Cell wall | structure composition and Functions
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. The cell wall has evolved many different times among different groups of organisms.
The cell wall has a few different functions. It is flexible, but provides strength to the cell, which helps protect the cell against physical damage. It also gives the cell its shape and allows the organism to maintain a certain shape overall. The cell wall can also provide protection from pathogens such as bacteria that are trying to invade the cell. The structure of the cell wall allows many small molecules to pass through it, but not larger molecules that could harm the cell.
The main component of the plant cell wall is cellulose, a carbohydrate that forms long fibers and gives the cell wall its rigidity. Cellulose fibers group together to form bundles called microfibrils. Other important carbohydrates include hemicellulose, pectin, and liginin.
3. THE OBJECTIVE ARE:
DEFINATION OF CELL WALL
LAYERS OF CELL WALL
FORMATION OF CELL WALL
STRUCTURE OF CELL WALL
THICKENING OF CELL WALL
COMPOSITION OF CELL WALL
FORMATION OF INTERCELLULAR SPACES
4. DEFINATION OF CELL WALL:
THE OUTERMOST LAYER OF CELL IN PLANT,BACTERIA FUNGI AND MANY ALGAE THAT
GIVE SHAPE TO THE CELL AND PROTECTS IT FROM INFECTION.IN PLANT THE CELL IS
MADE UP OF MOSTLY CELLULOSE DETERMINS TISSUE TEXTURE AND OFTEN IS CRUCIAL
TO CELL FUNCTION.COMPARE CELL MEMBRANE REFRENCE.
5. LAYERS OF CELL WALL:
There are four layers of cell wall:
1. Middle lamella
2. Primary wall
3. Secondary wall
4. Tertiary wall
6. LAYERS OF CELL WALL:
Middle lamella:
It is the cement that holds the individual cells together and it is
found between the primary cell walls of neighboring cells. It is an amorphous
substance and isotropic.
Primary wall:
It is the first wall that develops in the new cell by protoplasm.The
wall is formed cellouse.It is the wall that develops in growing cells.It is thin and
elastic.
Secondary wall:
It is formed on the inner surface of the primary wall.It begins to
develop in cells or parts of them which have ceased to grow.Generally,three
layers of secondary wall can be observed,the outer layer,the central layer and
the inner layer.
7. FORMATION OF CELL WALL:
During cell division,at telophase,new cell wall formation takes place .The
phragmoplast,a fibrous structure formed by macrotubules,appears during mitosis
between the two daughter nuclei and within the cell plate which divides the
parent cell in two.
8. STRUCTURE OF CELL WALL:
PRIMARY STRUCTURE:
Cell wall has complex structure and can be divided into
different layers:
1. PRIMARY WALL
2. MIDDLE LAMELLA
3. SECONDARY WALL
9. ULTRA-STRUCTURE OF CELL WALL:
1. Structurally the cell wall is
considered to be a fine interwoven
network of cellulose strands
varying complexity and size.
2. The compound microscope show
that the cell wall is composed of
large fibrils.
3. The electron microscope reveals
that the fundamental units of the
micellar system is chain like
cellulose molecules of diffrent
lengths formed of about (3000-
6000 glucose molecules linked
together)
4. Approximately 100 cellulose chains
aggregate to from an elementary
fibril or micelle.
10. THICKENING OF CELL WALL:
The secondary thickening due to
lignification is uniform all around the cell but in tracheids and vessels the
thickening due to localized to specific areas.The secondary thickenings of lignin
may form various patterns.
These are:
ANNULAR
SPIRAL
SCALARIFORM
RETICULATE
PITTED
11. THICKENING OF CELL WALL:
ANNULAR:
(ring-like);Ring of lignin are deposited one above the other in the interior
of the original cell wall.
SPIRAL:
(helices-like);Thickening of lignin are deposited in the form of spiral bands.
SCALARIFORM:
(ladder-type);Lignin is deposited in the form of the steps of a ladder
along the inner side of the cell wall.
RETICULATE:
The lignin thickening is deposited in irregular manner and a network is
formed.
