Cell walls

Cell Walls

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. Cell walls are present in plants, fungi, and certain prokaryotes, but are absent in animals.

Structure

The composition of cell walls varies between species and can also differ depending on the type of cell and its stage of growth. However, the primary components of most cell walls include:

• Cellulose: A polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. It is the main component of plant cell walls.
• Hemicellulose: A heterogeneous group of polysaccharides that bind with cellulose fibers to provide structural support.
• Pectin: A complex set of polysaccharides that are rich in galacturonic acid and are found in the primary cell walls of plants.
• Lignin: A complex organic polymer that is deposited in the cell walls of many plants, making them rigid and woody.

Function

The cell wall serves several important functions:

• Protection: It protects the cell from mechanical stress and provides a barrier against pathogens.
• Support: It maintains the shape of the cell and prevents it from bursting in hypotonic environments.
• Regulation: It controls the rate and direction of growth and regulates the diffusion of substances into and out of the cell.

Types of Cell Walls

Plant Cell Walls

Plant cell walls are primarily made of cellulose, hemicellulose, and pectin. They are divided into three layers:

• Primary Cell Wall: A thin, flexible, and extensible layer formed while the cell is growing.
• Secondary Cell Wall: A thick layer formed inside the primary cell wall after the cell has stopped growing. It is often lignified.
• Middle Lamella: A pectin-rich layer that glues adjacent plant cells together.

Fungal Cell Walls

Fungal cell walls are composed of chitin, glucans, and proteins. Chitin is a long-chain polymer of N-acetylglucosamine, a derivative of glucose.

Bacterial Cell Walls

Bacterial cell walls are made of peptidoglycan, a polymer consisting of sugars and amino acids. There are two main types of bacterial cell walls:

• Gram-positive: Thick peptidoglycan layer and teichoic acids.
• Gram-negative: Thin peptidoglycan layer and an outer membrane containing lipopolysaccharides.

Evolutionary Significance

The evolution of the cell wall was a crucial step in the development of life on Earth. It allowed cells to maintain their shape, protect themselves from environmental stresses, and interact with their surroundings in complex ways. The diversity of cell wall structures reflects the adaptation of organisms to different ecological niches.

See Also

• Cell membrane
• Cytoskeleton
• Photosynthesis

References

• Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). Molecular Biology of the Cell. Garland Science.
• Taiz, L., & Zeiger, E. (2010). Plant Physiology. Sinauer Associates.

External Links

• Wikipedia: Cell wall