Primary cell wall

CELL WALL
Cell wall
Cell wall
•It differentiates plant cells with respect to animal cells and it is responsible for
many characteristics of plant organisms.
•It is a strong, rigid, extraprotoplasmatic layer exherting a wall pressure, equal
in force and opposite in direction to the turgor pressure avoiding cell
disruption as a consequence of water absorption into the vacuole.
•It determines dimensions and shape of cell, texture tissues and structure and
functions of plant organs.
•It provides a skeletal support to the whole plant and also a barrier against
injury and infection, the latter also by the synthesis of phyto-alexins, gums and
lignins.
•It is considered an active cell compartiment; it contains numerous enzymes
and plays an important role in absorption, transport and secretion of
substances and in intracellular communication (it bears receptors).
•Excluding water (70% of the cell wall), the main constituent of the cell wall is
cellulose, an insoluble polysaccharide, considered to be the most abundant
organic compound in the World.
•It is composed of β-glucose molecules linked by beta-1,4 glycosidic bonds.
•Cellulose molecules are aggregated in microfribils (10-25 nm in diameter) by
hydrogen bonds between hydroxyl groups of C3 and C6 of parallell chains.
•Each microfibril consists of a hundred cellulose chains.
•In cellulose beta-1,4 glycosidic bonds alternate up and down
the plane of the molecule. In starch α-1,4 glycosidic bonds
are all on the same side.
•These chemical differences are at the base of the different
functions of these molecules in plant cells.
•Cellulose is not digested by animals.
•Polymerization of cellulose varies from 2000-6000 β-glucose
units in the primary cell wall, to more than 13000 β-glucose units
in the secondary cell wall.
•Microfibrils are in turn assembled in macrofibrils of 0.5 μm in diameter. This
increases the cell wall rigidity (comparable to that of steel of the same
thickness).
•Some regions of microfibrils are known as micelles, showing crystalline
properties due to the orderly arrangement of molecules.
•Cellulose forms a framework immersed in a matrix composed of
hemicelluloses, pectins, glycoproteins and enzymes.
•Cell wall structure can be compared to that of reinforced concrete; the steel
rods are the microfibrils, the concrete is the matrix.
xyloglucans
•Hemicelluloses are heterogeneous polysaccharides such as xyloglucans
(Dycotyledones) and xylans (Monocotyledones) that are linked to microfibrils
through hydrogen bonds. They reduce extensibility and cellular elongation.
•Pectins are hydrophilic polysaccharides giving plasticity and flexibility to the
cell wall, which in this way can elongates.
•They are constituents of the middle lamella, the outermost intercellular layer
forming the interface between adjacent plant cells and gluing them together.
•Glycoproteins are constituens of matrix.
•Primary cell wall contains also numeorus enzymes such as
hydrolases, cellulases, pectinases esterases, peroxidases,
and transglycosylases, that cut, trim and cross-link wall
polymers.
•Cell wall has variable thickness depending on cell function and age.
•Primary cell wall is a thin (1-3 µm), flexible and extensible layer, formed
while the cell is growing.
•Secondary cell wall is a thick (5-10 µm) layer, increasing wall rigidity,
formed inside the primary cell wall after the cell is fully grown; it is not
found in all cell types.
•Middle lamella is a layer rich in pectins forming the interface between
adjacent plant cells.
Primary cell wall
•It is composed of cellulose (30%), hemicellulose, pectins, protein, enzymes
and water.
•Sometimes it can contains cutin and suberin.
•It is found in cytokinetic cells and in those involved in metabolic activities such
as photosynthesis, respiration, secretion, storage, etc.
•The primary layer is not uniformly thickened, but it shows thin areas crossed
by plasmodesmata, which are inter-connecting channels of cytoplasm that
connect the protoplasts of adjacent cells across the cell wall.
Secondary cell wall
•It is composed of cellulose (50%) while pectins, proteins and water
are lacking.
•In some cases it contains lignin, such as the conducting cells in xylem,
which strengthens and waterproofs the wall, or suberin, such as cork
cell walls, which waterproofs the wall itself.
•It is found in cork, supporting (sclerenchyma) and conducting
(xylema) cells where the protoplast dissolves at cell maturity.
Stratified layer model
•In the secondary cell wall 3 different layers can be observed.
•Unlike the primary wall, the microfibrils are aligned mostly in
the same direction, and with each additional layer the
orientation changes slightly. This provides a higher tensile
strength.
