Question

The secondary wall material suberin deposited on the walls of
(a) Phellogen and phelloderm
(b) Epidermis of stem and endodermis of root
(c) Pericycle of stem and endodermis of root
(d) Phellem of stem and endodermis of root

Answer 1

Some complex macromolecules can be seen in higher plant epidermis and periderm cell-wall such as suberin, cutin, and lignins which form a protective barrier. This suberin is a complex polyester biopolymer and lipophilic which is composed of long-chain fatty acids called suberin acids and glycerol.

Complete answer:
So, this macromolecule suberin is highly hydrophobic and rubbery material. Suberin is deposited in the radial in roots and transverse cell walls of the endodermal cells which are known as the Casparian strip or Casparian band. It functions to prevent water and nutrients taken up by the root from entering the stele through the apoplast but water must bypass the endodermis via the symplast instead of that. So that this allows the plant to select the solutes that pass further into the plant. Thus, now it forms an important barrier to harmful solutes. The suberin is found in the phellem layer of the periderm (or cork) which is the outermost layer of the bark. The dead cells in this layer are abundant in suberin which prevents water loss from the tissues below. This macromolecule can also be found in various other plant structures. We can take an example that these are present in the lenticels on the stems of many plants, and the net structure in the rind of a netted melon is composed of suberised cells.

Additional Information:

1. Suberins and lignins are the two macromolecules which are considered covalently linked to lipids and carbohydrates, respectively, and lignin is covalently linked to suberin, and to a lesser extent, to cutting.
2. Suberin is a major constituent of cork and is named after the cork oak, Quercus suber. Its main function is as a barrier to the movement of water and solutes.
3. Suberin consists of two domains, a polyaromatic and a poly aliphatic domain. Within the primary cell wall, the polyaromatics are located predominantly, and the poly aliphatics are located between the primary cell wall and the cell membrane.
4. Casparian strips have pretty much the same function as tight junctions in animal epithelia. An extracellular (paracellular) diffusion barrier is shown by these strips to provide within the plant roots, forcing nutrients to pass into the cells and thus to be subjected to the action of plasma membrane transport proteins.
   So, the correct answer is, ‘(d) Phellem of stem and endodermis of root’.

Note: In addition to the aromatics and aliphatics components, glycerol has been reported as a major suberin component in some species. The glycerol interlink aliphatic monomers, and possibly also to link poly aliphatics to polyaromatics, during suberin polymer assembly.