
Some plant cells have
glyoxysomes.
Only present in lower
plant forms.
Lysosomes occur in
Lysosomes usually not
cytoplasm.
evident.
Yes; only cell
Yes; cell wall and a cell
membrane
membrane
None
Yes
Plant cells have
Chloroplast:
Animal cells don't have chloroplasts because
chloroplasts
they make their own
food
Plastids:
No
Yes
One, large central
vacuole taking up 90%
One or more small
Vacuole:
of cell volume.
vacuoles (much smaller
than plant cells).
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Table:
:The following is a glossary of plant cell anatomy.
Glossary of plant cell anatomy
No. Organelles
1
Cell wall
Functions
A thick, rigid membrane that surrounds a plant cell.
This layer of cellulose fiber gives the cell most of its
support and structure. The cell wall also bonds with
other cell walls to form the structure of the plant.
2
Cell membrane
The thin layer of protein and fat that surrounds the
cell, but is inside the cell wall. The cell membrane is
semipermeable, allowing some substances to pass
into the cell and blocking others.
3
Chloroplast
An elongated or disc-shaped organelle containing
chlorophyll. Photosynthesis (in which energy from
sunlight is converted into chemical energy - food)
takes place in the chloroplasts.
4
Chlorophyll
Chlorophyll is a molecule that can use light energy
from sunlight to turn water and carbon dioxide gas
into sugar and oxygen (this process is called
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photosynthesis). Chlorophyll is magnesium based
and is usually green.
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Granum (plural
A stack of thylakoid disks within the chloroplast is
grana
called a granum.
Photosynthesis A process in which plants convert sunlight, water,
and carbon dioxide into food energy (sugars and
starches), oxygen and water. Chlorophyll or closelyrelated pigments (substances that color the plant)
are essential to the photosynthetic process.
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Stroma
part of the chloroplasts in plant cells, located within
the inner membrane of chloroplasts, between the
grana.
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Thylakoid disk
Thylakoid
disks
are
disk-shaped
membrane
structures in chloroplasts that contain chlorophyll.
Chloroplasts are made up of stacks of thylakoid
disks; a stack of thylakoid disks is called a granum.
Photosynthesis (the production of ATP molecules
from sunlight) takes place on thylakoid disks.
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ATP
it is a high-energy molecule used for energy storage
by organisms. In plant cells, ATP is produced in the
cristae of mitochondria and chloroplasts.
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Christae
(Singular crista) the multiply-folded inner membrane
of
a
cell's
mitochondrion
that
are
finger-like
projections. The walls of the cristae are the site of
the cell's energy production (it is where ATP is
generated).
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Amyloplasts
An organelle in some plant cells that stores starch.
Amyloplasts are found in starchy plants like tubers
and fruits
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Cytoplasm
The jellylike material outside the cell nucleus in
which the organelles are located.
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Golgi body
(Also called the golgi apparatus or golgi complex) a
flattened, layered, sac-like organelle that looks like a
stack of pancakes and is located near the nucleus.
The
golgi
body
packages
proteins
and
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carbohydrates into membrane-bound vesicles for
"export" from the cell.
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Nuclear
The membrane that surrounds the nucleus.
membrane
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Nucleolus
an organelle within the nucleus - it is where
ribosomal RNA is produced.
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Nucleus
Spherical
body
containing
many
organelles,
including the nucleolus. The nucleus controls many
of the functions of the cell (by controlling protein
synthesis) and contains DNA (in chromosomes). The