Pits
•Regions in a cell wall where the primary wall is not
overlaid by secondary thickening, through which
substances can be exchanged between adjacent cells by
plasmodesmata. The pit consists of a cavity, which is the
area of thinning in the secondary wall, and a pit
membrane, which is the primary wall covering the
cavity. Pits usually occur in pairs.
Areolate pits of xylem cells (tracheids) of conifers.
Sclereids of pulp of pear fruit
•Orientation of microfibrils is influenced by microtubules and determines
direction of cellular elongation.
•In cells growing in all directions, microfibrils are deposited in a casual
manner; conversely, in cells growing along a specific direction, microfibrils
are deposited perpendicular to growth axis.
•Plant cell grows by the action of proteins (expansins),
which make wall less rigid so that the cell expands by
the pressure coming from within the vacuole.
•Together with the turgore
pressure, expansins break
bonds between microfibrils
and matrix polysaccharides
allowing
distension
of
tonoplast, as a consequence
of water assumption, and of
increase in the volume of
cell.
Formation of the
cell wall
•Cellulose
microfibrils
are
synthesized by enzyme complex
known as cellulose synthase, sited
on the phospholipid bilayer and
inserted by trans-Golgi vesicles.
•Their movement is guided by
cortical microtubules.
•Each complex extrudes numerous
glucanic chains which form
microfibrils by hydrogen bonds.
Formation of the cell wall
•Cortical microtubules play an important role in the alignment of cellulose
microfibrils. They run parallel to microfibrils deposited externally to the
membrane.
Formation of
the cell wall
•Framework of microfibrils in the cell wall reflects orientation of cortical
microtubules. The latter allow cellulose-synthase complex moving only
along specific directions.
Formation of the
cell wall
•Matrix constituents (hemicelluloses, pectins and glycoproteins) are
discharged into the wall by trans-Golgi secretion vesicles fusing with the
plasma membrane. This process is called exocytosis.
•Owing to the presence of the wall, in the plant cells endocytosis is
lacking.
PLASMODESMA
•Plasmodesmata are bridges (cytoplasmatic channels), lined with a plasma
membrane, that connect adjacent cells providing major pathways of
communication and transport between cells.
•They are crossed by a tube of endoplasmic reticulum (desmotubule) and are
running through pairs of pits in secondary cell wall or through the thin areas in
primary cell wall.
PLASMODESMA
PLASMODESMA
PLASMODESMA
•A pit is a region in a cell wall where the primary
wall is not overlaid by secondary thickening,
through which substances can be exchanged
between adjacents cells. It consists of a cavity,
which is the area of thinning in the secondary
wall, and a pit membrane, which is the primary
cell wall covering the cavity.
•Pits are crossed by plasmodesmata which are
formed by tubes of endoplasmic reticulum
(desmotubule) trapped across the cell plate
which is the partition formed between daughter
cells during cytokinesis.
•Alternatively, plasmodesmata can be inserted
into existing cell walls between non-dividing
cells.
Division of the plant cell
The presence of the cell wall differentiate significantly the division of
the plant cell with respect to that of animal cell.
Interphase of the plant cell is characterized by 2 events:
1. (G1) Nucleus moves to the center of cell through cytoplasmatic
bridles; the latter form a trasversal cytoplsmatic blade known as
phragmosome containing microtubules and actin filaments. This
blade determines the plan of the future cell division.
2. (G2) A ring-like band of microtubules, known as pre-prophase band
appears inside the membrane. This structure surrounds nucleus
determining a plan corresponding to the equatorial plan of the
future mitotic spindle. It disappears at the beginning of mitosis.
Division of the plant cell
Division of the plant cell
Division of the plant cell
•Pre-prophase band determines the
plan of the future cell division like
phragmosome.
•It disappears before metaphase.
Cytokinesis in the plant cell
Late Prophase
Middle Anaphase
Early Metaphase
Late Anaphase
Late Metaphase
Telophase-Cytokinesis
•Cytokinesis begins in early telophase with the formation of a cell plate.
Cytokinesis in the plant cell
Cytokinesis phases
This process characterizes terrestrial plants (from Bryophyta to
Magnoliophyta).
At the early telophase a microtubule system, called phragmoplast,
appears between the two new-formed nuclea.
Phragmoplast allows fusion of trans-Golgi secretion vesicles in the
middle of cell, leading to the formation of the cell plate.
During fusion of vesicles, a new mebrane with plasmodesma are
formed.
Once the cell plate reaches the cell wall (in a zone previously
determined by the pre-prophase band) the middle lamella is formed. It
is mainly composed of pectins. The primary cell wall is then deposited
inside the middle lamella.
