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Biology11 Notes for Biology Notes

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Cell wall

Cell wall is outermost, rigid, protective and semi-transparent covering of plant cells and cell of fungi, bacteria and some protists. It was discovered by Robert Hooke in 1665.

  • Fungal cell wall – of chitin
  • Bacterial cell wall – of peptidoglycan or mucopeptide or murein
  • Plant cell wall – of cellulose, hemicelluloses and protein.

Structure of cell wall:

cell wall structure

Cell wall has the following different layers:

Middle Lamella

  • It is outermost thin cementing layer between adjacent cells
  • Formed during cytokinesis
  • Composed of magnesium pectate and calcium.

Primary Wall

  • More or less elastic layer formed after the formation of middle lamella.
  • Composed of protein, hemicelluloses and loose network of microfibril of cellulose.
  • Primary wall and middle lamella are found in all types of plant cells.

Secondary wall

  • Much thicker, rigid and inelastic layer formed inner to primary wall.
  • Is formed only when cells attain maximum size
  • Is common in aclerenchyma, collenchymas, tracheid and vessels.
  • Composed of cellulose, hemicellular, pectin and lignin.
  • S1, S2, and S3 are its sublayers.
  • May contain wax, cutin and suberin and silica.

Plasmodesmata:
Primary and secondary walls are not formed continuously. They form gaps, known as plasmodesmata. It is a cytoplasmic bridge between two cells.

Desmosomes:
It is the protoplasmic bridge between two cells of animal cells.

Functions of Cell Wall:

  • It provides definite shape to cell due to its rigidity.
  • It protects the protoplasm against mechanical injury.
  • It prevents the cell from osmotic burstiong.
  • Cell wall of root hairs absorbs water.
  • It has some enzymatic activity.
  • Cutin and suberin of cell wall of stem and leaves help to reduce the rate of transpiration.
  • The permeable nature of cell wall allows the exchange of any substance through it.

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