nucleus is surrounded by the nuclear membrane.
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Centrosome
(Also called the "microtubule organizing center") a
small body located near the nucleus - it has a dense
center and radiating tubules. The centrosomes is
where microtubules are made. During cell division
(mitosis), the centrosome divides and the two parts
move to opposite sides of the dividing cell. Unlike
the
centrosomes
in
animal
cells,
plant
cell
centrosomes do not have centrioles.
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Ribosome
Small organelles composed of RNA-rich cytoplasmic
granules that are sites of protein synthesis.
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Rough
A vast system of interconnected, membranous,
Endoplasmic
infolded and convoluted sacks that are located in the
reticulum
cell's cytoplasm (the ER is continuous with the outer
nuclear membrane). Rough ER is covered with
ribosome that gives it a rough appearance. Rough
ER
transport
materials
through
the
cell
and
produces proteins in sacks called cisternae (which
are sent to the Golgi body, or inserted into the cell
membrane).
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Smooth
A vast system of interconnected, membranous,
Endoplasmic
infolded and convoluted tubes that are located in the
reticulum
cell's cytoplasm (the ER is continuous with the outer
nuclear membrane). The space within the ER is
called the ER lumen. Smooth ER transport materials
through the cell. It contains enzymes and produces
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and digests lipids (fats) and membrane proteins;
smooth ER buds off from
rough ER, moving the newly-made proteins and
lipids to the Golgi body and membranes.
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Vacuole
A large, membrane-bound space within a plant cell
that is filled with fluid. Most plant cells have a single
vacuole that takes up much of the cell. It helps
maintain the shape of the cell.
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Mitochondriya
Spherical to rod-shaped organelles with a double
membrane. The inner membrane is infolded many
times, forming a series of projections (called
cristae). The mitochondrion converts the energy
stored in glucose into ATP (adenosine triphosphate)
for
the
cell.
Cell wall
A cell wall is a tough, flexible and sometimes fairly rigid layer offering
protection against mechanical stress. It is located outside the cell membrane and
provides these cells with structural support and protection. A major function of the
cell wall, in multicellular organisms, is to act as a pressure vessel, preventing overexpansion when water enters the cell. In other word the creation of a stable osmotic
environment by preventing osmotic lysis and helping to retain water. It permits the
organism to build and hold its shape (morphogenesis). The cell wall also limits the
entry of large molecules that may be toxic to the cell. They are found in plants,
bacteria, fungi, algae, and some archaea. Animals and protozoa do not have cell
walls.
The materials in a cell wall vary between species, and in plants and fungi also
differ between cell types and developmental stages. In plants, the strongest
component of the complex cell wall is a carbohydrate called cellulose, which is a
polymer of glucose. In bacteria, peptidoglycan forms the cell wall. Archaean cell
walls have various compositions, and may be formed of glycoprotein S-layers,
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pseudope
ptidoglyca
n,
or
polysacch
arides.
Fungi
possess
cell walls
made
of
the
glucosami
ne
polymer chitin, and algae typically possess walls made of glycoproteins and
polysaccharides. Unusually, diatoms have a cell wall composed of silicic acid. Often,
other accessory molecules are found anchored to the cell wall.
Plant cell wall Composition
The major carbohydrates making up the primary (growing) plant cell wall are
cellulose, hemicellulose and pectin. The cellulose microfibrils are linked via
hemicellulosic tethers(rope/chain) to form the cellulose-hemicellulose network, which
is embedded in the pectin matrix.
Up to three strata or layers may be found in plant cell walls:

