The side chains (R) of seven amino acids are given below.
HOOC–CH2–CH2–, H2N–(CH2)4–, HOOC–CH2–, H2N–CO–CH2–CH2–, HO–CH2–, HS–CH2–, (p)HO–C6H4–CH2–
How many of these amino acids are acidic?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 2
An amino acid is acidic when it has more carboxyl groups than amino groups. The common part of every amino acid already has one –NH2 and one –COOH, so it is the side chain that decides: an extra –COOH in R makes the amino acid acidic.
Acidic:
HOOC–CH2–CH2– (glutamic acid) (extra –COOH)
HOOC–CH2– (aspartic acid) (extra –COOH)
Not acidic:
H2N–(CH2)4– (lysine) — extra –NH2, so basic.
H2N–CO–CH2–CH2– (glutamine) — an amide group, not a carboxyl group.
HO–CH2– (serine) — an –OH group, so neutral.
HS–CH2– (cysteine) — an –SH group, so neutral.
(p)HO–C6H4–CH2– (tyrosine) — a phenolic –OH, not a carboxyl group, so neutral.
Acidic amino acids: glutamic acid and aspartic acid = 2.
NCERT (2026-27 edition):
10.2.2 Classification of Amino Acids – “Amino acids are classified as acidic, basic or neutral depending upon the relative number of amino and carboxyl groups in their molecule. Equal number of amino and carboxyl groups makes it neutral; more number of amino than carboxyl groups makes it basic and more carboxyl groups as compared to amino groups makes it acidic.”