The Cell Wall - Practice Questions & MCQ

1. Structure of the cell wall
Cell wall is the outermost non-living covering in fungal and plant cells. 
Cell wall of algae is made of cellulose, galactans, mannans and minerals like calcium carbonate, while in other plants it consists of cellulose, hemicellulose, pectins and proteins. 
Fungal cell wall is made up of chitin (acetylglucosamine polymers), glucans, polysaccharides and mucopolysaccharides, waxes, and pigments.

A typical cell wall is made up of:
a) Matrix: It is a gel-like ground substance consisting of 30 - 60% water, Hemicellulose, 2 - 8% pectin, 0.5 - 3 % lipids, 1 - 2 % proteins. 

b) Microfibrils: It is approximately 20-40%. It is a structural element of the cell wall, also called as the steel bars of the cell wall.  In plants, microfibrils are composed of cellulose. In fungi, it is composed of fungal cellulose, which is equivalent to chitin.

c) Depositions on the cell wall:  

1. Lignin: 
- associated with sclerenchyma cells, tracheids, and vessels. 
- it is a non-polysaccharide compound 
- provides hardness, and stiffness to the cell wall 

2. Suberin: 
- it is a hydrophobic substance 
- prevents water loss from the organ 
- examples: cork cells, endodermis (casparian strips) 

3. Cutin: 
- it is a waxy substance 
- hydrophobic in nature 
- present on epidermal cells 
- reduce water loss from the leaves 

4. Silica: 
- It is associated with grasses, Equisetum (pteridophytes) 
- provides protection, hardness/stiffness 
- protects the grass from grazing animals 

5. Calcium, Iron deposition 
Example: Chara 

2. Components of the cell wall
A cell wall is made up of the following components:

a) Primary Cell Wall: It is thin and elastic in young cells and diminished gradually in mature cells. It contains more hemicellulose as compared to cellulose. It is prominent in meristematic cells and parenchymatous cells.

b) Secondary Cell Wall: It contains more cellulose. It is rigid, non-elastic and contains pit at the regular intervals. It is found in collenchyma, sclerenchyma and xylem vessels. It can have many layers as S1, S2, S3 etc.

c) Tertiary Cell Wall: It is completely made up of cellulose and is found in xylem vessels and tracheids of gymnosperms.

d) Middle Lamella: It is the connecting layer between two cells. It is made up of calcium and magnesium pectate. It ruptures to form intercellular spaces. 

3. Functions of the Cell Wall:
i) Cell wall gives shape to the cell 
ii) It protects the cell from mechanical damage and infection
iii) it also helps in cell-to-cell interaction 
iv) it provides a barrier to undesirable macromolecules.