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Cell Wall

  2. CELL WALL  A cell wall is an outer layer surrounding certain cells that is outside of the cell membrane.  All cells have cell membranes, but generally only plants, fungi, algae, most bacteria, and archaea have cells with cell walls.  The cell wall provides strength and structural support to the cell, and can control to some extent what types and concentrations of molecules enter and leave the cell.  The materials that make up the cell wall differ depending on the type of organism.
  3. PROPERTIES  Cell walls serve similar purposes in those organisms that possess them.  They may give cells rigidity and strength, offering protection against mechanical stress.  In multicellular organisms, they permit the organism to build and hold a definite shape (morphogenesis). Cell walls also limit the entry of large molecules that may be toxic to the cell.  They further permit the creation of stable osmotic environments by preventing osmotic lysis and helping to retain water. Their composition, properties, and form may change during the cell cycle and depend on growth conditions.
  4. RIGIDITY OF CELL WALL  In most cells, the cell wall is flexible, meaning that it will bend rather than holding a fixed shape, but has considerable tensile strength.  The apparent rigidity of primary plant tissues is enabled by cell walls, but is not due to the walls' stiffness.  Hydraulic turgor pressure creates this rigidity, along with the wall structure.
  5. RIGIDITY OF CELL WALL  In plants, a secondary cell wall is a thicker additional layer of cellulose which increases wall rigidity.  Additional layers may be formed by lignin in xylem cell walls, or suberin in cork cell walls.  These compounds are rigid and waterproof, making the secondary wall stiff.  Both wood and bark cells of trees have secondary walls .
  6. PERMEABILITY  The primary cell wall of most plant cells is freely permeable to small molecules including small proteins, with size exclusion estimated to be 30-60 kDa.  The pH is an important factor governing the transport of molecules through cell walls
  7. Cell Wall is present in : Fungi, Algae, Most bacteria, and Archaea, Plants, etc.
  8. FUNGAL CELL WALL In Fungi, the mycelial wall is composed of : oChitin : occur in bundles like cellulose molecule. oA polymer of acetylglucosamine units.
  9. ARCHAEA CELL WALL Like other living organisms, archaea have a semi-rigid cell wall that protects them from the environment. oThe cell wall of archaea is composed of S-layers and olack peptidoglycan molecules with the exception of methanobacteria who have pseudopeptidoglycan in their cell wall.
  10. BACTERIAL CELL WALL oWall of the bacteria consists of peptidoglycan which contains polymers of modified sugars (N- acetyl glucosame and N- scetyl muramic acid) cross linked by short peptides.
  11. PLANT CELL WALL  Plant cells are eukaryotic cells that differ in several key aspects from the cells of other eukaryotic organisms. Their distinctive features include:  A large central vacuole, a water-filled volume enclosed by a membrane known as the tonoplast that maintains the cell's turgor, controls movement of molecules between the cytosol and sap, stores useful material and digests waste proteins and organelles.
  12. PLANT CELL WALL STRUCTURE  The cell wall is the rigid, semi-permeable protective layer in some cell types.  This outer covering is positioned next to the cell membrane (plasma membrane) in most Plant cell, bacteria, algae.  Animal cells , do not have a cell wall.  The cell wall conducts many important functions in a cell including protection, structure, and support.  Cell wall composition varies depending on the organism. In plants, the cell wall is composed mainly of strong fibers of the carbohydrate polymer cellulose.  Cellulose is the major component of cotton fiber and wood and is used in paper production.
  14. The plant cell wall is multi-layered and consists of up to three sections. From the outermost layer of the cell wall, these layers are identified as the middle lamella, primary cell wall, and secondary cell wall. While all plant cells have a middle lamella and primary cell wall, not all have a secondary cell wall. STRUCTURE
  15. Middle lamella - outer cell wall layer that contains polysaccharides called pectins. Pectins aid in cell adhesion by helping the cell walls of adjacent cells to bind to one another.​ Primary cell wall - layer formed between the middle lamella and plasma membrane in growing plant cells. It is primarily composed of cellulose microfibrils contained within a gel-like matrix of hemicellulose fibers and pectin polysaccharides. The primary cell wall provides the strength and flexibility needed to allow for cell growth.​ STRUCTURE
  16. Secondary cell wall - layer formed between the primary cell wall and plasma membrane in some plant cells. Once the primary cell wall has stopped dividing and growing, it may thicken to form a secondary cell wall. This rigid layer strengthens and supports the cell. In addition to cellulose and hemicellulose, some secondary cell walls contain lignin. Lignin strengthens the cell wall and aids in water conductivity in plant vascular tissue cells. STRUCTURE
  17. PLANT CELL WALL FUNCTIONS A major role of the cell wall is to form a framework for the cell to prevent over expansion. Cellulose fibers, structural proteins, and other polysaccharides help to maintain the shape and form of the cell. Additional functions of the cell wall include: Support - the cell wall provides mechanical strength and support. It also controls the direction of cell growth.​ Regulate growth - sends signals for the cell to enter the cell cycle in order to divide and grow
  18. Withstand turgor pressure - turgor pressure is the force exerted against the cell wall as the contents of the cell push the plasma membrane against the cell wall. This pressure helps a plant to remain rigid and erect, but can also cause a cell to rupture.​ Regulate diffusion - the cell wall is porous allowing some substances, including proteins, to pass into the cell while keeping other substances out.​ Storage - stores carbohydrates for use in plant growth, especially in seeds. PLANT CELL WALL FUNCTIONS
  19. Communication - cells communicate with one another via plasmodesmata (pores or channels between plant cell walls that allow molecules and communication signals to pass between individual plant cells).​ Protection - provides a barrier to protect against plant viruses and other pathogens. It also helps to prevent water loss.​ PLANT CELL WALL FUNCTIONS