Give the plausible explanation for the following :
(a)Glucose doesn’t give 2,4-DNP test.
(b)The two strands in DNA are not identical but are complementary.
(c) Starch and cellulose both contain glucose unit as monomer, yet they are structurally different.

Solution:

(A)Although, glucose in its open chain structure contains a free aldehydic group yet it does not give 2,4-DNP test, a characteristic reaction of aldehydes, i.e. -CHO groups. This is because glucose actually exists in the cyclic hemiacetal form with only a small amount of the open chain form in equilibrium. Since, the concentration of the open chain form is very low and its reactions with 2,4-DNP are reversible, thus its 2,4-DNP adduct is generally not observed.

(b) The two stands are complementary to each other because the hydrogen bonds are formed between specific pairs of bases. Adenine forms hydrogen bond with thymine whereas cytosine forms hydrogens bonds with guanine.

(c) ) Starch consists of amylose and amylopectin. Amylose is a long unbranched chain of α−D−(+)−α-D-(+)-glucose units held together by 1, 4-glycosidic linkage. Amylopectin is a branched chain polymer of α−D−α-D-glucose units in which the chain is formed by C₁-C₄ glycosidic linkage. Cellulose is a straight chain polysaccharide composed of β−D−β-D-glucose units which are joined by glycosidic linkage between C₁ of one glucose unit and C₄ of the next glucose unit.