Cell Wall



Why is the cell important in plant?

  CELL WALL:

The cells of bacteria, fungi, and plants have a thick but porous coat, the cell wall,  outside the plasma membrane. The composition of ce wall varies in different groups.

Plant Cell Wall :

Presence of cell wall distinguishes plant cells from animal cells. 
 Cell wall, A Distinct Organelle:The cell wall was first observed by Robert Hooke in 1665  in a thin slice of cork.Certain cells, such as gametes and zoospores, lack a cell wall. That the cell membrane is distinct from the cell wall can be experimentally.Place a group of cells in a concentrated solution of sodium chloride. The cell contents along with the cell membrane will shrink and gather at the centre, while the cell wall will remain intact ( plasmolysis).

Thickness:The cell wall varies in thickness from 0.1ųm to 10 ųm in different cells.

Source:The cell wall is secreted by the cytoplasm. 


1.Chemical Composition:

In plants, the cell wall is chiefly composed of the insoluble polysaccharide, cellulose.Certain other compounds such as hemicellulose, pectin and proteins also occur in the cell wall.

 ●Cellulose:A Cellulose molecule is a long, unbranched chain of glucose molecules. There are about 6,000 glucose units in each chain. About 30 to 60 cellulose molecules form a bundle called microfibril, about 5__ 10nm thick.The Cellulose molecules in a microfiber are oriented parallel to one another and held together by hydrogen bonds. The walls of many plant cells are composed of layers in which the microfibrils are oriented parallel to one another and held together by hydrogen bonds.The walls of many plant cells are composed of layers in which the microfibrils of one layer are oriented at approximately 90° to those of adjacent layers. The microfibrils have a high tensile strength which approaches that of steel .They occur as a Framework embedded in a ground substance or matrix.




● Metrix: The matrix is an amorphous ¹, gel_ like material. It is composed chiefly of complex polysaccharides named hemicellulose ( 5__ 15%) and pectins (2_8%), but also contains a good deal of water material between the cell and its environment. Thus, the cell wall lacks selective permeability, which is the function of the cell membrane as in animal cells.

● Hemicelluloses:Hemicelluloses are branched polysaccharides having a backbone composed of one sugar, such as glucose, and side chains of other sugars, such as xylose. 

 ●pectins:Pectins are branched or unbranched polysaccharides rich in galacturonic acid. Pectin holds water and forms a gel that fills the spaces between the fibrous elements. When a plant is attacked by pathogens, fragments of pectins released from the wall trigger a defensive response by the plant cell. Purified pectin is used to give gel_ like consistency to grams and jellies. 

●Proteins:Proteins interweave through Cellulose and pectin networks. One class, the  extensions,provide structural support and may form a barrier to invading microbes. A new type of protein, the expansion, is known to loosen the cell wall for cell expansion by the addition of cellulose molecules to the cellulose microfibrils in mature cells that otherwise cannot grow.
    
       The percentage of the cell wall components highly varies, depending upon the type of plant, the type of cell,  and stage of the cell.
Water Contents:The cell wall is hydrated and 60 to 70% of its mass usually comprises water.

2. Physical Structure 

The cell wall consists of two regions: primary wall and secondary wall. In some plants, the cell adds a tertiary wall also.

Types of the cell Wall:

 (i) Primary Wall: 

A young plant cell forms a single layer of wall material. This layer is known as the primary cell wall. It is about 1___ 3 ųm thick. The primary wall is thin, elastic and capable of expansion in a growing cell.It grows by addition of more wall material within the existing one. Such a growth is termed intussusception¹.Meristematic and parenchymatous cells have primary ce wall only. The cells of leaves and fruits too have only primary wall.


