Question

Compare primary and secondary cell walls.

Answer 1

Primary cell wall:

1. It is the first formed wall of the cell which is produced inner to the middle lamella.
2. It is commonly thin and capable of extension.
3. It is found in all plant cells and some cells consist of primary cell wall only, e.g., leaf cells, fruit cells, cells of cortex and pith.
4. The matrix of wall consists of galactants, mannans, minerals like calcium carbonate, pectin, hemi cellulose etc.,
5. Micro fibrils, pectin polysaccharides and structural proteins of the cell wall form the first continuous lattice or network by forming hydrogen bonds with xyloglucan end of hemicelluloses.
6. Pectin polysaccharides form second network, which is formed by cross-linking of galacturonic acid residues through calcium bridges and ionic interactions.
7. Structural proteins or glycoprotein form third network which interweaves through other network like wrap and weft structure.

Secondary cell wall:

1. Secondary cell wall is produced in some mature cells when the latter have stopped growth, e.g., tracheids, vessels elements, fibres, collenchymas.
2. Secondary cell wall is laid inner to the primary wall by deposition of materials over the surface of secondary structures.
3. The composition of secondary wall is basically similar to the primary wall in having cellulose micro fibrils embedded in a matrix of pectin and hemicelluloses.
4. Cellulose content is usually high in secondary cell wall n comparison to primary cell wall hemicelluloses content is comparatively low rather than primary cell wall.
5. A number of different materials may be deposited in secondary wall.
6. It reduces the water content of the wall matrix and increases its hardness.
7. Other substances which can be deposited in secondary cell wall matrix are silica, minerals, waxes, tannins, resins, etc.,