Cell is the structural, functional and heritable unit of living organisms. It consists of a membrane bound mass of protoplasm called Protoplast. This protoplast includes selectively permeable plasmamembrane, nucleus/nuclear material, cytoplasm, vacuoles and inclusions. A term extracellular matrix (ECM) has been used for the outermost boundary of animal cells i.e., the material present outside the plasma membrane. In present day context, however, it is used in plants as well. In plant cells, fungi, prokaryotes (monerans) like (bacteria and cyanobacteria— blue green algae), and few protists, this cell membrane is covered by a nonliving, porous, permeable, inert, hydrophilic, inelastic, thick, rigid, semi-transparent protective covering called cell wall. This wall is secreted by cytoplasm (protoplasm) i.e., cell wall is a form of extracellular matrix but actually it is metabolically active and is capable of growth. Plant cells show little variations in shape due to cell wall. Animals lack cell wall because
(i) they are adapted to move their body.
(ii) they have muscles and nerves.
(iii) they have to change their size of body during locomotion.
Animals differ from plants mainly in the absence of cell wall (Schwann 1839). Among prokaryotes/ monerans, cell wall is absent in mycoplasma (PPLO). It is also absent in gametes (gymnogametes) and zoospores of lower plants. Animal cells burst in hypotonic solution due to absence of cell wall.
Cell wall was first observed by Robert Hooke (1665) when he observed dead empty cork cells of Spanish oak under his microscope. Though cell wall is nonliving but is not completely dead.
Functionally, it is highly active and performs a number of functions like
(1) protection from injury and attack of pathogens.
(2) provide shape and rigidity to cell.
(3) counteract osmotic/turgor pressure by developing wall pressure.
(4) provide strength to land plants to withstand gravitational force.
(5) help in cell expan¬sion and growth.
(6) because of having a number of hydrophilic substances like proteins and pectin, it imbibes water and thus helps in absorption of water by root hairs and germination of seeds.
(7) plasmodesmata in cell wall maintain cytoplasmic continuity between cells.
(8) some enzymes are found in cell wall for defence and offence.
(9) secondary wall materials like cutin and suberin check loss of water.
(10) lignin in secondary wall provides hardness, tensile strength and stiffness to bear load.
Structure. The thickness of cell wall varies from 0.1 to 10 µm. Young meristematic parenchymatous cells have thinnest and xylem vessels have thickest cell wall. A cell wall contains
(i) Matrix is non-cellulosic. It is amorphous, gel like ground substance of the cell wall and consists of water (30 to 60%), hemicellulose (5-15%), pectin (2-8%), lipids (0.5-3%) and glycoproteins (1-2%). This matrix is free from cellulose. Hemicellulose is the main carbohydrate in the matrix. It is a heteropolysaccharide formed of arabinose, xylose, mannose and galactose. It acts as binder and binds cellulose fibres to matrix. Pectin is a heteropolysaccharide and chiefly made up of galacturonic acid, galactose and arabinose. It acts as filler substance as well as a cementing substance in middle lamella. It aiso determines hydration of cell wall, its elasticity, growth and orientation of microfibrils in the matrix. Lipids help in the transport of hydrophobic substances across the cell wall as well as provide linkage to wall depositions. Proteins in the cell wall form enzymes and with pectins help to imbibe water. Glyco-proteins in the matrix control orientation of microfibrils.
(ii) Fibrils (= macrofibrils). In the matrix are found embedded macrofibrils. These are made up of
cellulose and form 20-40% part of cell wall. Each fibril (macrofibril) is 0.5 µm in width and made up of 250 microfibrils . They provide rigidity to cell wall just as steel bars in ceilings. These microfibrils are arranged in layers. Each microfibril is 260 A0 in thickness and consists of 20 micelles (elementary fibrils).These micelles are smallest structural units of cell wall. Each micelle is 100 A0 in diameter and made up of 100 cellulose chains. Each cellulose chain consists of 500 to 10,000 glucose units joined by β, 1-4 glycosidic bonds, in primary wall, the fibrils are short (0.5-0.7 µm), wavy and loosely scattered while they are straight, long (4-7 µm), close and parallely arranged in secondary wall.
The microfibrils in the matrix form three types of intricate networks:-
(a) Cellulose microfibrils connected with the help of xyloglucan chains through H-bonds. This network is embeded in second network.
(b) Pectin polysaccharides forming second network rich in galactouronic acid.
(c) Structural proteins forming third interlocking network that interweaves through the above two net works.
(iii) Depositions in the cell wall. Secondary cell wall has deposits of:-
- Silica (e.g., grass, sugar cane and wheat leaves, Equisetum) for protection and stiffness;
- Cutin (a fatty substance) on living epidermal cells to prevent loss of water.
- Suberin (a fatty substance) in cork cells and endodermis to make cell wall impermeable for checking loss of water and entry of pathogens.
- Wax (lipid) on surface to act as water repellent in floating leaves and to check transpiration
- Lignin (a hydrophobic polymer of coneferyl alcohol) in fibres and xylem/wood to provide hardness, stiffness and tensile strength. Lignin reduces hydration to make cell wall hard without affecting water permeation of the cell wall. Beside these depositions, tannins, resins and other substances are also deposited in secondary walls.
- Cutin and suberin check transpiration and protect leaves from rain water
A cell wall consists of one, two or all the following three layers:
1. Middle Lamella. This is thin, amorphous, cementing layer that joins (cements) two adjacent plant cells together. It is just like brick work of the common wall between two adjacent rooms. It is absent on the outer free surface of cell. In plasmodesmata region, it is absent....... (Rest part of study materials after registration).
Details heading of rest part study materials:-
2. Primary Wall
3. Secondary Wall
4. Chemical composition
5. Microchemical , Chemical & Stain
6. Origin of Cell Wall
9. Types of pit
10. Different pit in Gymnosperm & Angiosperm
11. ECM of animal cell
12. Chemistry of ECM
13. Cell Coat