The middle lamella, a layer rich in pectins. This outermost layer forming the
interface between adjacent plant cells and glues them together.

The primary cell wall, generally a thin, flexible and extensible layer formed
while the cell is growing.

The secondary cell wall, a thick layer formed inside the primary cell wall
after the cell is fully grown. It is not found in all cell types. In some cells, such
as found xylem, the secondary wall contains lignin, which strengthens and
waterpoofs the wall.
Cell walls in some plant tissues also function as storage depots for
carbohydrates that can be broken down and resorbed to supply the metabolic and
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growth needs of the plant. For example, endosperm cell walls in the seeds of cereal
grasses,
nasturtium,
and
other species,
are
rich
in
glucans and
other
polysaccharides that are readily digested by enzymes during seed germination to
form simple sugars that nourish the growing embryo. Cellulose microfibrils are not
readily digested by plants, however.
The most common hemicellulose in the primary cell wall is xyloglucan. 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.
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 crosslinked 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-Dgalacturonic 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
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modified to allow cells to expand and grow. Several models have been proposed to
account for the mechanical properties of the wall.
Some of the 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 and form.

regulate diffusion of material through the apoplast.

carbohydrate storage - walls of seeds may be metabolized.

protect against pathogens, dehydration, and other environmental factors.

source of biologically active signalling molecules.

cell-cell interactions.
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.
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Secondary cell walls contain cellulose (35 to 50%), xylan, a type of
hemicellulose, (20 to 35%) and a complex phenolic polymer called lignin (10 to
25%). Lignin penetrates the spaces in the cell wall between cellulose, hemicellulose
and pectin components, driving out water and strengthening the wall. The walls of
cork cells in the bark of trees are impregnated with suberin, and suberin also forms
the permeability barrier in primary roots known as the Casparian strip. Secondary
walls - especially in grasses - may also contain microscopic silica crystals, which
may strengthen the wall and protect it from herbivores.
Plant cells walls also contain numerous enzymes, such as hydrolases,
esterases, peroxidases, and transglycosylases, that cut, trim and cross link wall
polymers. Small amounts (1-5%) of structural proteins are found in most plant cell
walls;
they
are
classified
as
hydroxyproline-rich
glycoproteins
(HRGP),
arabinogalactan proteins (AGP), glycine-rich proteins (GRPs), and proline-rich
proteins (PRPs). Each class of glycoprotein is defined by a characteristic, highly
repetitive protein sequence. Most are glycosylated, contain hydroxyproline (Hyp) and
become cross-linked in the cell wall.
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Properties.
The composition, properties, and form of the cell wall may change during the cell
cycle and depend on growth conditions.
1. Rigidity
The rigidity of cell walls is often over-estimated. In most cells, the cell wall is
flexible, and has considerable tensile strength. The apparent rigidity of primary plant
tissues is a function of hydraulic turgor pressure of the cells and not due to rigid cell
walls. This flexibility is seen when plants wilt, so that the stems and leaves begin to
droop, or in seaweeds that bend in water currents. The rigidity of healthy plants
results from a combination of the wall construction and turgor pressure.
In plants, a secondary cell wall is a thicker additional layer of cellulose which
increases wall rigidity. Additional layers may be formed containing lignin in xylem cell
walls, or containing suberin in cork cell walls. These compounds are rigid and
waterproof, making the secondary wall stiff. Both wood and bark cells of trees have
secondary walls. Other parts of plants such as the leaf stalk may acquire similar
reinforcement to resist the strain of physical forces.
Certain single-cell protists and algae also produce a rigid wall. Diatoms build a
frustule from silica extracted from the surrounding water; radiolarians also produce a
test from minerals. Many green algae, such as the Dasycladales encase their cells in
a secreted skeleton of calcium carbonate. In each case, the wall is rigid and
essentially inorganic.
2. Permeability
The primary cell wall of most plant cells is semi-permeable and permit the
passage of small molecules and small proteins, with size exclusion estimated to be
30-60 kDa. Key nutrients, especially water and carbon dioxide, are distributed
throughout the plant from cell wall to cell wall in apoplastic flow.
Algal cell walls : like plants, algae have cell walls. Algal cell walls contain cellulose
and a variety of glycoproteins. The inclusion of additional polysaccharides in algal
cells walls is used as a feature for algal taxonomy.
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
Manosyl form microfibrils in the cell walls of a number of marine green algae
including those from the genera, Codium, Dasycladus, and Acetabularia as
well as in the walls of some red algae, like Porphyra and Bangia.