Cytokinesis in the plant cell
Cytokinesis in the plant cell
Formation of the cell plate
Cytokinesis in the plant cell
Formation of the cell plate: fusion of trans-Golgi vesicles containing pectins
Cytokinesis in the plant cell
Formation of the cell plate and plasmodesmata
Cytokinesis in the plant cell
Formation of the middle lamella and of the new cell wall
Cytokinesis in the plant cell
Distibution of microtubules during the cell cycle and formation of the cell wall
(formazione del fragmosoma)
Cell wall modifications
Cell walls contain a wide range of additional compounds that modify their
mechanical properties and permeability:
•Deposit of lignin (lignification).
•Deposit of suberin (suberification).
•Deposit of cutin and waxes.
•Deposit of minerals such as carbonates, oxalates and silica (mineralization).
•Production of gums and mucilages (gelification).
•Deposit of pigments such as phlobaphenes, tannins, flavonoids etc.
(pigmentation).
Lignin
It penetrates the spaces in the cell wall between cellulose,
hemicellulose and pectin components, driving out water and
strengthening the wall.
Its function appears to be to cement together and anchor
cellulose microfibrils and to stiffen the cell wall.
It waterproofs wall allowing conducting function in xylem cells.
It gives rigidity to the wall of sclerenkyma cells.
It can be produced as a phyto-alexin as a consequence of a
damage or infection (defense role).
It is among the most chemically inert of plant substances,
resistant to enzymes, and survives in fossils of woody stems.
Lignin
phenylpropanoid precursors
•Lignin is a complex, cross-linked
polymer, comprising phenylpropanoid
units (C6-C3) synthesized in cytoplasm.
Its composition can vary with the
species.
waxes
Cutin and
waxes
•Waxes are mixture of long-chain fatty acids (e.g. palmitic, stearic,
cerotic) with aliphatic alcohols (e.g. cerilic, myristic)
•Cutin consists of long-chain hydroxy acids and their derivatives, with
waterproofing qualities, which are interlinked via ester bonds, forming a
polyester polymer of indeterminate size.
•Together, cutin and waxes form a barrier against pathogens and hydric
stress, known as cuticle, in the outer part of the primary cell wall of the
plant epidermis (leaves, flowers, fruits, young stems and roots).
Cutin and waxes
•Carnauba wax, is the major source
of wax for polishes, obtained from
the leaf surfaces of wax palm
(Copernicia cerifera), a tree native
to Brazil.
•At industrial level, it is employed in
the
ointment
and
tablets
manufacturing.
Copernicia cerifera
Suberin
•Suberin is a fatty substance found in or on the
surface of cell walls in cork, usually associated
with waxes, and exodermis and endodermis
where it forms the Casparian strip.
•It renders tissues waterproof and protect them
from decay.
•It is a complex polymer of fatty acids (e.g.
suberic acid) esterified by phenylpropanoids
linked to glycerol units.
Gums and mucilages
•As a consequence of specialization, damage or climatic
stress, cell wall can produce heterogenous polysaccharides
such as gums and mucilages which are in turn can be
easily collected (e.g. incision of the trunk) and used in
pharmacy because of the gelling, emulsifier and stabilizing
properties.
Mineralization
•Salts such as carbonates and silicates can give
rigidity and protection to the wall. This
modification occurs in the family of Poaceae,
Moraceae and Equisetaceae.
•Mineralization has pharmacognostic importance
for the plant drug authentication.
Pharmaceutical application of
cell wall products
•Cellulose is used to make cotton wool
and sterile gauzes. At the industrial
level, it is extracted from the genus
Gossypium (Malvaceae).
Pharmaceutical application of
cell wall products
•From brown algae it is extracted alginic
acid, a heterogeneous polysaccharide,
employed as emulsifier, excipient and
stabilizing.
•From red algae it is extracted agar, a
sulfur polymer of galactose, employed as
emulsifier, excipient and as main
component of media used in cell cultures.
Pharmaceutical application of
cell wall products
•As a consequence of climatic or mechanical stress,
plants produce exudates containing gums,
heterogeneous polysaccharides composed of
galactose, fructose, xylose, arabinose, ramnose,
galacturonic acid etc. They are used as excipient,
emulsifier and laxative.
•Mucilages are chemically similar to gums, but they
are not stress-induced products. They are normal
constituents of the cell wall. They are employed as
anti diarrhea, laxative, emollient, decongestant and
expectorant.
da: “Biologia delle piante” (P.H. Raven et alii)
da: “Biologia delle piante” (P.H. Raven et alii)