**Animal Kingdom: The Hardest Substance**

The hardest substance in the animal kingdom is undoubtedly the tooth enamel. Tooth enamel is the outer layer of the teeth, and it is considered the hardest and most highly mineralized substance in the bodies of animals. Enamel is primarily composed of hydroxyapatite, a crystalline calcium phosphate mineral that provides exceptional hardness and resilience.

**Composition and Structure of Tooth Enamel:**

Tooth enamel is primarily composed of inorganic minerals, making it highly resistant to wear and decay. It consists of approximately 96% mineral content, including hydroxyapatite crystals, which are arranged in a highly organized structure. The remaining 4% of enamel is comprised of water and organic materials.

**Properties of Tooth Enamel:**

1. Hardness: Tooth enamel is extremely hard and durable, ranking 5 on the Mohs scale of mineral hardness. This means it can withstand significant mechanical stress without easily fracturing.

2. Strength: Enamel is a remarkably strong substance due to its tightly packed hydroxyapatite crystals. This strength allows tooth enamel to protect the underlying dentin and dental pulp from external forces.

3. Wear Resistance: The hardness and strength of tooth enamel make it highly resistant to wear caused by chewing, biting, and other oral activities. Enamel protects the teeth from mechanical abrasion and damage.

4. Acid Resistance: Enamel has excellent resistance to acidic substances, such as those found in certain foods and beverages. This characteristic prevents the enamel from eroding and protects the teeth from decay and cavities.

**Plant Kingdom: The Hardest Substance**

In the plant kingdom, the hardest substance is lignin. Lignin is a complex organic polymer that provides structural support and rigidity to plant cell walls. It is responsible for the strength and durability of various plant structures, such as tree trunks, stems, and branches.

**Composition and Structure of Lignin:**

Lignin is composed of a complex arrangement of phenolic compounds, including coniferyl alcohol, sinapyl alcohol, and p-coumaryl alcohol. The precise arrangement and composition of these compounds determine the physical properties of lignin.

**Properties of Lignin:**

1. Structural Support: Lignin contributes to the rigidity and strength of plant structures, allowing them to stand upright and resist mechanical forces, such as wind and gravity.

2. Water Resistance: Lignin is highly hydrophobic, meaning it repels water. This property helps to prevent plant cell walls from absorbing excessive moisture, which could lead to structural weakness or decay.

3. Chemical Resistance: Lignin is resistant to degradation by many chemical agents, including enzymes and microorganisms. This resistance allows plant structures to maintain their integrity and durability over extended periods.

4. Insulation: The presence of lignin in plant cell walls provides insulation against temperature fluctuations. This insulation helps protect the plant from extreme heat or cold, ensuring its survival in diverse environments.

In conclusion, tooth enamel is the hardest substance in the animal kingdom, known for its exceptional hardness, strength, wear resistance, and acid resistance. On the other hand, lignin is the hardest substance in the plant kingdom, providing structural support, water resistance, chemical resistance, and insulation to plant cell walls.

1 enamel
2 sporopollenin