COMPOSITION OF CELL WALL
Cell wall composed of cellulose, hemicellulose, pectin, lignin, cutin & chitin.
Cell wall consists of 3 parts -
1. Matrix
- Highly cross-linked amorphous gel-like ground substances.
- Water = 30-60%, forms dispersion medium, hydration.
- Pectin = 2-8%, heteropolysaccaride, helps in cross-linking, determines hydration of cell wall, elasticity, growth.
- Hemicellulose = 5-15%, Lipid = 0.5-3%, Protein = 1-2%
2. Microfibrils
- Structural elements of cell wall. Provide rigidity of cell wall.
- In plants microfibrils are formed of cellulose.
- They are 10-25 mm in thickness, 0.5-0.7 mm in length.
- Cellulose forms the backbone of the plant cell.
- Cellulose polymer of b-D-glucose.
- A cellulose microfibril consists of about 3000 glucose molecules.
- Many chains of cellulose molecules lie parallel to each other to form the bundles.
- A bundles of 100 molecules of cellulose forms the one elementary fibril known as the micelle.
- The 20 micelle when get arraged parallely, form the fibrils of 250Å thick, known as microfibrils.
- Microfibril forms large size bundle of cellulose fibres to form the macrofibrils. (macrofibril form skeletal of cell wall).
- Microfibrils are loose & wavy in primary wall.
- Microfibrils are close & parallel arrangement in secondary wall.
- The hemicellulose is composed of monosaccharide units like arabinose, xylose, mammose & galactose.
- The pectin chemically composed of glucuronic and galacturonic acid.
- The lignin chemically composed of confieryl alcohol.
- The cutin composed of many fatty acids.
- In fugal cell wall formed of chitin material.
- The chitin is a polymer of b (1® 4) acetyl glucosamine.
- Chitin also occurs in Exoskeleton of Arthropods (insects + crustaceans)
- Chitin is second most abundant organic material.
- In bacterial cell wall made up of peptidoglycan mucopeptide (murein).
- Murein is made of N-acetyl glucosamine, acetyl muramic acid and small peptide chains.
- i.e. Murein is polymer of NAG and NAM
3. Deposition on the cell wall
(a) Lignin
- Deposition of lignin on cell wall is called lignification.
- After lignification cell becomes dead & impermeable to water.
- It provides hardness to the wall.
- Lignification commonly occur in seconday cell wall.
- Lignin is the derivative of cellulose.
- Lignification developed with evolution of land plants.
(b) Cutin
- Depostion of cutin is called cutinization.
- Cutin derivative of lipid. Polymer of Hydroxy fatty acids.
- Deposition on epidermal cells of leaf.
- Epidermal layer form a layer "cuticle" outside the cell wall.
- Formation of cuticle layer is called "Cuticularisation".
- Cutin reduces the rate of transpiration. (Loss of water in vapour form)
(c) Suberin
- Deposition of suberin is called Suberization.
- Suberin derivative of fatty acids. Consists of Phellonic acid and Glycerol.
- After suberization cell becomes dead and impermeable to water.
- It occurs in cork cells and endodermal cells.
- Commercial cork is obtained from Quercus suber (Spanish oak)
(d) Silica
- Deposition occur in Atropa, Grasses, Equisetum, Diatoms.
- It provides stiffness to wall. Protects the plants from fungal attack. Irritation to grazing animals.
(e) Nonsiliceous Minerals
- Deposition of Fe and Cu occurs in aquatic habitat plants, e.g. Chara
(f) Wax
- Polymer of fatty acid and alcohol.
- Wax occurs as component of cuticle and surface bloom.
- Water repellent and control transpiration.