A 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 most prokaryotesexcept mycoplasma bacteria, in algae, plants and fungi but rarely in other eukaryotes including animals. A major function is to act as pressure vessels, preventing over-expansion of the cell when water enters.
The composition of cell walls varies between species and may depend on cell type and developmental stage. The primary cell wall of land plants is composed of the polysaccharides cellulose,hemicelluloseand pectin. Often, other polymers such as lignin, suberin or cutin are anchored to or embedded in plant cell walls. Algae possess walls made of glycoproteins and polysaccharides such as carrageenan andagar that are absent from land plants. In bacteria, the cell wall is composed of peptidoglycan. The cell walls of archaea have various compositions, and may be formed of glycoprotein S-layers,pseudopeptidoglycan, or polysaccharides. Fungi possess cell walls made of the glucosamine polymerchitin. Unusually, diatomshave a cell wall composed of biogenic silica.
PLANT CELL WALL STRUCTURE:
The plant cell wall is multi-layered and consists of up to three sections. From the outermost layer of the cell wall, these layers are identified as the middle lamella, primary cell wall, and secondary cell wall. While all plant cells have a middle lamella and primary cell wall, not all have a secondary cell wall.
- ¬ Middle lamella: Outer cell wall layer that contains polysaccharides called pectins. Pectins aid in cell adhesion by helping the cell walls of adjacent cells to bind to one another.
- ¬ Primary cell wall: Layer formed 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 needed to allow for cell growth.
- ¬ Secondary cell wall: Layer formed between the primary cell wall and plasma membrane in some plant cells. Once the primary cell wall has stopped dividing and growing, it may thicken to form a secondary cell wall. This rigid layer strengthens and supports the cell. In addition to cellulose and hemicellulose, some secondary cell walls contain lignin. Lignin strengthens the cell wall and aids in water conductivity in plant vascular tissue cells.
PLANT CELL WALL FUNCTION:
A major role of the cell wall is to form a framework for the cell to prevent over expansion. Cellulose fibers, structural proteins, and other polysaccharides help to maintain the shape and form of the cell. Additional functions of the cell wall include:
- ¬ Support - the cell wall provides mechanical strength and support. It also controls the direction of cell growth.
- ¬ Withstand turgor pressure - turgor pressure is the force exerted against the cell wall as the contents of the cell push the plasma membrane against the cell wall. This pressure helps a plant to remain rigid and erect, but can also cause a cell to rupture.
- ¬ Regulate growth - sends signals for the cell to enter the cell cycle in order to divide and grow.
- ¬ Regulate diffusion - the cell wall is porous allowing some substances, including proteins, to pass into the cell while keeping other substances out.
- ¬ Communication - cells communicate with one another via plasmodesmata (pores or channels between plant cell walls that allow molecules and communication signals to pass between individual plant cells).
- ¬ Protection - provides a barrier to protect against plant viruses and other pathogens. It also helps to prevent water loss.
- ¬ Storage - stores carbohydrates for use in plant growth, especially in seeds.