Xylanes

Alginic acid is a common polysaccharide in the cell walls of brown algae

Sulfonated polysaccharides occur in the cell walls of most algae; those
common in red algae include agarose, carrageenan, porphyran, furcelleran
and funoran.
Other compounds that may accumulate in algal cell walls include sporopollenin and
calcium ions.
The group of algae known as the diatoms synthesize their cell walls (also
known as frustules or valves) from silicic acid (specifically orthosilicic acid, H4SiO4).
The acid is polymerised intra-cellularly, then the wall is extruded to protect the cell.
Significantly, relative to the organic cell walls produced by other groups, silica
frustules require less energy to synthesize (approximately 8%), potentially a major
saving on the overall cell energy budget and possibly an explanation for higher
growth rates in diatoms.
Fungal cell walls : Chemical structure of a unit from a chitin polymer chain.There
are several groups of organisms that may be called "fungi". Some of these groups
have been transferred out of the Kingdom Fungi, in part because of fundamental
biochemical differences in the composition of the cell wall. Most true fungi have a cell
wall consisting largely of chitin and other polysaccharides. True fungi do not have
cellulose in their cell walls, but some fungus-like organisms do.
True fungi : Not all species of fungi have cell walls but in those that do, the plasma
membrane is followed by three layers of cell wall material. From inside out these are:

a chitin layer (polymer consisting mainly of unbranched chains of N-acetyl-Dglucosamine)

a layer of β-1,3-glucan

a layer of mannoproteins (mannose-containing glycoproteins) which are
heavily glycosylated at the outside of the cell.
Fungus-like protists :The group Oomycetes, also known as water molds, are
saprotrophic plant pathogens like fungi. Until recently they were widely believed to
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be fungi, but structural and molecular evidence has led to their reclassification as
heterokonts, related to autotrophic brown algae and diatoms. Unlike fungi,
oomycetes typically possess cell walls of cellulose and glucans rather than chitin,
although some genera (such as Achlya and Saprolegnia) do have chitin in their
walls. The fraction of cellulose in the walls is no more than 4 to 20%, far less than
the fraction comprised by glucans. Oomycete cell walls also contain the amino acid
hydroxyproline, which is not found in fungal cell walls.
The dictyostelids are another group formerly classified among the fungi. They
are slime moulds that feed as unicellular amoebae, but aggregate into a reproductive
stalk and sporangium under certain conditions. Cells of the reproductive stalk, as
well as the spores formed at the apex, possess a cellulose wall. The spore wall has
been shown to possess three layers, the middle of which is composed primarily of
cellulose, and the innermost is sensitive to cellulase and pronase.
Prokaryotic cell walls:
Bacterial cell walls
Diagram of a typical gram-negative bacterium, with the thin cell wall
sandwiched between the red outer membrane and the thin green plasma membrane.
Schematic of typical gram-positive cell wall showing arrangement of NAcetylglucosamine and N-Acetlymuramic acid
Further information: Cell envelope
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Around the outside of the cell membrane is the bacterial cell wall. Bacterial
cell walls are made of peptidoglycan (also called murein), which is made from
polysaccharide chains cross-linked by unusual peptides containing D-amino acids.
Bacterial cell walls are different from the cell walls of plants and fungi which are
made of cellulose and chitin, respectively. The cell wall of bacteria is also distinct
from that of Archaea, which do not contain peptidoglycan. The cell wall is essential to
the survival of many bacteria, although L-form bacteria can be produced in the
laboratory that lack a cell wall. The antibiotic penicillin is able to kill bacteria by
preventing the cross-linking of peptidoglycan and this causes the cell wall to weaken
and lyse.
There are broadly speaking two different types of cell wall in bacteria, called
Gram-positive and Gram-negative. The names originate from the reaction of cells to
the Gram stain, a test long-employed for the classification of bacterial species.
Gram-positive bacteria possess a thick cell wall containing many layers of
peptidoglycan and teichoic acids. In contrast, 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. Most
bacteria have the Gram-negative cell wall and only the Firmicutes and Actinobacteria
(previously known as the low G+C and high G+C Gram-positive bacteria,
respectively) have the alternative Gram-positive arrangement. These differences in
structure can produce differences in antibiotic susceptibility, for instance vancomycin
can kill only Gram-positive bacteria and is ineffective against Gram-negative
pathogens, such as Haemophilus influenzae or Pseudomonas aeruginosa.
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