 (ii) Secondary Wall:

In mature cells, more layers of wall material are added internal to the primary wall.These layers are called the secondary cell wall. Growth by addition of new wall material on the existing one is called accretion². The secondary wall is 5___ 10ųm  thick and rigid. It usually consists of three layers, which are often named S1, S2, and S3, but there may be more layers as in the latex tube of Euphorbia milli.The Cellulose microfibrils of secondary wall are compactly arranged with different orientation in different layers. Otherwise the structure of the secondary wall is similar to that of the primary wall. The matrix contains lignin and Suberin. Addition of secondary wall brings about thickening of the cell wall
 Thickening of cell wall occurs particularly in cells that form the harder woody parts of plants. 



      The formation of secondary wall is not uniform in all the cells.This results in the formation of various tissues such as parenchyma, sclerenchyma, collenchyma, fibres, vessels, and trachieds.

●Pits:  At certain places, secondary wall is not laid down. Such areas are called simple pits. The pits in the walls of adjacent cells are often opposite to each other. These pits are separated by a  pit membrane composed of middle lamella and primary walls. The pit membrane is permeable and allows passage of materials. In the tracheids of gymnosperms, the secondary wall partly overhangs the Pits.Such pits are known as the bordered pits.



(iii) Tertiary Wall:

Tracheids of gymnosperms form another layer of wall material internal to the secondary wall.It is called tertiary cell wall .It is composed of cellulose and xylan, another polysaccharide.



1. L.intus= within, sus__ cipio= to take up.
2. L. accretio= to grow.

Flexibility. 

The plant cell wall is rigid enough to support  the plant body, and flexible enough to permit the plant to bend in the strong wind instead  of breaking. 

Metabolic Activity. 

The cell wall was earlier considered to be nonliving. Now it is not regarded as entirely nonliving because it is capable of growth, is metabolically active and a cell cannot live without its wall.

● Middle Lamella.

Adjacent cells in a plant tissue are held together by a thin, sticky, amorphous layer of cementing material. This layer is called the  middle lamella.It is lacking on the outer ( exposed) side of the surface cells. Middle lamella is chiefly made up of calcium and magnesium lactate.It is laid down by the joint activity of the cells between which it lies. It is formed at the time of cytokinesis as a layer of pectin. In old tissues, soft pectin may become hardened by deposited of calcium to form calcium pectate.In ripening fruits, the.pectate compounds of the middle lamella solubilize to a jelly__ like material, making the fruits soft.



3. Depositions:

Certain additional materials may be deposited in the cell for specified purposes. The common materials are mentioned below__
(i) Lignin: It is a tough, glue_ like substance composed of aldehydes and coniferl alcohol units.It is impregnated in the primary and secondary walls of the cells of woody plants.In some cells, such as protoxylem, the lignin is laid down in annular,  spiral or reticulate patterns. In others, it form a complete layer except at the Pits. Lignin cements and anchors the Cellulose fibres together to form a solid material for rigidity and strength. Lignin along with Cellulose makes wood fit for use as a construction material. Addition of Lignin is called lignification.The latter evolved with the evolution of land plants.Water and dissolved materials can pass through a lignified wall.

(ii) Cutin:  It is a fat _ life substance deposited on the exposed surface of the cell as a distinct layer called  cuticle in addition to cell wall. It makes the exposed surface of the cell wall relatively impermeable to water and checks evaporation. Its addition is called cutinisation.

(iii) Suberin:  It is a fat _ material deposited in the secondary wall of the cork cells. It makes the cell walls impermeable to water and checks the loss of water by evaporation. Its addition is called suberisation.

(iv) Silica: It is deposited in the cell walls of grasses.
Some other substances, such as minerals, waxes, tanins, resins, gums, etc., may also be deposited in the cell wall for specific purposes. 

● Wilting:The plant cells must be well filled with water so that they exert an outward pressure on their walls. If the plant loses too much water, the cells do not fill the space enclosed by their walls fully, and the and wilts.

