Covering of cell

Cell covering :- 

     Generally eukaryotic cell is cover by 2 layer the outermost layer is known as cellwall and inner to the cellwall cellmembrane is present. So we will first discuss about cell wall and then cell membrane

1.CELL WALL:-
         It is the outer rigid protective supportive and semitransparent covering of plant cells,fungi,and some protists.cell wall was first seen in "cork cell" by Hooke in 1665.its thickness varies in different types of cell from "0.1micrometer to 10micrometer".cell wall is a nonliving extracellular secretion or matrix of the cell which is closely appressed to it. It is metabolically active and is capable of growth.cell wall performs a number of functions like -

(I). Protects the protoplasm against mechanical injury

(II). protects the cell from attack of pathogens

(III).provides rigidity and shape to the cell

(IV).counteracts osmotic pressure

(V).pits present in the wall help to produce a protoplasmic countinum or symplast amongst the cells
It
(VI).walls prevent bursting of plant cells by inhibiting excessive endosmosis

(VI).wall has some enzymatic activity connected with metabolism

Chemical composition of cell wall:- 
    1)matrix-- water -60%
                         Hemicellulose--5-10%
                       Pectin substances--2-8%
                              Lipids --0.5-3.0%
                           Proteins--1-2%
    2)microfibrils--cellulose and fungus cellulose--10-15%
   3)other ingredients--lignin,cutin,suberin,silica,minerals (e.g. iron,calcium,carbonate) ,waxes,tannins,resins,gum

STRUCTURE OF CELLWALL:--
 The cell wall consists upto 3 parts -
         1) middle lamella
         2) primary wall 
         3) secondary wall 

1) middle lamella:- it is a thin amorphous  and cementing layer between two adjacent cells.middle lamella is the first layer which is deposited at the time of cytokinesis.it is just like brick work of the common wall between two adjecent rooms.middle lamella is absent on the outerside of the surface wall.It is made up of calcium and magnesium pectates .The softening of ripe fruit is caused by partial solubilization of pectic compounds to produce jelly-like consistency.

2)primary wall:-it is the first formed layer of the cell which is produced inner to the middle lamella.the primary wall is commonly thin (0.1-3.0 micrometer)and capable of extension.it grows by intussusception or addition of materials with in the existing wall.some cells posses only primary wall. e.g. leafcells,fruit cells,cells of cortex and pith.

    Primary wall consists of number of microfibril embedded in the amorphous gel like matrix or ground substance .in the majority of plants ,the microfibrils are formed of cellulose.they are synthesised at the plasma membrane by particle rossettes having cellulose synthatase enzyme .the wall is made up of a polymer of "beta 1,4 acetyl glucosamine " or fungus cellulose in many fungi.fungus cellulose is similar to chitin present in the exoskeleton of insects .

3)Secondary wall:--it is produced in some mature cells when the latter have stopped growth, e.g., tracheids,vessel elements fibers , collenchyma. Secondary wall is laid inner to the primary wall by accretion or deposition of material over the surface of existing structure .It is thick (3-10micrometer) and made up of atleast three layers,sometimes more .The inner most layer of the secondary wall is sometime distinct both chemically as well as in staining properties due  to the presence of xylans.It is then called tertiary wall,e.g. tension wood in gymnosperms.

(Plasmodesmata- it is the cytoplasmic bridges between adjacent plant cells which develop in the minute pore of their walls .They form a protoplasmic continuum called "SYMPLAST" )

2.CELL MEMBRANE:--
         
           Hugo de veries was the first to study the osmotic pressure with in the cells and formulated the idea that the protoplasm if each and every cell is sorrounded by a thin layer known as the plasma membrane.

      The physical organisation and functioning of all biological membranes depends on their constituents ,e.g. lipids,proteins,and carbohydrates. Proteins embedded in the phospholipid bilayer have a number of functions including moving material through the membrane and receiving chemical signals from the cell's external environment. The carbohydrates associated with the membrane are attached either to the lipid or to the proteins molecule .

(Structure of plasma membrane)

     They are located on the outside of the plasma membrane.carbohydrates are crucial in recognising specific molecules.membrane associated carbohydrates may be covalently bound to lipids or to proteins ,these are

(I) Glycolipids-  glycolipids consists of a carbohydrate covalently bound to a lipid; they serve as recognition signals for interaction between cells .
(II) Glycoproteins- glycoproteins consists of a carbohydrate covalently bound to a protein .The bound carbohydrates are oligosaccharide chains.