CELL WALL

CELL WALL
 A cell wall is an outer layer surrounding certain cells that
is outside of the cell membrane.
 All cells have cell membranes, but generally only plants,
fungi, algae, most bacteria, and archaea have cells with
cell walls.
 The cell wall provides strength and structural support to
the cell, and can control to some extent what types and
concentrations of molecules enter and leave the cell.
 The materials that make up the cell wall differ
depending on the type of organism.

PROPERTIES
 Cell walls serve similar purposes in those organisms that
possess them.
 They may give cells rigidity and strength, offering
protection against mechanical stress.
 In multicellular organisms, they permit the organism to
build and hold a definite shape (morphogenesis). Cell
walls also limit the entry of large molecules that may be
toxic to the cell.
 They further permit the creation of
stable osmotic environments by preventing osmotic
lysis and helping to retain water. Their composition,
properties, and form may change during the cell
cycle and depend on growth conditions.

RIGIDITY OF CELL WALL
 In most cells, the cell wall is flexible, meaning that it
will bend rather than holding a fixed shape, but has
considerable tensile strength.
 The apparent rigidity of primary plant tissues is
enabled by cell walls, but is not due to the walls'
stiffness.
 Hydraulic turgor pressure creates this rigidity, along
with the wall structure.

RIGIDITY OF CELL WALL
 In plants, a secondary cell wall is a thicker
additional layer of cellulose which increases wall
rigidity.
 Additional layers may be formed by lignin
in xylem cell walls, or 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 .

PERMEABILITY
 The primary cell wall of most plant cells is
freely permeable to small molecules including
small proteins, with size exclusion estimated to
be 30-60 kDa.
 The pH is an important factor governing the
transport of molecules through cell walls

Cell Wall is present in :
Fungi,
Algae,
Most bacteria, and
Archaea,
Plants, etc.

FUNGAL CELL WALL
In Fungi, the mycelial
wall is composed of :
oChitin : occur in
bundles like cellulose
molecule.
oA polymer of
acetylglucosamine
units.

ARCHAEA CELL WALL
Like other living organisms,
archaea have a semi-rigid cell
wall that protects them from the
environment.
oThe cell wall of archaea is
composed of S-layers and
olack peptidoglycan molecules
with the exception of
methanobacteria who have
pseudopeptidoglycan in their
cell wall.

BACTERIAL CELL WALL
oWall of the bacteria
consists of
peptidoglycan which
contains polymers of
modified sugars (N-
acetyl glucosame and
N- scetyl muramic
acid) cross linked by
short peptides.

PLANT CELL WALL
 Plant cells are eukaryotic cells that differ in several
key aspects from the cells of other eukaryotic
organisms. Their distinctive features include:
 A large central vacuole, a water-filled volume
enclosed by a membrane known as
the tonoplast that maintains the cell's turgor,
controls movement of molecules between
the cytosol and sap, stores useful material and
digests waste proteins and organelles.

PLANT CELL WALL STRUCTURE
 The cell wall is the rigid, semi-permeable protective layer
in some cell types.
 This outer covering is positioned next to the cell
membrane (plasma membrane) in most Plant
cell, bacteria, algae.
 Animal cells , do not have a cell wall.
 The cell wall conducts many important functions in a cell
including protection, structure, and support.
 Cell wall composition varies depending on the organism.
In plants, the cell wall is composed mainly of strong
fibers of the carbohydrate polymer cellulose.
 Cellulose is the major component of cotton fiber and
wood and is used in paper production.

STRUCTURE

The plant cell wall is multi-layered and
consists of up to three sections. From the
outermost layer of the cell wall, these layers
are identified as the middle lamella, primary
cell wall, and secondary cell wall. While all
plant cells have a middle lamella and
primary cell wall, not all have a secondary
cell wall.
STRUCTURE

Middle lamella - outer cell wall layer that contains
polysaccharides called pectins. Pectins aid in cell
adhesion by helping the cell walls of adjacent cells to
bind to one another.​
Primary cell wall - layer formed between the middle
lamella and plasma membrane in growing plant cells. It
is primarily composed of cellulose microfibrils contained
within a gel-like matrix of hemicellulose fibers and pectin
polysaccharides. The primary cell wall provides the
strength and flexibility needed to allow for cell growth.​
STRUCTURE

Secondary cell wall - layer formed between
the primary cell wall and plasma membrane in
some plant cells. Once the primary cell wall has
stopped dividing and growing, it may thicken to
form a secondary cell wall. This rigid layer
strengthens and supports the cell. In addition to
cellulose and hemicellulose, some secondary cell
walls contain lignin. Lignin strengthens the cell
wall and aids in water conductivity in plant
vascular tissue cells.
STRUCTURE

PLANT CELL WALL FUNCTIONS
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.
Additional functions of the cell wall include:
Support - the cell wall provides mechanical strength
and support. It also controls the direction of cell growth.​
Regulate growth - sends signals for the cell to enter
the cell cycle in order to divide and grow

Withstand turgor pressure - turgor pressure is the
force exerted against the cell wall as the contents of the
cell push the plasma membrane against the cell wall.
This pressure helps a plant to remain rigid and erect, but
can also cause a cell to rupture.​
Regulate diffusion - the cell wall is porous allowing
some substances, including proteins, to pass into the
cell while keeping other substances out.​
Storage - stores carbohydrates for use in plant growth,
especially in seeds.
PLANT CELL WALL FUNCTIONS

Communication - cells communicate with one
another via plasmodesmata (pores or channels
between plant cell walls that allow molecules
and communication signals to pass between
individual plant cells).​
Protection - provides a barrier to protect
against plant viruses and other pathogens. It
also helps to prevent water loss.​
PLANT CELL WALL FUNCTIONS