Composition of cell wall, Biology

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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.

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