Which chemical substance present in cork cells of tree bark makes them impervious to gases and water?

Introduction / Context:
Cork is the outer protective tissue of tree bark and is widely used for stoppers, insulation, and notice boards. It is lightweight and does not allow water or gases to pass through easily. From a biology and chemistry perspective, this property of cork cells is due to specific substances deposited in their cell walls. This question asks you to identify the chemical that makes cork cells impervious to gases and water.

Given Data / Assumptions:

• The tissue in question is cork, also called phellem, found in tree bark.
• We are interested in the chemical present in the cell walls of cork cells.
• Options include cellulose, suberin, pectin, lignin, and cutin.
• We assume basic knowledge of plant tissue structure from school level biology.

Concept / Approach:
Cork cells are dead at maturity and have walls impregnated with a waxy substance called suberin. Suberin is a complex hydrophobic material that makes the walls waterproof and gas resistant. This helps protect the plant from water loss and invasion by pathogens. Cellulose is the main structural component of all plant cell walls but is not by itself waterproof. Pectin and lignin also occur in plant tissues, but in cork it is suberin that is most closely associated with impermeability. Cutin is related to the cuticle on leaf surfaces rather than cork.

Step-by-Step Solution:
Step 1: Recall that cork cells form the outer bark and are arranged in layers, giving mechanical protection and insulation. Step 2: These cells are dead and have walls thickened with a waxy, hydrophobic material that prevents water and gases from crossing easily. Step 3: The name of this material is suberin, which is characteristic of cork tissues and parts of roots. Step 4: Cellulose forms the basic framework of plant cell walls but does not alone provide complete impermeability. Step 5: Pectin is mainly present in the middle lamella, helping to bind cells together, while lignin strengthens walls in xylem but is different from suberin. Step 6: Therefore, the correct chemical responsible for cork being impervious to gases and water is suberin.

Verification / Alternative check:
Botany textbooks that describe secondary growth and bark structure mention suberised walls in cork cells. They explain that suberin deposits make the cells impermeable and help reduce water loss. In contrast, discussions of xylem emphasise lignin, and the leaf cuticle is described as rich in cutin, not suberin. The term suberised is directly linked to cork, providing a strong clue that suberin is the relevant substance. This consistent association in plant anatomy literature supports the answer.

Why Other Options Are Wrong:
Cellulose: A universal component of plant cell walls, providing strength but not specifically responsible for making cork cells waterproof and gas proof.
Pectin: Found in the middle lamella and important for cell adhesion, not for impermeability in cork.
Lignin: Provides rigidity and strength in xylem vessels and sclerenchyma but is not the main waxy barrier material in cork.
Cutin: A waxy substance present mainly in the cuticle on leaf surfaces, not in cork cells of bark.

Common Pitfalls:
Students sometimes select lignin because they remember it as a strengthening material, or cellulose because it is the most common wall component. Others may choose cutin because it sounds waxy and water resistant, but it belongs mainly to leaves and herbaceous stems. To avoid confusion, connect the term cork specifically with suberin and remember that suberised cells are those impregnated with suberin, giving them water resistant properties.

Final Answer:
The chemical substance that makes cork cells impervious to gases and water is suberin.