Which of the following is a water proofing substance secreted by the cork and endodermal cells ?

Question

Accepted Answer

The correct option is (2): Suberin

Suberin is the correct answer because it is the specific, highly effective water-proofing substance characteristic of both cork and endodermal cells in plants. Chemically, suberin is a complex lipophilic polyester biopolymer, meaning it is fat-like and composed of long-chain fatty acids, fatty alcohols, and phenolic compounds. In **cork cells** (phellem), which form the outermost protective layer of the bark, suberin is heavily deposited into the cell walls. This deposition causes the cells to die and creates a tough, impermeable barrier that prevents water loss from the tissues beneath and protects the plant from mechanical injury and pathogen invasion. Similarly, in the **endodermis of roots**, suberin is selectively deposited on the radial and transverse cell walls to form a ring-like structure known as the Casparian strip. This strip forces water and dissolved minerals to enter the vascular cylinder (stele) via the cytoplasm (symplastic pathway) instead of passing through the cell walls (apoplastic pathway), allowing the plant to actively regulate what is absorbed, a critical function that relies entirely on suberin’s water-repelling property.

The incorrect option is (1): Glycocalyx

Glycocalyx is incorrect because it is a term from animal and microbial biology, not plant water-proofing. It is a carbohydrate-rich layer (composed of glycoproteins and glycolipids) found on the exterior of virtually all animal cells and on the cell walls of many bacteria. Its primary functions include cell-to-cell recognition, cell adhesion, and providing a protective shield for the plasma membrane. It has no role in forming the structural water barrier in the cork or endodermal cells of plants, making it irrelevant to the context of the question.

The incorrect option is (3): Tannin

Tannins are incorrect as they are a class of secondary plant metabolites, specifically polyphenolic compounds. Their primary roles are defensive, such as deterring herbivores through their bitter taste (astringency) and acting as anti-microbial agents against pathogens. They are typically stored in the cell vacuoles, and sometimes in the cell walls, but they do not serve as the primary, structural water-proofing polymer that constitutes the bulk of the barrier in cork and endodermis. Therefore, while a plant defense component, they are not the water-proofing material in question.

The incorrect option is (4): Cutin

Cutin is also an incorrect option, although it is a water-proofing substance and is chemically related to suberin. Cutin is the major component of the **cuticle**, the waxy, protective layer found exclusively on the outer surface of epidermal cells of leaves and young stems. Its function is indeed to reduce water loss (transpiration) from aerial parts of the plant. However, it is not the substance secreted by the inner protective layers mentioned in the question: the **cork** (periderm) and the **endodermis** of the root have suberin, not cutin, as their primary water-proofing material.