Plant Cell Wall- Definition, Structure, Functions, Diagram

  • A cell wall is an outer rigid semi-elastic supportive and protective layer.
  • It is present around the plasma membrane.
  • It provides mechanical support and helps in maintaining the shape of the plant cell.
  • The cell wall is present in the plant cell and absent in the animal cell which distinguishes them from each other.
  • The cell wall is formed by the protoplast. Any plant cell which is devoid of the cell wall is called the protoplast.
  • The plant cell is mostly made up of the following components:
    • Cellulose
    • Hemicellulose
    • Pectin
    • Protein
Plant Cell Wall
Plant Cell Wall Diagram
  • In both the primary and secondary cell walls of the plant, cellulose is present.
  • Cellulose is an insoluble carbohydrate.
  • The fibrous structure present in the cell wall maintains the integrity of the structure.
  • In the primary cell wall, Pectin is present predominantly.
  • It plays the important role in:
    • Expansion
    • Strength
    • Porosity
    • Adhesion
    • Intercellular signaling
  • Other non-cellulosic polysaccharides include xyloglucan, glucan, xylan, mannan, and callose.
  • Based on the sugar substitutes and side chains, pectic and non-cellulosic polysaccharides can be distinguished further too.
  • During biosynthesis, these components are attached to the polysaccharides.
  • These substituents are important in determining the solubility and viscosity within the cell wall.
  • They are also responsible for determining the interaction between polysaccharides and proteins.
  • The cell wall of fungi is made of chitin.
  • The cell wall of bacteria is made of the protein, lipid, and polysaccharides complex.

Structure of Plant Cell wall

  • It is derived from the living protoplast.
  • It consists of the middle lamella, primary cell wall, plasmodesmata, secondary cell wall, and pits.

Middle lamella

  • After the cytokinesis, it is the first-formed layer.
  • It is present in between the two adjacent cells. 
  • It is made up of calcium and magnesium pectate.
  • It helps to join the two adjacent cells.

Primary cell wall

  • It is the first formed cell wall.
  • It is present in the inner side of the middle lamella.
  • It is the thin and permeable layer that can be expanded.
  • Cutin and cutin waxes are present in some epidermal cells of the leaf and stem. It makes the primary cell wall impermeable.
  • It is formed before the growth and development of the cell.
  • It is made up of matrix and microfibrils.
  • Matrix is made up of water, hemicelluloses, pectin, lipids, and proteins.
  • Microfibrils are embedded in the gel-like matrix.
  • The primary cell wall of the plant is made of cellulose.
  • In the fungi, chitin makes the primary cell wall, and in bacteria murein makes it.
  • Primary cell wall forms the only cell wall in the immature meristematic and parenchymatous cells. 

Plasmodesma (plural: plasmodesmata)

  • Plasmodesmata are cytoplasmic or protoplasmic bridges present in the primary cell wall of adjacent cells.
  • They form a protoplasmic continuum called symplast.
  • They transfer cytoplasmic materials among adjacent cells.
Structure of Plant Cell wall Diagram
Structure of Plant Cell wall Diagram. Source: LadyofHats.

Secondary cell wall

  • The secondary cell wall is situated inner to the primary cell wall.
  • This is the thick layer, permeable, and cannot be expanded.
  • It forms after the growth and development of the cell.
  • It is present in the cells of the thick-walled dead tissue of the plant. Eg: Cells of sclerenchyma, tracheids, and vessels.
  • It is differentiated into the outer layer (S1), middle layer (S2), and inner layer (S3).
  • Each layer is made up of a matrix and microfibrils.
  • The chemical composition of the matrix is almost similar to the matrix of the primary cell.
  • Microfibrils of the secondary cell wall is made up of cellulose and lignin.
  • Some chemicals like suberin, silica, wax, resins, oils, etc. are also deposited in the secondary cell wall.

Pits

  • In the secondary cell wall, pits are the unthickened areas or depressed areas.
  • A pit consists of a pit cavity or pit chamber and pit membrane.
  • The pit membrane consists of the primary cell wall and middle lamella.
  • The pit membrane is permeable.
  • So pit helps in rapid translocation of materials between two adjacent cells.

Tertiary cell wall

  • In some plant cells, there is the presence of another cell wall beneath the secondary cell wall. It is known as the tertiary cell wall.
  • The morphology, chemistry, and staining properties of the tertiary cell wall are different from the primary and secondary cell walls.
  • In the tertiary cell wall, xylan is also present in it.

Functions of plant cell wall

  • It provides mechanical support as the skeletal framework in the plant. 
  • It protects the inner components of the cell from mechanical injuries.
  • It is permeable to the water and solutes. It is the presence of the water-filled channels which allows the free diffusion of water and water-soluble substances. Eg: gas, salt, sugar, hormones.
  • It prevents entry of the pathogenic agents inside the cell acting as the first line of defense.
  • When the cell is kept in the hypotonic solution, it prevents the osmotic bursting of the cell.
  • In the cell wall, cutin, wax, silica, and suberin is present which reduces the rate of transpiration.
  • The cell wall of root hairs helps in the absorption of sap from the soil.
  • Walls of tracheids and vessels help in the conduction of sap.
  • Middle lamella helps to join the adjacent cells.
  • Plasmodesmata help in the transfer of cytoplasmic materials among adjacent cells.

Cell wall in the defense mechanism

  • During the infection, oligosaccharides elicitors can be released.
  • These substances can be released from the host plant’s cell wall i.e DAMPs (Damage-associated Molecular patterns) or they can be from the pathogen cell wall i.e PAMPs ( Pathogen-associated Molecular patterns). 
  • It occurs during the process of degradation.
  • In the plasma membrane, immune receptors are present which receive these elicitors.
  • It then activates the defense responses of DAMP or PAMP-triggered immunity.

References

  1. Verma, P. S., & Agrawal, V. K. (2006). Cell Biology, Genetics, Molecular Biology, Evolution & Ecology (First edition). S . Chand and company Ltd.
  2. Houston, K., Tucker, M. R., Chowdhury, J., Shirley, N., & Little, A. (2016). The plant cell wall: A complex and dynamic structure as revealed by the responses of genes under stress conditions. Frontiers in Plant Science, 7(AUG2016), 1–18. https://doi.org/10.3389/fpls.2016.00984 
  3. Shakya, M., Mehta, D. K. R., Gautam, M., Pokharel, K. R., & Khanal, K. (2077). Principles of Biology (First edition). Asmita Books Publisher and Distributors Ltd.

Leave a Comment