Question

What are essential amino acids? How do you explain the amphoteric behaviour of amino acids?

Hint:

Amphoteric behaviour is entirely due to the formation of a Zwitterion, where the molecule carries both positive and negative charges internally.

Answer 1

Step 1: Concept
Nutritional classification and fundamental chemical properties of amino acids.

Step 2: Meaning
Essential amino acids are physiologically required but cannot be synthesized internally. Amphoteric substances can react as both acids and bases.

Step 3: Analysis
Essential amino acids must be obtained through the diet (e.g., Valine, Leucine). Structurally, amino acids contain an acidic carboxyl group ($-COOH$) and a basic amino group ($-NH_{2}$). In an aqueous solution, the carboxyl group loses a proton to the amino group, forming an internal salt known as a zwitterion (dipolar ion). This zwitterion can accept a proton in acidic conditions and donate a proton in basic conditions.

Final Answer: Essential amino acids are those which cannot be synthesized in the human body and must be obtained through the diet. Amino acids exhibit amphoteric behaviour because they exist as dipolar zwitterions in aqueous solutions. The basic $-NH_{3}^{+}$ part can react with acids, and the acidic $-COO^{-}$ part can react with bases.