Cell Wall: Structure, Composition, and Functions

The cell wall is a rigid and protective outer covering commonly found in both prokaryotic and eukaryotic organisms such as plants, fungi, algae, bacteria, and archaea, but is absent in all animal cells. It is a non-living structure that varies in color and composition depending on the type of organism, and is freely permeable to natural substances. The cell wall performs many important purposes in a cell, including protection, structure, and support.

The cell wall was discovered by Robert Hooke in 1665, while its name came from the Latin word ‘cella’, meaning a ‘small room’.

Plant Cell Wall

It is multilayered and consists mainly of three distinct components. If a plant cell looks like a water balloon, the cell wall is analogous to a cardboard box that protects the balloon from damage. When viewed from outside, they are arranged as middle lamella, primary cell wall, and secondary cell wall. Although all plant cells have a middle lamella and primary cell wall, they may not have a secondary cell wall.

1. Middle Lamella: The outermost layer of the cell wall, made of polysaccharides called pectins that help adjacent cells to bind with one another.
2. Primary Cell Wall: Located between the middle lamella and plasma membrane in growing plant cells. It is primarily composed of cellulose microfibrils contained within a gel-like matrix of hemicellulose fibers and pectin polysaccharides. The primary cell wall provides the strength and flexibility to the cell. A minute pore called plasmodesmata creates channels that cross the cell walls and allow direct communication between adjacent cell cytoplasms, thus allowing cell-cell communication.
3. Secondary Cell Wall: Develop once the primary cell wall has stopped dividing and growing, it may thicken to form a secondary cell wall that provides additional strength and protection to the cell. The secondary cell wall is formed between the primary cell wall and plasma membrane in some plant cells. In addition to cellulose and hemicellulose, some secondary cell walls contain lignin that increases its hardness and also aids in water conductivity in plant tissues

Fungal Cell Walls       

In general, fungi with cell wall have three layers that together increase the rigidity and strength of the cell wall:

1. Chitin: The innermost layer of a fungal cell wall, it is fibrous in nature, and made up of polysaccharides.
2. Glucans: Found next to the chitin layer, these are glucose polymers that are interlinked with chitin.
3. Proteins: The final layer of the cell wall in fungi, it is made of structural proteins called mannans, commonly called mannoproteins that are attached to mannose sugar. The mannoproteins, along with the mannose sugar, also helps in cell signaling.

Cell Walls in Algae

Algae being a widely diverse group, their cell walls also vary in their composition. In algae it consists of polysaccharides, like cellulose or glycoproteins, while others contain both of them. In addition, algal cell walls may have mannans, xylans, alginic acid, and sulfonated polysaccharides. While mannans are proteins that make microfibrils in some green and red algae, xylans are complex polysaccharides that are sometimes found to replace cellulose in algae. Another type of polysaccharide, the alginic acid is often found in brown algae.

Cell Walls in Bacteria

The most commonly studied prokaryotic cell wall found outside the cell membrane; it is primarily made of a unique molecule called peptidoglycan or murein. Peptidoglycan consists of alternating sugars and amino acids arranged in a mesh-like layer that helps to provide shape and protection to the cell. Based on its composition and the ability to retain dyes, the bacterial cell wall is classified into two distinct groups.

Gram-positive bacterial cell wall: Contains thick, multiple layers of peptidoglycan that are arranged stack by stack

Gram-negative bacterial cell wall: Contains a thin layer of peptidoglycan, that is surrounded by an outer layer of lipopolysaccharides (LPS). The LPS protects gram-negative bacteria against certain antibiotics, such as penicillins. 

Cell Walls in Archaea

Semi-rigid in nature, it is mostly composed of surface-layer proteins called S-layers, and lack peptidoglycan molecules, with the exception of methanobacteria that have pseudopeptidoglycan in their cell wall. The pseudopeptidoglycan is distinctly different in composition from the peptidoglycan layer found in the prokaryotic bacterial cell wall.

Main Functions

• Shape Maintenance: Forms a framework for the cell and prevents it’s over expansion 
• Support: Provides mechanical strength and rigidity to the cell
• Protection: Forms a barrier to protect internal cellular components from physical shock and also against viruses and other pathogens that may cause disease

Other Functions

• Storage: Helps to stores carbohydrates in seeds that plants use as food
• Turgidity Maintenance: Prevents water loss through evaporation that helps the cell to remain erect and perform food production through photosynthesis. The turgor pressure also helps a cell to break open or rupture.​
• Growth: Turgidity of the cell helps in its elongation and expansion along a direction that helps a plant to grow
• Communication: Helps in establishing communication between cells that allows passing and exchanging signals and other molecules across the cell through diffusion