CELL WALL Structure

PRIMARY STRUCTURE

Cell wall is complex in nature .It remains differentiated in the following layers:

(1) Middle Lamella

(ii)Primary cell wall

(iii)Secondary cell wall

(iv)Tertiary cell wall

(i) Middle Lamella-

The cells of plant tissue generally remain cemented together by an intercellular matrix known as middle lamella.
It is composed of pectin, lignin & some other proteins.
They are dissolved in strong acids.

(ii) Primary Cell Wall-

It first formed a cell wall. Its thickness is about 100-200nm.It is the outermost layer of the cell & it forms the only cell wall in the immature meristematic & parenchymatous cells.
It is comparatively thin & permeable.
It is composed of a loosely organized network of cellulose microfibrils associated with hemicellulose pectins & glycoproteins.
Pectins are important in imparting flexibility & makes the primary cell wall to expand during cell growth.
The cellulose synthesizing enzymes that synthesize cellulose microfibrils are localized within the plasma membrane.
These are called rosettes; they add glucose molecules to growing microfibrils.
It is composed of polysaccharide cellulose but other substances may be incorporated in it.
Lignin or suberin may also present in it & epidermal cells of leaves & stem also possess cutin & cuter waxes & are impermeable to water.
Primary cell wall of yeast and fungi is composed of chitin.

(iii) Secondary Cell Wall-

It is made by the deposition of other substances such as lignin on plasma membranes.
The cellulose fibers are arranged parallel in this wall and embed in hemicellulose and lignin.
It is comparatively thicker than the primary cell wall.
It is generally present in mature permanent or non growing cells
It is composed of cellulose, Pectins non cellulose polysaccharides, Lignin and a Phenolic polymer which give hardness and mechanical rigidity to the wood.
In mature cells it appears to be a 9 -10µm thick layer.
It is divided into three layers
A- Outer layer – S1
B- Middle layer – S2
C- Inner Layer – S3

The outer S1 layer is attached with a primary cell wall.
The middle S2 layer comparatively thick
The inner layer S3 is very thin and delicate.
The secondary cell wall gives extra protection and strength to the cell.

(iv) Tertiary Cell Wall-

It is found beneath the secondary cell wall.
It differs from primary and secondary cell wall, and contains xylans.
Generally they are present in tracheids of gymnosperms particularly in coniferous woods.

CHEMICAL NATURE

Plant’s cell walls are composed of carbohydrates known as cellulose.
Besides cellulose various chemical substances such as hemicelluloses, pectin, lignin, cutin & chitin.
The cell wall also contains certain minerals like calcium & magnesium in the form of carbonates & silicates.
The cellulose is a polysaccharide & it is the most abundantly occurring chemical substance of most plant cells.
Chemically cellulose contains long chains of glucose molecules.
Glucose is the structural unit of cellulose & about 3000 glucose molecules are linked together to form a long chain of cellulose molecules.
A bundle of 100 chain molecules of cellulose forces the elementary fibril known as micelle.
The 20 micelle when arranged parallel form the fibrils of 25 nm thick known as microfibrils (5-15 nm in diameter).
These microfibrils form large sized bundles 0.5micrometre thick of cellulose fiber to form the microfibrils.
The microfibrils consist of many cellulose fibrils.
The hemicellulose is composed of monosaccharide units such as arabinose, xylose, and mannose galactose.
It occurs sometimes in between the microfibrils of cellulose.

HEMICELLULOSE

They are heterogeneous groups of carbohydrates polymers constructed from various 5 & 6 carbons sugars including xylose, arabinose, mannose & galactose.
The hemicellulose xylan which utilizes the pentose xylose as its main building block, accounts as much as 50% of the cell wall in woody tissues.
Hemicellulose molecules bind to the cellulose microfibrils & to each other creating a coating that helps to bond these microfibrils together into a rigid interconnected network of cellulose & hemicellulose.

PECTINS

These are the polymers of carbohydrates D-galacturonic acid & D-glucuronic acid units .
Like the glycosoaminoglycans of animal cells, they readily form hydrated gels.
This property of pectin molecules is responsible for gelatin that occurs during the process of making jams and jellies .
Pectin molecules are involved in binding adjacent cell wall together & in forming the matrix in which cellulose microfibrils are embedded.

LIGNINS

They are group of polymerized aromatic phenols. They occurs mainly in woody tissues, as cross linked network that contributes to the hardening of the cell wall. Lignin molecules are localized between the cellulose fibrils, where they function to rest compression forces . (FIG II)

GLYCOPROTEINS

They are important constituent of plant cell wall, accounting as much as 10% of the total mass.
Among the wall glycoprotein a group of common glycoprotein are called extensins.
Extensins & other glycoprotein forms cross linked networks with each other as well as with cellulose microfibrils

OTHER COMPONENT

In addition to the preceding components a small percent of the total mass of the cell wall is accounted for the lipids, including waxes & other complex polymers .
Minerals such as Ca & K also occurs in the plant cell wall in the form of inorganic salts.