PITTED:
The entire inner surface of the cell wall is tkickened,leaving small
unthickenend areas called pits.
12. CHEMICAL COMPOSITION OF CELL WALL:
The cell wall is composed of the following chemical compounds:
CELLULOSE
HEMICELLULOSES
PECTIC SUBSTANCES
LIGNIN
CUTIN,SUBERIN,WAXES
GUMS AND MUCILAGES
13. COMPOSITION OF CELL WALL:
CELLULOSE:
It is a carbohydrate and is the most common compound present in the
cell walls.It is a hydrophilic,crystalline,compound polysaccharide.The molecules of
the cellulose are chain-like and are formed by glucose residues held together by
oxygen.
14. COMPOSITION OF CELL WALL:
HEMICELLULOSES:
It is a group of polysaccharides with different solubilities,such as
xylans,mannans,glucans,etc.
PECTIC SUBSTANCES:
The are closely related to the hemicellulose.They found in three
forms,protopectin,pectin and pectic acid and are polymers compound of uronic acid.The
pectin compound are amorphous and hydrophilic.They form middle lamella.
LIGNIN:
It is a polymer of high carbon content,distinct from carbohydrates.The lignin of
conifers and dicot from one another.
CUTIN,SUBERIN,WAXES:
These are fatty substance.Cutin and suberin are closely
related,polymerized compound consisting of fatty acids.suberin occurs in association with
cellulose in cork cells of periderm.
GUMS AND MUCILAGE:
These are related to pectin substances and swell up in water.Gums
appear in plant due to break-down of walland cell components.
15. FORMATION OF INTERCELLULAR SPACES:
The intercellular spaces are formed by two methods:
1.The cell wall is sparated from each other along areas of their contact.The
mechanism is thought to involve spilting of the middle lamella.Such intercellular
spaces are called Schizogenous intercellular spaces.
2.Sometimes the intercellular space is formed by the dissolution of entire
cell.Such intercellular spaces are formed as Lysigenous intercellular spaces.
FOR EXAMPLE:
Large air spaces are present in aquatic plant and in some monocot
roots,and secretory cavities found in citrus.
16. PLASMODESMATA:
Cytoplasmic strand-like structures exending
from protoplasts into cell wall and interconnecting the living protoplasts of the
plant body are called Plasmodesmata.These are found in red
algae,mosses,gymnosperms and angiosperms.
PITS:
Secondary walls are commonly characterized by presence of depressions or
cavities called pits .such depressions are also present on the primary walls and
these differ from the pits present in the –secondary walls in structure and
development these depressions are known as primary pit-fields the primary wall
is thin and continuous across the primary pit-field area .while in pits ,the
secondary wall is not deposited in the pit region.
17. TYPES OF PITS:
The pits are of two types:
1.Simple pits
2.Bordered pits
3.Simple pit-pair
4.Bordered pit-pair
5.Half bordered pit-pair
6.Blind pit
7.Unilateral compound pit
8.Torus
18. TYPES OF PITS:
SIMPLE PITS:
The secondary walls are deposited in such a way that it does not overarching
the pit cavity.Such pits are termed as simple pits.
BORDERED PITS:
The secondary walls develops over the pit cavity to form an overarching
hood with a harrow pore in the centre.
SIMPLE PIT-PAIR:
If the two pit-pairs are simple,they are knows as simple pit-pairs.
BORDERED PIT-PAIRS:
If the two pit-pairs are bordered,they are called bordered pit-pairs.
HALF BORERDED PIT-PAIRS:
If one of the pits is simple and other is bordered,it is knows as
half bordered pit-pairs.
19. TYPES OF PITS:
BLIND PITS:
If the pit is without its opposite partner it an intercellular space
is present,it called blind pit.
UNILATERAL COMPOUND PIT:
Sometime,two or more pits are found opposite
one large pit,such an arrangement is called unilateral pitting.
TORUS:
In some plants,pit membrane of a bordered pit-pair is thickened in
the centre in the form of a disc.This thickening is called torus.
