Proteins are most abundant biomolecules of the living organism. The main source of proteins are milk, cheese, pulses, peanuts, fish meat etc. Amino acids are building block of proteins. Amino acids contain amino (–NH2) and carboxyl (–COOH) functional groups. Depending upon the relative position of amino group with respect to carboxyl group, the amino acids can be classified as α, β, γ, δ and so on. Only alpha amino acid are obtained on hydrolysis of protein. Amino acids are classified as acidic, or basic or neutral depending upon the relative number of amino and carboxyl group in their molecule.
Amino acids which can be synthesised in the body are known as non-essential amino acids. On the other hand, those which can not be synthesised in the body and must be obtained through diet are known as essential amino acids. Proteins are condensation product of amino acids which are linked through peptide bond or peptide linkage.
Proteins can be classified into two types on the basis of their molecular shape. (a) Fibrous and (b) Globular proteins. Proteins found in a biological system with unique three-dimensional structure and biological activity is called a native protein. When a protein in its native form is subjected to physical changes, such as change in temperature, or chemical changes, such as change in pH, hydrogen bonds are disturbed. As a result, globules unfold, helices uncoil, and protein loses its biological activity. This is called denaturation of protein.
The zwitterionic form of glycine is represented as
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → +NH3-CH2-COO−
An amino acid contains both an acidic –COOH group and a basic –NH2 group in the same molecule. In aqueous solution, the carboxyl group loses a proton and the amino group gains a proton, giving a dipolar ion called a zwitterion: it carries both a positive and a negative charge but has no net charge.
\( \displaystyle \ce{H2N-CH2-COOH <=> H3N^+-CH2-COO^-} \)
So the zwitterion of glycine is +NH3–CH2–COO−.
Why the other options are incorrect
Option (1) +NH3-CH2-COOH — Incorrect. This is the cation formed in acidic solution; it has only a positive charge.
Option (3) NH2-CH2-COOH — Incorrect. This is the neutral, un-ionised formula; it carries no charges.
Option (4) NH2-CH2-COO− — Incorrect. This is the anion formed in basic solution; it has only a negative charge.
NCERT (2026-27 edition):
Classification of Amino Acids – …
