Cell Wall – Structure, Composition, Functions & Types (Biology Class 11 Notes)

 

3.4 – Structure of Cell (Cell Wall)

Origin of Cell Wall

• First described by Robert Hooke (1665) while studying cork.
• • Biogenesis:
• Cell wall material originates from Golgi bodies (dictyosomes) → secretory vesicles carry cellulose, hemicellulose, pectin precursors.
• Vesicles fuse at cell plate during cytokinesis → form middle lamella.
• Primary wall develops first → later secondary wall deposited (if needed).

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Composition of Plant Cell Wall

1. Primary Cell Wall

• Found in young, growing cells; thin & flexible.
• Molecules:

• Cellulose (structural polysaccharide of β-1,4 glucose units; forms strong chains)

1. Chains assemble into microfibrils.

2. Each microfibril has ~36 cellulose chains.
3. Provides tensile strength.

• Hemicellulose (branched polysaccharide, e.g., xylans, mannans, glucans)

1. Cross-links cellulose fibrils.

2. Acts as filler and stabilizer.

• Pectin (heteropolysaccharide rich in galacturonic acid)

1. Forms gel-like matrix.

2. Provides flexibility, porosity, water retention.

• Proteins

1. Extensins (glycoproteins; strengthen wall)

2. Expansins (enzymes; loosen wall for growth)

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2. Middle Lamella

• First formed layer at cell plate.
• Cementing material between adjacent cells.
• Composition: Calcium & magnesium pectates (salts of pectic acid).

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3. Secondary Cell Wall

• Formed inside primary wall in mature cells (fibers, tracheids, sclerenchyma).
• Thicker & rigid due to lignin.

• Molecules:

1. Cellulose (strength)
2. Hemicellulose (cross-linking)
3. Lignin (complex phenolic polymer; waterproof, rigid)
4. Cutin/Suberin (lipid polymers; prevent water loss)
5. Silica (SiO₂; hardness, e.g., grasses)

Layers of Secondary Wall

• S1 (outer): Microfibrils oblique.
• S2 (middle, thickest): Microfibrils nearly parallel → main strength layer.
• S3 (inner): Microfibrils transverse.

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Pits in Secondary Walls

• Definition: Unthickened areas of secondary wall for transport.

Types:

• Simple pits → no border (common in parenchyma).
• Bordered pits → secondary wall arches over cavity (in xylem, tracheids).
• Special case: Torus (thickened central part of pit membrane in gymnosperms).

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Plasmodesmata

• Discovered by Eduard Tangl (1879).
• Cytoplasmic channels connecting adjacent cells.
• Formed when strands of ER get trapped during cell wall formation.
• Function: Transport & communication (symplastic movement).

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Cell Walls in Other Organisms

• Algae: Cellulose + glycoproteins.
• Fungi: Chitin (N-acetylglucosamine polymer; also in insect exoskeleton).
• Bacteria: Peptidoglycan (polymer of sugars + amino acids).
• Archaea: Polysaccharides & proteins, no peptidoglycan (sometimes pseudopeptidoglycan).
• Diatoms (algae group): Silica cell wall (frustule; lightweight, energy-saving).

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Functions of Cell Wall

• Provides strength & rigidity.
• Maintains shape and prevents bursting in hypotonic conditions.
• Directs cell growth pattern.
• Protects against mechanical damage & pathogens.
• Stores polysaccharides (reserve food).
• Enables cell-to-cell communication (plasmodesmata).

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Exam-Oriented Key Facts

• Term “cell wall” → Robert Hooke, 1665.
• Plasmodesmata discovered by Eduard Tangl, 1879.
• Microfibrils → ~36 cellulose chains.
• Primary wall → cellulose, hemicellulose, pectin.
• Middle lamella → Ca & Mg pectates.
• Secondary wall → cellulose, hemicellulose, lignin.
• S2 layer → thickest, strongest.
• Pits → simple (parenchyma), bordered (xylem).
• Fungal wall → chitin.
• Bacterial wall → peptidoglycan.
• Archaeal wall → polysaccharides/proteins, no peptidoglycan.
• Diatoms → silica.

✅ Relevant Links from PreachBio.com

1. Plasma Membrane – Essential Components, Structure, and Functions
   → https://www.preachbio.com/2024/01/plasma-membrane-essential-components.html PreachBio

2. Understanding Bacillariophyta (Diatoms – silica cell walls)
   → https://www.preachbio.com/2024/03/understanding-bacillariophyta.html PreachBio

3. Structural and Functional Components of Bacterial Cells
   → https://www.preachbio.com/2023/11/Structural-functional-components-bacterial-cells-comprehensive-overview.html PreachBio

Plasmodesmata have caused debate among scientists regarding cell theory. Some scientists suggest that the cells of higher plants are not really cells since they are not physically separated or independent from one another.

The group of algae called diatoms synthesize their cell walls from silicic acid. The acid is polymerized inside cells, then the wall is extruded to protect the cell. The synthesis of Silica cell walls requires less energy. That is why there are higher growth rates in diatoms.