Q.Growth of cell wall during cell elongation takes place by .............

The cell wall is the outermost, actively growing protective covering of a cell, present in bacteria, fungi and plants but completely absent in animal cells - the single feature most consistently used to tell a plant cell from an animal cell. Its chemistry differs by group: peptidoglycan in bacteria, chitin and fungal cellulose in fungi, cellulose/galactans/mannans in algae, and cellulose, hemicellulose, pectin, lignin, cutin, suberin and silica in plants. A mature plant cell wall has three distinct regions laid down in order - the outermost middle lamella (calcium/magnesium pectate, cementing adjacent cells), the primary wall (a loose, elastic network of cellulose microfibrils in a hemicellulose/pectin/glycoprotein matrix), and the thick, inelastic secondary wall (laid during maturation, itself divided into three sub-layers S1, S2, S3 with differently oriented microfibrils). Channels called plasmodesmata run through the wall connecting adjacent cells' protoplasm, and thin, unevenly-thickened regions called pits (with a pit chamber and porous pit membrane) provide further local connections. The wall gives the cell definite shape and rigidity, screens out unwanted molecules, protects the protoplasm from mechanical injury, prevents osmotic bursting, and contributes to the plant's defence against pathogens.