Question: What is meant by essential amino acids? Why are amino acids amphoteric in nature?
Step 1 (Definition of Essential Amino Acids): [1 Mark]
Essential amino acids are those amino acids which cannot be synthesized in the human body and must be obtained through the diet. If they are not provided in the diet, they can cause deficiency diseases. Examples include Valine, Leucine, and Lysine.
Step 2 (Explanation of Amphoteric Nature): [0.5 Mark]
Amino acids are amphoteric because they contain both an acidic group ($-COOH$) and a basic group ($-NH_2$) within the same molecule. In aqueous solution, the carboxyl group can lose a proton and the amino group can accept a proton, giving rise to a dipolar ion known as a zwitterion.
The equilibrium reaction is represented as:
$$H_2N-CH(R)-COOH \rightleftharpoons H_3N^+-CH(R)-COO^-$$Step 3 (Final Concluded Answer): [0.5 Mark]
Due to the zwitterionic structure, amino acids react with both acids and bases:
- With base: The $-NH_3^+$ group loses a proton to form a salt.
- With acid: The $-COO^-$ group accepts a proton to form a salt.
Since they can neutralize both acids and bases, they are classified as amphoteric.
💡 EL10 Concept Insight: Think of an amino acid like a person who can speak two different languages; because it has both an "acid" part and a "base" part, it can "talk" (react) to both acids and bases, making it a chemical chameleon!