4.  Intercellular Connections. 

At places, the cell wall remains thin as secondary wall is not formed here. Such areas have fine,  simple or branched canals in the primary cell wall and middle lamella. Through these canals, plasma membrane, cytoplasm and endoplasmic reticulum of adjacent cells are continuous. The canals are called plasmodesmata (singular plasmodesma) and the ER tubules extending through them are termed desmotubles.This arrangement unified  most of the plant into one living continuum. Such a continuous cytoplasm is called symplasm.Water and small solutes, and even some protein and RNA molecules, may pass from cell to cell through the plasmodesmata.
Intercellular spaces may contain some nonliving matter, termed apoplasm.

Fungal Cell Wall

In fungi, the mycelial wall is generally composed of chitin, a polymer of acetylgucosamine units. Chitin molecules occur in bundles like cellulose molecules. 

Prokaryotic Cell Wall

Wall of the prokaryotic cells consists of peptidoglycan which contains polymers of modified sugars ( N _ acetyl glucosamine and N_ acetyl muramic acid) across_ linked by short peptides. Peptidoglycan is also called murein, or mucopeptid. It may also contain teichoic acid. Penicillin and related drugs interfere with the building of these walls and therefore inhibit the growth of bacteria. 



● Rigidity of plant Body: Each plant cell is firmly cemented to its neighbours so that the cells and the organs of plants cannot move much with respect to one another. Thick and tough cell walls provide rigidity and structural support to the plant cells and to the entire plant body.

Functions of Cell Wall

The cell wall serves many functions __ 
(i) It maintains shape of the cells.

(ii) It protects the cells from mechanical injury. 

(iii) It wards off the attacks of pathogens ( viruses, bacteria, fungi, protozoans).

(iv) It provides mechanical support against gravity. It is due to the rigid cell walls that the aerial parts of the plants are able to keep erect and expose their leaves to sunlight.

(v) The cell wall prevents undue expansion of the cell when water enters by osmosis to compensate for the lack of contractile Vacuole. This prevents bursting of cells. 

(vi) It allows the material to pass in and out of the cell.

(vii) Though permeable, the cell wall plays some regulatory role on the passage of materials into and out of the cell.

(viii) Many enzymic activities associated with metabolism are known to occur in the cell wall.
(ix) Cutin and Suberin deposits check loss of water from the cell surface by evaporation. 

(x) The cell wall helps in the maintenance of balance of intercellular osmotic pressure with that of its surroundings. 

(xi)  Pores in the cell walls permits plasmodesmata to link up all the protoplasts into a system called  Symplast  (symplasm).

(xii) The walls of xylem vessels, tracheids and sieve tubes allow movement of materials. 

(xiii) The wall in some cases has a role in defence and offence by means of spines.

(xiv) Growth of the cell wall enables the cells to enlarge in size.

Difference between Primary and secondary cell wall

Primary Cell Wall 

1. It is formed in a growing cell.

2. It lies internal to the middle lamella. 

3. It is present in all plant cells.

4. It is elastic and capable of expansion in a growing cell.

5. It grows in thickness by intussusception. 

6. It consists of a single layer of wall material. 

7. It is 1__ 3 ųm thick.

8. Its cellulose microfibrils are short, wavy loosely arranged. 

9. It lacks Pits.

10. It lacks additional materials. 

11. Its water content is about 60%. 

12. It has relatively low Cellulose content. 

13. It has relatively high hemicellulose, protein and lipid contents. 

Secondary Cell wall

1. It is formed in a mature cell.

2. It lies internal to the primary wall. 

3. It is present in certain cells  only.

4. It is rigid and incapable of expansion. 

5. It grows in the thickness by accretion. 

6. It consists of 3 or more layers of wall material. 

7. It is 5__ 10 ųm thick.

8. Its cellulose macrofibrils are long, straight,  compacity arranged. 

9. It has pits at certain places.

10. It had additional material such as lignin, suberin.

11. Its water content is about 30_ 40%.

12. It has relatively high cellulose content. 

13. It has relatively low hemicellulose, protein and lipid contents.















































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