Which of the following represent disaccharides?

Correct — C, Maltose and lactose. A disaccharide is a sugar that gives exactly two monosaccharide units when it is hydrolysed, the two being joined through an oxygen atom in what is called a glycosidic linkage. Maltose, the malt sugar formed when starch is broken down, hydrolyses to two molecules of glucose. Lactose, the sugar of milk, is composed of galactose and glucose joined between carbon 1 of the galactose and carbon 4 of the glucose. Both leave an aldehyde group free, so both are reducing sugars — unlike the third common disaccharide, sucrose, in which the reducing groups of glucose and fructose are locked into the linkage. Each of the other three options is drawn from a different rung of the same classification, which is what makes the question a test of the whole scheme rather than of two names: two polysaccharides, two monosaccharides that happen to be the building blocks of the correct answer, and two pentose sugars.

• (a)Starch and glycogen — Both are polysaccharides — long chains of many glucose units, starch as the storage carbohydrate of plants and glycogen as its animal counterpart. Polysaccharides are not sweet, which is why NCERT calls them non-sugars.
• (b)Glucose and galactose — The most tempting option, because these two are exactly the units that make up lactose. But each is a single six-carbon sugar that cannot be hydrolysed further, so both are monosaccharides.
• (d)Ribose and xylose — Both are pentoses, five-carbon monosaccharides. Ribose is the sugar of RNA — a nucleic acid containing ribose is ribonucleic acid — and xylose is the wood sugar obtained from plant hemicellulose.

Carbohydrates are polyhydroxy aldehydes or ketones, or molecules that yield such units on hydrolysis, and they are classified by how many units they give. Monosaccharides cannot be hydrolysed to anything simpler — glucose, fructose, galactose, ribose. Oligosaccharides give two to ten units, and the commonest are the disaccharides — sucrose, maltose and lactose. Polysaccharides give many units — starch, glycogen, cellulose. A second cut runs across the first: sugars that reduce Fehling's or Tollens' reagent are reducing sugars, and among the disaccharides maltose and lactose are reducing while sucrose is not.

Read the four options as a set and the design becomes obvious: the question has put one pair on each level of the classification and asked you to pick the middle one. That means a candidate who knows only that starch is a polysaccharide and glucose a monosaccharide has already halved the paper, and the choice between the two remaining pairs turns on recognising ribose as the sugar in RNA. It is worth learning the three common disaccharides together with what each hydrolyses into, because that single line answers a whole family of questions: sucrose gives glucose and fructose, maltose gives glucose and glucose, lactose gives glucose and galactose. Adults who cannot digest milk lack the enzyme lactase that splits that last linkage — the everyday face of this chemistry.

• A disaccharide yields two monosaccharide units on hydrolysis, joined through an oxygen atom in a glycosidic linkage.
• Sucrose gives glucose and fructose; maltose gives two molecules of glucose; lactose gives galactose and glucose.
• Maltose and lactose are reducing sugars because a free aldehyde group remains; sucrose is non-reducing because both reducing groups are tied up in the linkage.
• Starch, glycogen and cellulose are polysaccharides; glycogen is the animal storage form and starch the plant one, both built from glucose.
• Ribose is a five-carbon monosaccharide, and it is the sugar that distinguishes RNA from DNA, which carries 2-deoxyribose instead.

• Choosing glucose and galactose because they are the components of lactose; components of a disaccharide are monosaccharides by definition.
• Treating every sweet-tasting carbohydrate as a sugar in the chemical sense — polysaccharides are not sweet, and NCERT calls them non-sugars.
• Forgetting that maltose is made of two units of the same sugar; a disaccharide's two units need not be different.

Either as this classification item, or by naming one disaccharide and asking what it hydrolyses into, or by asking which of a list is a non-reducing sugar.

• practice — not a real PYQ

  On hydrolysis, sucrose yields which pair of monosaccharides?

  • (a)Glucose and glucose
  • (b)Glucose and galactose
  • (c)Glucose and fructose
  • (d)Fructose and galactose

  Answer(c) Glucose and fructose — sucrose gives an equimolar mixture of the two, and because both reducing groups are involved in the glycosidic bond, sucrose itself is a non-reducing sugar.

• practice — not a real PYQ

  Which one of the following is a polysaccharide?

  • (a)Maltose
  • (b)Glycogen
  • (c)Galactose
  • (d)Lactose

  Answer(b) Glycogen — it is the storage polysaccharide of animals, a branched chain of many glucose units, while maltose and lactose are disaccharides and galactose a monosaccharide.