Cell Wall

Rigid outer layer of a cell, found in plants, fungi, and some prokaryotes

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The cell wall is a structural layer surrounding some types of cells, situated outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mechanism. The cell wall is present in plants, fungi, and certain prokaryotes, but is absent in animals.

Structure

The composition of the cell wall varies between species and can also differ depending on the type of organism. In general, the cell wall is composed of carbohydrates such as cellulose, hemicellulose, and pectin in plants, chitin in fungi, and peptidoglycan in bacteria.

Plant Cell Walls

In plants, the cell wall is primarily made of cellulose, a complex carbohydrate that provides tensile strength. The plant cell wall is divided into three layers:

• Primary cell wall: This is the outermost layer, which is flexible and allows for cell growth.
• Secondary cell wall: Found in some plant cells, this layer is thicker and more rigid, providing additional strength.
• Middle lamella: A pectin-rich layer that glues adjacent plant cells together.

Fungal Cell Walls

Fungal cell walls are primarily composed of chitin, a long-chain polymer of a glucose derivative. This provides rigidity and structural support to the fungal cells.

Bacterial Cell Walls

Bacterial cell walls are made of peptidoglycan, a polymer consisting of sugars and amino acids. The structure of the bacterial cell wall is crucial for the classification of bacteria into Gram-positive and Gram-negative groups, based on their reaction to the Gram stain.

Function

The cell wall serves several essential functions:

• Protection: It protects the cell from mechanical stress and prevents the cell from bursting in hypotonic environments.
• Support: Provides structural support and maintains the shape of the cell.
• Regulation: Acts as a barrier to control the movement of substances in and out of the cell.
• Communication: In plant cells, plasmodesmata are channels through the cell wall that allow for communication and transport between cells.

Biosynthesis

The synthesis of the cell wall involves the assembly of its components, which are produced in the cell and transported to the cell surface. In plants, cellulose is synthesized by cellulose synthase complexes located in the plasma membrane.

Evolution

The evolution of the cell wall is a significant event in the history of life, allowing for the development of complex multicellular organisms. The presence of a cell wall is a key feature distinguishing plant cells from animal cells.

Also see

• Cell membrane
• Cellulose
• Chitin
• Peptidoglycan
• Plasmodesmata
• Gram stain