About Cell Wall
Discovery of Cell Wall
It was first discovered by Robert Hooke in 1665.Cell wall is the outer most, rigid, protective, non living and supportive layer found in all the plant cells, bacteria, cyanobacteria and some protists. It is not found in animal cells.
Chemical composition of Cell Wall
Mainly cell wall consists of two parts, matrix and cellulosic fibres (microfibriles). Matrix consists of hemicellulose, pectin, glycoproteins, lipids and water. A cellulose molecule is long unbranched chain of glucose molecules. There are about 6,000 glucose units in each chain. In most of the plants cell wall is made up of cellulose a polymer made-up of unbranched chain of glucose molecule linked by glycosidic bond. About 100 molecules of cellulose form a micelle, about 20 micelle form a microfibril and approx 200 microfibril form a fibril. The cell wall of bacteria and the inner layer of blue green algae is made-up of mucopeptide and not of cellulose. The mucopeptide is a polymer of two amino sugars namely N-acetyl glucosamine (NAG) and N-acetyl muramic acid (NAM) held alternately in –1,4- linkage. In higher fungi, the cell wall is made up of chitin, polymer of glucosamine.Pectin is a mixture of polymerised and methylated galacturans, galacturonic acid and neutral sugars. Hemicellulose is a mixture of polymerised xylans, mannans, glucomannans, galactans, xyloglucans and arabinogalactans. Glycoproteins are known to influence metabolic activities of the wall. A glycoprotein called extensin or expansin takes part in loosening and expansion of cell was through incorporation of cellulose molecules to cellulose microfibrils.Plant cell wall may have lignin for strength (e.g., woody tissue), silica for stiffness and protection (e.g., epidermal cells of grasses, Equisetum), cutin for preventing loss of water (e.g., epidermal cells), wax as component of cuticle and surface bloom as water repellent (floating leaves) and checking transpiration, suberin for impermeability (e.g., cork cells, endodermal cells), etc.
Structure of Cell Wall
Cell wall consists of middle lamella, primary wall, secondary wall, tertiary wall.
(i) Middle lamella of cell wall : Middle lamella is the outermost region which functions as a cementing layer between two cells. It is absent on the outer free surface. It ruptures to create intercellular spaces. Middle lamella is formed of calcium and magnecium pectate. Fruit softening is due to gelatinisation of pectic compounds of middle lamella. Pectin is used as commercial jellying agent. Which is present outside the primary wall.
(ii) Primary wall : A young plant cell forms a single layer of wall material. This layer is known as the primary cell wall. The primary wall is thin, elastic and capable of expansion in a growing cell. It grows by intussusception. Meristematic and parenchymatous cells have primary cell wall only. The cells of leaves and fruits too have only primary wall.
(iii) Secondary wall : In mature cell, more layers of wall material are added internal to the primary wall. These are called the secondary cell wall. Growth by addition of new wall material on the primary wall is called accretion. The secondary wall is thick and rigid. It usually consists of three layers, which are often named It is found in collenchyma and sclerenchyma cells, xylem vesseles.
(iv) Tertiary wall : Sometimes tertiary wall is laid down on secondary wall, e.g., tracheids of gymnosperms. It is composed of cellulose and xylan, another ploysaccharides.
Origin of Cell Wall
A cell wall is organised at telophase stage of cell division. The plane and place of cell wall is determined by the microtubules. Fragments of ER and vesicles of golgi body alligned at the equator, called as phragmoplast, later which forms the cell plate. The synthesis of cellulose takes place by the help of enzyme cellulose synthase present in the plasma membrane. The cell plate forms the cell wall. A cell posses three phases of growth namely cell formation, cell elongation and cell maturation. The formation of new cells occurs by mitotic activity. The cell elongation is initiated by an increase in cell turgor. It is brought about by special proteins called expansion. They are of two types expansion and expansion. As a result, lacunae or gaps appear in between the cellulose micelle. There are two possibilities for the deposition of new wall material.
(i) By intussuception : As the cell wall stretches in one or more directions, new cell wall material secreted by protoplasm gets embedded within the original wall.
(ii) By apposition : In this method new cell wall material secreted by protoplasm is deposited by definite thin plates one after the other.
Differences between primary and secondary cell wall
Primary cell wall Secondary cell wall
Primary wall is laid inner to middle lamella Secondary wall is laid inner to primary wall.
It is formed in a growing cell. It is formed when the cells have stopped growing.
It is capable of extension. Extensibility is absent except in collenchyma cells.
It is single layered. It is three or more layered.
Cellulose content is comparatively low (5 – 20%). Cellulose content is comparatively high (20 – 90%).
Cellulose microfibrils are shorter, wavy and loosely arranged. They are longer, closely arranged straight and parallel.
Protein content up to 5%. Protein content up to 1%.
Hemicellulose content is high up to 50%. It is 25% of the total.
Lipid content up to 5 – 10%. Lipid is absent.
Primary wall is comparatively thin 1 – 5
It is comparatively thick 5 – 10
Thickenings of cell wall
In many secondary walls specially those of xylem the cell wall becomes hard and thick due to the deposition of lignin. With the increasing amount of lignin, deposition protoplasm is lost. First the lignin is deposited in middle lamella and primary wall and later on in secondary wall. Like cellulose lignin is permeable to water and substances dissolved in it. Lignin is deposited at specific places of the cell walls due to which xylem tracheids and trachea take up following forms:
(i) Annular thickenings : Deposition of lignin takes place in the form of rings on the inner surface of protoxylem cells. These rings are placed one above the other leaving some space in between each other.
(ii) Spiral thickenings : In these thickenings deposition of lignin takes place in the form of complete spiral bands and are formed in tracheids and trachea of protoxylem.
(iii) Scalariform (Ladder like) thickenings : In these thickenings lignin is deposited in the form of transverse rods of the ladder. The unthickened areas between the successive thickenings appear as elongated transverse pits. This type of thickening is common in protoxylem.
(iv) Reticulate (Net like) thickenings : The lignin is deposited in the form of a net or reticulum. The unthickened areas are irregular in shape. These are found in metaxylem.
(v) Pitted thickenings : These are found in metaxylem. In such thickening the whole inner wall is more or less uniformly thickened leaving here and there some unthickened areas called pits.
Pits Cell Wall
Secondary walls may have irregular thickenings at some places and these places are called pits. Pits are of two types :–
(i) Simple pit : In which pit chamber is uniform in diameter.
(ii) Bordered pit : In which pit chamber is flask shaped in tracheids of gymnosperm and vessels of angiosperms.
Plasmodesmata of Cell Wall
Tangle (1879) first of all discovered them and were studied elaborately by Strasburger (1901). A number of plasmodesmata or cytoplasmic strands are present in pit through which the cytoplasm of one cell is in contact with another. Endoplasmic reticulum plays a role in origin of plasmodesmata.