Cell organelles- 1.THE CELL WALL

                                                

             Cells  together  to   form  tissues   and  tissues  together to form  organs. Cells  are almost microscopic. Plants,fungi and  bacteria etc.. have cell wall. It  is  the  outer most covering of their  cells. Cell  wall   is  0.2 µm  thick.

          Animals and man do not  have cell wall.  It  is a  non living  layer.     It  gives   shape  and protection  to  the cell. It  separates  one cell to  another.

Cell wall

                                         

Structure  and  function:

                                                                                                                   

                  

            


            In  the  younger  stage  the   cell wall  is very  thin, delicate and   elastic   this   gives  the  cell wall  the  capacity  to  stretch . According  to the  age  of  cell wall    its  wall  increases  in  its  elasticity  and  thickness.  The  cell wall   is  formed  by  protoplasm . Cell wall is made up  of   substance called  cellulose, which  is  carbohydrate  having   carbon ,hydrogen  and   oxygen. The  cell  wall  consists of  three layers.There are many theories  about formation of  cellwall.


 1.The Middle lamella:

          It  is  the  outer most  layer  binds two  adjacent  cells  in which  wall formation  is yet  to be completed. It  laid  down  first. It   is rich  layer  in  pectin.

2.The   Primary cell wall:

 

                                   

 

 

 

             This  layer  is  generally  thin and  deposited inside the middle lamella layer. It  is  formed  while the  cells  are  growing. This  wall  is  made  up  of  carbohydrates. The major  carbohydrates  are  cellulose, hemicelluloses  and  pectin. The  outer  most covering   of  Primary cell  wall  of  plant  epidermis  is  usually  filled with  cutin and  wax  forming   permeability layer known  as  plant  cuticle. It  gives   protection from  water  and sunlight.

  3.The  secondary cell wall :

 

                    The secondary  cell wall    layer   is formed  inside the  primary wall after  the  cells are fully grown. It  strengthens  the  cell  wall because  of  presence  of   lignin. The  conducting  cells in  xylum  possesses  lignin. The  secondary  wall  contains  a compounds   like  polymers  that  made  up  of wood. That  includes  cellulose, xylum and  lignin.     The  lignin  it penetrates   the  cell wall  between  cellulose  hemicellulose  and pectin compounds  driving  out  water  and  strengthening  the cell wall. The  cell  wall  of  epidermis  and  endodermis   may  also contain cutin  that  protects it  from  herbivore.
             

         In  secondary cell  wall  there  are  cavities  varying  in  depth  and  depressions are  called  pits. Cell  to cell communication  is  possible through  pits that  allow  Plasmodesma.  Plasmodesma   are microscopic  channels which  traverse   the  cell  walls of   plant cell. There  are  two kinds  of  pits.  Simple  and  bordered  pits.

          Bordered  pits are  common in  the  gymnospermous  woods.  while  the  simple  pits  are common in  the  angiospermous  woods.

            The  cell  walls  in some  plant  tissues  contain rich  storage of  carbohydrates. That  can  be  used  for  metabolic   and growth needs  of  plants . Plant  cell wall  contain structural  protein  and  enzymes like hydrolases, esterases. In  grasses , the cell wall  become   silicified  due  to the deposition  of  silica  and  this   gives  rigidity  to the  leaf blades.

               Cell  wall  serves  as  a  protective layer  against  mechanical  stress. The  Primary  wall  of  the  most  plant  cell  is  semipermeable  and  permits the  passage of  small  molecules and  proteins. Water  and carbon-di- oxide  is   distributed  throughout the  plant  from  cellwall  to cell  and  PH  balance  is controlled  by  the  cell wall.