Give a plausible explanation for each of the following:
(a) Glucose doesn't give the 2,4-DNP test.
(b) The two strands in DNA are not identical but are complementary.
(c) Starch and cellulose both contain glucose units as monomer, yet they are structurally different.
(a) Glucose doesn't give the 2,4-DNP test.
(b) The two strands in DNA are not identical but are complementary.
(c) Starch and cellulose both contain glucose units as monomer, yet they are structurally different.
Solution
(a) Glucose exists overwhelmingly as the pyranose hemiacetal; with no free aldehyde, 2,4-DNP finds nothing to attack.
(b) The bases pair strictly A with T and G with C across the helix — so the strands carry different, mutually determined sequences: \textbf{complementary}, not identical.
(c) \textbf{Starch} is built of α-D-glucose (α-1,4 links, plus α-1,6 branches) and coils helically; \textbf{cellulose} is built of β-D-glucose (β-1,4 links) in straight chains that hydrogen-bond into rigid fibres — same monomer, different linkage, different structure.
(b) The bases pair strictly A with T and G with C across the helix — so the strands carry different, mutually determined sequences: \textbf{complementary}, not identical.
(c) \textbf{Starch} is built of α-D-glucose (α-1,4 links, plus α-1,6 branches) and coils helically; \textbf{cellulose} is built of β-D-glucose (β-1,4 links) in straight chains that hydrogen-bond into rigid fibres — same monomer, different linkage, different structure.
CBSE Class 12 Board · 2020 · Subjective