Characteristics of cell wall
- The cell wall is a highly complex and well-organized structure.
- It gives a definite shape to the plant cell (cell wall is also present in fungal and bacterial cell).
- It is the rigid outer covering of the cell.
- It lies outside the plasma membrane of some cells (plant cell, bacterial cell and fungal cell).
- Different cell walls are composed of different components such as the major component of plant cell wall is cellulose and that of fungi is chitin.
- A plant cell wall has three different layers, these are:
- Primary cell wall
- Secondary cell wall
- Middle lamella
NOTE: Animal cells does not have a cell wall around the plasma membrane of their cell.
Functions of cell wall
- It gives a definite shape to the plant cells.
- It maintains the structural integrity of a cell.
- Being an outer covering, it provides protection to the cell against a variety of external factors
- It also provides various channels, receptors and pores which regulates different functions of a cell.
- Therefore, cell wall is a multifunctional structure in plant cells, thus it contributes to the overall growth of a plant.
Characteristics of the primary cell wall:
- It is present in all plant cell wall.
- It is elastic in nature and its thickness ranges between 1 and 3 um (micrometer).
- Its fibrils are loosely arranged.
- It gradually elongates over a period of time.
- The primary wall of cell wall of dicot plant consists of about 30 % pectic polysaccharides.
- It consists of about 15 to 30 % of cellulose.
- It consists of about 20 % of proteins.
Some of the other components of the primary cell wall are:
- Phenolic esters
- Minerals such as boron and calcium
Functions of primary cell wall:
i. In addition to cellulose, the primary cell wall also contains high amount of lignin which has been shown to form covalent cross-links with pectin and protein in the layer.
ii. The primary cell wall also contains xyloglucan in the layer is bound to cellulose (through hydrogen bonds) and to protein and pectin through covalent bonds.
NOTE: In primary cell wall covalent bonds have also been identified between carbohydrates and lignin forming lignin-carbohydrate complexes.
Characteristics of the secondary cell wall:
- Its thickness ranges between 5 and 10um (micrometer).
- It has a large number of pores.
- It is present only in some cells.
- Water concentration of this layer ranges between 30 and 40 % .
- Microfibrils of this layer are compact and elongate.
Some of the major components of the secondary cell wall include:
Functions of secondary cell wall:
i. Being lignified the secondary cell wall provides mechanical strength and contributes to the water transportation process in a plant.
ii. Addition of lignin (which is hydrophobic and inert in nature) to cellulose and hemicellulose provides mechanical strength as well as hydrophobicity to the secondary cell wall.
iii. In secondary cell wall, cellulose acts as the load-bearing unit where its microfibrils bind to the hemicellulose (such as: xylan) and forms a basic framework of the layer.
iv. The secondary walls exist in the form of fibers/woods and thus they are used in our daily life (especially of carpenters).
- It is a cementing layer that connects the cell walls of two adjoining plant cells together.
- It is the first layer which gets deposited at the time of cytokinesis (i.e., cellular division).
- Middle lamella develops from the cell plate which forms during cell division.
- This layer is mainly made of magnesium and calcium pectates.
- It is the outermost layer of the cell wall of a mature plant cell.
- The pectins are the main component that forms a unified and continuous layer between the adjacent cells.
- Often the first layer of the secondary cell wall may be called a compound middle lamella.
- If sometimes the middle lamella gets degraded by enzymes, as happens during ripening of fruit, the adjacent cells gets separated.
Some of the other components of the middle lamella include:
- Calcium ions
- Magnesium ions
Function of middle lamella
- Being a cementing layer the middle lamella plays an important function in maintaining the structural integrity of the cell (mainly the plant cell).
- It helps in providing stability to the plant cell and tissue.
- It plays a very important role in the formation of plasmodesmata between the adjacent cells through which various cellular components are exchanged between the cells.
- It helps the plant cell in growth and division by forming the cell plate that divides a plant cell into two parts during cell division. Hence, middle lamella aids in cytokinesis of plant cell.
It is the phenomenon in which a living plant cell loses water through osmosis as a result there is shrinkage or contraction of the contents of the cell away from the cell.
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