MCQ’s

1. Structure and Composition of the Cell Wall

The plant cell wall is mainly composed of which polysaccharide? (NEET 2017)
a) Chitin
b) Cellulose ✅
c) Starch
d) Glycogen
Which layer of the plant cell wall cements adjacent cells together? (NEET 2020)
a) Primary cell wall
b) Secondary cell wall
c) Middle lamella ✅
d) Plasma membrane
The middle lamella is mainly composed of: (AIIMS 2016)
a) Cellulose
b) Pectin ✅
c) Lignin
d) Chitin
The first layer to be formed during cytokinesis in plant cells is: (NEET 2019)
a) Primary cell wall ✅
b) Middle lamella
c) Secondary cell wall
d) Tertiary cell wall
Which layer provides mechanical strength to plant cells? (NEET 2018)
a) Middle lamella
b) Primary cell wall
c) Secondary cell wall ✅
d) Tertiary cell wall

2. Primary and Secondary Cell Wall

The primary cell wall is mainly composed of: (NEET 2016)
a) Chitin
b) Cellulose and pectin ✅
c) Starch
d) Glycoproteins
Which of the following is absent in the primary cell wall? (AIIMS 2017)
a) Cellulose
b) Hemicellulose
c) Lignin ✅
d) Pectin
The secondary cell wall is deposited: (NEET 2021)
a) Outside the plasma membrane
b) Outside the primary cell wall
c) Beneath the primary cell wall ✅
d) Between the cytoplasm and nucleus
Which of the following layers is the thickest in the secondary cell wall? (NEET 2022)
a) S1
b) S2 ✅
c) S3
d) Pectin layer
The main function of the secondary cell wall is to provide: (NEET 2019)
a) Water absorption
b) Rigidity and mechanical strength ✅
c) Photosynthesis
d) Storage of nutrients

3. Tertiary Cell Wall and Special Components

The tertiary cell wall is mainly found in: (NEET 2015)
a) Parenchyma
b) Gymnosperm tracheids ✅
c) Xylem vessels
d) Epidermal cells
Which compound is unique to the tertiary cell wall? (NEET 2017)
a) Xylans ✅
b) Lignin
c) Chitin
d) Pectin

4. Chemical Nature of the Cell Wall

The major component of the plant cell wall is: (NEET 2016)
a) Protein
b) Lipid
c) Cellulose ✅
d) DNA
Which of the following is a polymer of glucose found in the cell wall? (NEET 2018)
a) Chitin
b) Cellulose ✅
c) Hemicellulose
d) Lignin
Lignin provides: (NEET 2020)
a) Flexibility
b) Rigidity and hardness ✅
c) Elasticity
d) Protein storage

5. Hemicellulose, Pectins, and Lignins

Hemicellulose consists of: (NEET 2017)
a) Monosaccharides ✅
b) Fatty acids
c) Lipids
d) Amino acids
Which sugar is a major component of hemicellulose? (NEET 2018)
a) Xylose ✅
b) Glucose
c) Fructose
d) Ribose
Pectin helps in: (NEET 2016)
a) Cell rigidity
b) Cell adhesion ✅
c) Photosynthesis
d) DNA replication
Pectin is responsible for the formation of: (NEET 2019)
a) Lignin network
b) Gelatin in jams and jellies ✅
c) Cutin layer
d) Suberin deposits

6. Microfibrils and Structural Organization

The smallest unit of cellulose organization is: (NEET 2021)
a) Micelle ✅
b) Microfibril
c) Fibril
d) Cellulose fiber
How many glucose molecules form a single cellulose polymer? (NEET 2017)
a) 100
b) 1000
c) 3000 ✅
d) 5000

7. Cell Wall Modifications

The waterproof substance found in the plant cell wall is: (NEET 2018)
a) Lignin
b) Cutin ✅
c) Pectin
d) Cellulose
Cutin is most abundant in: (AIIMS 2015)
a) Xylem
b) Epidermal cells ✅
c) Parenchyma
d) Phloem

8. Additional Components of the Cell Wall

Glycoproteins in the plant cell wall are called: (NEET 2016)
a) Extensins ✅
b) Plasmodesmata
c) Suberin
d) Xylans
Extensins help in: (NEET 2022)
a) Nutrient transport
b) Strengthening the cell wall ✅
c) Protein synthesis
d) DNA replication
Which mineral is found in plant cell walls? (NEET 2019)
a) Sodium
b) Calcium ✅
c) Iron
d) Zinc

9. Miscellaneous

Which plant cell wall component provides resistance against microbial attack? (NEET 2020)
a) Cellulose
b) Lignin ✅
c) Cutin
d) Pectin
Cellulose microfibrils are embedded in which matrix? (NEET 2018)
a) Pectin and hemicellulose ✅
b) Lignin and chitin
c) Starch and glycoproteins
d) Protein and lipids
Xylans are mainly found in: (NEET 2015)
a) Phloem
b) Gymnosperms tracheids ✅
c) Epidermal cells
d) Parenchyma