Eukaryotic cell wall

Plant cell wall

1. EUKARYOTIC CELL WALL By Akanksha Jain Ph.D. Biotechnology SSMV Bhilai

2. PLANT CELL WALL Cellular exoskeleton or extracellular matrix (ECM) of plant cell. Robert Hooke (1665) discovered cell wall when he observed dead empty cells in a very thin slice of cork under his microscope. Outer boundary of plant cell Non- living structure seen outside the plasma membrane Secreted by the cytoplasm Varies in thickness in different cells – 0.1 to 10 µm

3. Definition: Cell wall is the thick, rigid, non-living, semi-elastic, transparent, specialized form of protective extra-cellular matrix that present outside the plasma lemma of cells. Occurrence : Found in plant cells, fungal cells, some protists and prokaryotes except a few lower plants, gametes and in animal cells.

4. Thickness: The thickness varies from 0.1 to 10/µm and xylem vessels have thickest cell wall, while thinnest cell wall found in meristematic and parenchymatous cells. Chemical composition: In plants, cell wall composed of cellulose micro-fibrils embedded in the matrix. Matrix is the gel-like ground substance which consists of water, hemicellulose, pectin, glycoproteins and lipids. The cell wall may have lignin for hardness, silica for stillness and protection, cut in to prevent water loss and suberin for impermeability. In fungi, cell wall composed of chitin or fungal cellulose, a polymer of NAG. In bacteria, cell wall composed of peptidoglycan which consists of polymers of NAG (N-acetyl glucosamine) and NAM (N-acetyl muramic acid) cross-linked by short peptides.

5. Cell wall - Chemical composition Varies in different cells. Mostly composed of cellulose Also contain Hemicellulose Pectin Lignin Suberin Waxes Mineral deposits in the form of Ca and Mg carbonates Silicates etc.

6. CELLULOSE Long, unbranched chain of glucose molecules linked by Beta 1,4 glycosidic bonds. About 6000 glucose units in a cellulose chain. 100 cellulose chain form a bundle called micelle – cellulose chains in a micelle are cross linked by hydrogen bonds. 20 micelle form a larger bundle – microfibril.

7. 250 microfibrils form a still large bundle - macrofibril . Fibrils have high tensile strength. They associate with a gel like matrix containing pectin, hemicellulose and lignin to form the cell wall. In primary wall , microfibrils are arranged at random. In the secondary wall, they are closely packed and are arranged parallel to one another. These are oriented in different directions in the different layers of the cell wall.

8. Arrangement of cellulose, microfibrils and macrofibrils in plant cell walls

9. HEMICELLULOSE : Composed of glucose, xylose, mannose and glucuronic acid . PECTIN : Contain galactose, arabinose and galacturonic acid and are soluble in water. LIGNIN : Found only in mature cell walls – made up of an insoluble aromatic polymer resulting from the polymerization of phenolic alcohols.

10. SUBERIN : A fat like material deposited in the secondary wall of cork cell (Suberisation) - makes the cell wall impermeable to water and checks the loss of water by evaporation. CUTIN : A fat like substance deposited on the exposed surface of cell as a distinct layer called cuticle – makes the exposed surface of the cell wall relatively impermeable to water and check evapouration (Cutinisation ).

11. PHYSICAL STRUCTURE OF CELL WALL Th r ee l a y e r s – middle lamella , primary wall and secondary wall . Occasionally, a tertiary wall may be present.

12. 1. Middle lamella: Development of the cell wall starts with the formation of middle lamella by the cell plate after cell division. Middle lamella is the first formed cell wall com p osed pectate. o f Calciu m and Ma g nes i um I t i s a vis c ous a nd jell y li k e substance and acts like a cementing material between the primary walls of adjacent cells.

13. 2. Primary wall : Formed during early stages of growth and development Found in all plant cells 1 to 3 µm thick Composed mostly of loosely interwoven fibrils of cellulose, hemicellulose and pectic substances. Thin, elastic and undergoes extension with the growth of the cell.

14. 3. Secondary wall : Laid down on the primary wall when its growth is over. 5 to 10 µm thick Has three layers- outer layer (S1), middle layer (S2) and the inner layer (S3).

15. Consists of many layers of closely packed microfibrils of cellulose, hemicellulose and in many cases, lignin. – lignification imparts strength and rigidity to the cell wall. Suberin and waxes are also present. Addition of secondary wall brings about thickening of the cell wall – occurs particularly in cells that form the harder woody parts of plants.

16. At certain places, secondary wall is not laid down and such areas are simple pits – separated by a pit membrane composed of middle lamella and primary walls. Through these fine cytoplasmic strands called plasmodesmata extend from the cytoplasm of one cell to that of neighbouring cell – facilitates movement of materials between adjacent cells. In tracheids of Gymnosperms, the secondary wall partially overhangs the pits – bordered pits.

17. Simple and bordered pits Imag e:h t tp://www.tutorsglobe.com

18. Simple pits Imag e:h t tp://www.expertsmind.com

19. Bordered pits Imag e:h t tp://www.expertsmind.com

20. Tertiary wall : In some cells, a tertiary wall is formed on the inner surface of the secondary wall – found in the xylem tracheids of Gymnosperms. Thin Composed mainly of a polysaccharide known as xylan and cellulose.

21. The cell wall is hydrated Water content of the wall accounts for about 90 % to 95 % of the fresh weight. Functions of Cell wall : Gives protection and mechanical support to the plant cell. Provides definite shape to plant cell. It is permeable and allows materials to pass in and out of the cell. It helps in the maintenance of balance of intracellular osmotic pressure with that of its surroundings.