The incorrect statement from the following with respect to carbohydrates is :
- All monosaccharides are reducing sugars.
- The monosaccharide units obtained from hydrolysis of oligosaccharides are always the same.
- Starch and cellulose are typical examples of polysaccharides, which are very high molecular weight compounds of more than ten monosaccharide units.
- Open chain and cyclic structures co-exist at equilibrium that are responsible for certain properties as in the case of D-(+)-glucose.
Answer
(2) The monosaccharide units obtained from hydrolysis of oligosaccharides are always the same.
Solution
(A) All monosaccharides, aldoses and ketoses, reduce Fehling's solution and Tollens' reagent ⇒ reducing sugars ⇒ correct.
(B) Oligosaccharides give 2–10 monosaccharide units on hydrolysis; these may be the same (maltose → 2 glucose) or different (sucrose → glucose + fructose, lactose → galactose + glucose) ⇒ 'always the same' is incorrect.
(C) Starch and cellulose are polysaccharides made of a large number of monosaccharide units ⇒ correct.
(D) D-(+)-glucose exists as the open chain form and cyclic α- and β-forms in equilibrium, which explains properties such as no reaction with NaHSO3 and mutarotation ⇒ correct.
∴ The incorrect statement is (B).
Answer: (2)