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Cell Wall -Tony Eom, Ty Dupree

A cell wall is a tough flexible layer of polysaccharides, but sometimes fairly rigid that surrounds some types of cells. It is the outer covering of the cell membrane and provides the cells with structural support and protection, and also acts as a filtering mechanism.


Plant_cell_wall_diagram.png

Layers

  • The middle lamella, a layer rich in pectins. This outermost layer forms the interface between adjacent plant cells and glues them together.
  • The primary cell wall, generally a thin, flexible and extensible layer formed while the cell is growing.
  • The secondary cell wall, a thick layer formed inside the primary cell wall after the cell is fully grown. It is not found in all cell types. Some cells, such as the conducting cells in xylem, possess a secondary wall containing lignin, which strengthens and waterproofs the wall

Composition

  • cellulose, 35-50%
  • xylan, 20-35%, a type of hemicellulose
  • lignin, 10-25%, a complex phenolic polymer that penetrates the spaces in the cell wall between cellulose, hemicellulose and pectin components, driving out water and strengthening the wall.
  • Pectic acid
    - polymer of around 100 galacturonic acid molecules
    - very hydrophilic and soluble - become very hydrated
    - forms salts and salt bridges with Ca++ and Mg++ that are insoluble gels
    - major component of middle lamella but also found in primary walls


Functions of cell walls:

  • Provide tensile strength and limited plasticity which are important for:
    • keeping cells from rupturing from turgor pressure
    • turgor pressure provides support for non-woody tissues
  • Thick walled cells provide mechanical support
  • Tubes for long-distance transport
  • Cutinized walls prevent water loss
  • Provide mechanical protection from insects & pathogens
  • Physiological & biochemical activities in the wall contribute to cell-cell communication