Lactose is made up of:

Correct — D, Glucose and Galactose. Lactose is the sugar of milk, and it is a disaccharide: one unit of galactose joined to one unit of glucose through a beta-1,4 glycosidic bond, giving the formula C12H22O11. It is broken back into its two halves in the small intestine by lactase, an enzyme sitting in the brush border of the intestinal lining. Where lactase production falls away after childhood, the undigested lactose passes on to the colon and is fermented by gut bacteria, which is what lactose intolerance is. One more property is worth carrying: lactose is a reducing sugar, because the linkage leaves the glucose unit with a free anomeric carbon that can open into an aldehyde. Sucrose, whose two units are locked to each other through both their anomeric carbons, is not. The three common dietary disaccharides are best learned as a set — lactose is galactose plus glucose, sucrose is glucose plus fructose, and maltose is glucose plus glucose.

• (a)Glucose only — Two glucose units joined together give maltose, the sugar of malt and of starch digestion, not lactose. Lactose is built from two different monosaccharides.
• (b)Glucose and Fructose — That combination is sucrose — ordinary table sugar, obtained from sugarcane and sugar beet. Fructose has no part in lactose.
• (c)Glucose and Sucrose — A category error, and the most instructive wrong answer here. Sucrose is itself a disaccharide, so it cannot serve as a building block of another disaccharide. Only monosaccharides can.

Carbohydrates are grouped by how many sugar units they contain. Monosaccharides are single units — glucose, fructose, galactose, ribose. Disaccharides are two units joined by a glycosidic bond, with a molecule of water removed: sucrose, lactose and maltose. Polysaccharides are long chains — starch and glycogen as stores, cellulose as structure. Lactose belongs to the middle group and occurs naturally in the milk of mammals, at roughly 4 to 5 per cent in cow's milk and higher in human milk.

The question is really testing whether the three household disaccharides are kept apart, so learn them by their source as well as their parts. Cane sugar is sucrose and it carries fructose. Malt sugar is maltose and it is two glucoses. Milk sugar is lactose and it is the only one of the three that contains galactose — which is a useful anchor, because galactose almost never appears in the diet except as half of lactose. Option (c) rewards a different kind of thinking altogether: even a candidate who has forgotten which sugar is which can strike it out on the ground that a disaccharide cannot be a subunit of a disaccharide.

• Lactose is a disaccharide of galactose and glucose joined by a beta-1,4 glycosidic bond; its formula is C12H22O11.
• Lactase, in the brush border of the small intestine, hydrolyses lactose into galactose and glucose.
• Sucrose is glucose plus fructose; maltose is glucose plus glucose. All three share the formula C12H22O11.
• Lactose and maltose are reducing sugars; sucrose is not, because both its anomeric carbons are tied up in the linkage.
• Cow's milk contains roughly 4 to 5 per cent lactose, and it is the principal carbohydrate of mammalian milk.

• Swapping lactose and sucrose because both are white, sweet and familiar — fructose belongs to sucrose, galactose to lactose.
• Picking an option that names a disaccharide as a building block; only monosaccharides can be.
• Assuming all disaccharides are reducing sugars. Sucrose is the exception, and questions do turn on it.

Either as a direct 'X is made up of' item like this one, or as a matching set pairing sucrose, maltose and lactose with their monosaccharide pairs, or through the enzyme that digests each.

• practice — not a real PYQ

  Which one of the following disaccharides is composed of two glucose units?

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

  Answer(c) Maltose — two glucose units joined by an alpha-1,4 bond. Galactose is not a disaccharide at all but a single sugar unit.

• practice — not a real PYQ

  Deficiency of which enzyme in the small intestine causes lactose intolerance?

  • (a)Sucrase
  • (b)Lactase
  • (c)Amylase
  • (d)Pepsin

  Answer(b) Lactase — without it, lactose is not split into galactose and glucose and passes undigested into the colon, where bacteria ferment it.