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.