Cell wall

Structural Organization

1. In Prokaryotes (Bacteria)

• Peptidoglycan (Murein) Layer
  • Composed of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked by β-1,4 glycosidic bonds.
  • Cross-linked with short peptide chains.
    
• Gram-positive bacteria
  • Thick peptidoglycan layer (20–80 nm).
  • Contain teichoic acids for rigidity and ion regulation.
    
• Gram-negative bacteria
  • Thin peptidoglycan layer (2–7 nm).
  • Surrounded by outer membrane containing lipopolysaccharides (LPS), porins, and proteins.
    
• Archaea
  • Lack peptidoglycan; instead may have pseudopeptidoglycan (pseudomurein), proteins, or polysaccharides.
    

2. In Fungi

• Primarily composed of chitin (β-1,4 N-acetylglucosamine), β-glucans, and glycoproteins.
• Provides rigidity, protection against environmental stress, and fungal pathogenicity.
  

3. In Algae

• Composed of cellulose, glycoproteins, pectins, and in some groups, silica (diatoms) or calcium carbonate.
• Highly diverse depending on species.

In Higher Plants

PRIMARY STRUCTURE

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

(i) 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.

Functions of Cell Wall

1. Mechanical Support & Shape
   • Provides rigidity and maintains cell shape.
     
2. Protection
   • Prevents osmotic lysis in hypotonic environments.
   • Acts as a barrier against pathogens and toxins.
     
3. Regulation of Growth
   • Controls cell expansion by loosening cellulose-hemicellulose network (acid growth hypothesis).
     
4. Transport & Communication
   • Contains plasmodesmata in plants → cytoplasmic connections between adjacent cells.
     
5. Storage
   • Some walls store carbohydrates (hemicellulose, pectins) which may be metabolized.
     
6. Specialized Functions
   • In xylem → lignified secondary walls enable water conduction.
   • In fungi/bacteria → essential for pathogenicity and interaction with hosts.

In algae/diatoms → silica/calcium carbonate deposition helps in protection and buoyancy.