The lipid-protein plasma membranes are thinner and provide minimal protection for the inner components of the cell. So, a thicker outer envelope is required to provide the utmost protection to the cell. The outer envelope is the cell wall and it encloses almost all cells except animal cells.
Therefore, the cell wall is the outermost protective layer of the cell in plant cells, bacteria, fungal cells, and protists. It’s components vary among different organisms. Besides the vital protection function, it has other tasks like providing support to the cell and helping in cell-cell interactions.
Table of Contents
Composition of Cell Wall
The plant cell walls are made of cellulose that acts as the skeleton of the cell wall. The cellulose is arranged into rod-shaped called microfibrils. The plant cell wall matrix comprises three macromolecules; hemicellulose, proteins, and pectins.
- Hemicellulose: These are branched polysaccharides with a backbone of a sugar-like glucose and side chains of other sugars like xylose. The hemicellulose binds to the cellulose micro-fibrils forming a resilient structural network.
- Proteins: The cell wall consists of a smaller amount of proteins. The proteins are predominantly hydroxyproline-rich glycoproteins. The cell wall also has enzymes like extensin, lignin, cutin, and suberin.
- Pectins: These are heterogeneous classes of negatively charged polysaccharides with galacturonic acid. Pectins hold water. These also form an extensively hydrated gel filling the spaces between the fibrous elements.
Structure of Cell Wall
The cell wall arises as a thin cell plate formed between the plasma membranes of newly formed daughter cells after cell division. Then it matures after incorporating additional materials assembled inside the cell and secreted in the extracellular space. The cell wall structure has three layers; middle lamella, primary, and secondary cell wall.
- Middle lamella is the outer layer of the plant cell wall. It works as an interface between neighboring cells and attaches them. The layer has pectin, lignin, and some proteins. The middle lamella lacks cellulose and is composed of lignin and hemicellulose.
- The primary cell wall is the first wall formed by the cell that gets deposited on either side of the middle lamella of the adjacent cells. It is composed of xyloglucans that bond with the microfibrils by H-bond. The xyloglucans help in connecting microfibrils to the pectin. Hence the microfibril is loosely packed. All the meristematic cells consist of the primary walls and many mature cells with living components. The thickness of the primary wall will increase markedly, but surface growth may not occur.
- A secondary cell wall forms inside the primary cell wall after complete maturation. Some cells, like the cells of xylem tissues, consist of cellulose and lignin to provide additional rigidity and waterproofing. The xyloglucan content decreases and xylan increases leading to the formation of tighter microfibrils and less branched polymers. This layer provides the rectangular or square shape of the cell. It is the thickest layer.
Types of Cell Wall
Based on the composition, the cell wall comprises five types; plant, fungal, algal, bacterial, and algal.
- Plant cell wall: As discussed earlier, the primary component of plant cells is cellulose. It forms the skeleton where pectin, lignin, and hemicellulose interlink to create two types of cell walls; the primary and secondary cell walls (which have larger lignin content). These support cells and act as protective barriers in the young and mature plant cells.
- Fungal cell wall: The primary component of the fungal cell wall is chitin. Chitin is a polysaccharide similar to cellulose, but the acetyl amine group replaces the hydroxyl group. Along with the fungal cell wall, chitin is also present in the exoskeleton of the arthropods.
- Bacterial cell wall: The cell wall of bacteria is composed of peptidoglycan. The amount of peptidoglycan in the cell wall is the determinant factor for separating bacteria as Gram-negative (stained red colored and has 10% peptidoglycan in the cell wall) and Gram-positive (stained purple color and has 90% peptidoglycan in the cell wall). The peptidoglycan consists of the chains polysaccharide-glycan bound together by amino acids.
- Archeal cell wall: Like the bacterial cell wall, many archaeal cell walls are composed of the pseudo-peptidoglycan, a component similar to the peptidoglycan.
- Algal cell wall: The composition of the algal cell wall consists of cellulose like the plant cells. Other groups of polysaccharides like mannan and xylan are also present in the algal cell wall. In addition to the polysaccharides, some algal cell wall consists of alginic acid (present in brown algae) or silica (present in the diatoms).
The cell wall performs various essential functions. The functions are as follows:
- Support: The cell wall gives mechanical support to the plant cell. It is also responsible for providing the well-known rectangular shape of the plant cell.
- Protection: The cell wall protects against attack from foreign agents like plant pathogens and viruses. It also protects osmotic and mechanical strength. The proteins in the cell wall help in localized relaxation, which prevents cell rupture due to turgor pressure.
- Cell signaling: The proteins present in the cell wall span the plasma membrane. This spanning helps pass signals from the cell wall to the cytoplasm.
- Cell-cell communication: The plasmodesmata, pores in the cell wall that forms a channel, helps the adjacent cells of any plant tissue to communicate.
- Verma, P., & Agarwal, V. (2005). Cell biology, genetics, molecular biology, evolution and ecology (23rd ed). S Chand and Company Ltd.
- Iwasa, J., & Marshall, W. Karp’s Cell and Molecular Biology (8th ed., pp. 254–256). Wiley.