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Fructose and the disaccharides: sucrose, maltose and lactose sheet
3-mark answer frame

What is a glycosidic linkage? Name the monosaccharides and the linkage in sucrose and maltose, and explain why only one of them is a reducing sugar.

How to lay out the answer. Follow the frame, then write it in your own words.

  1. 1 · Define. A glycosidic linkage is the oxide link joining two monosaccharide units, formed with the loss of a water molecule.
  2. 2 · Sucrose. α-D-glucose + β-D-fructose, joined C1 of glucose to C2 of fructose.
  3. 3 · Maltose. Two α-D-glucose units, joined C1 of one to C4 of the other.
  4. 4 · Explain the reducing nature. In sucrose both reducing groups are tied up, so it is non-reducing. In maltose, C1 of the second glucose is free and can open to CHO, so it is reducing.
  5. 5 · Close in one line. So whether a disaccharide reduces Fehling's solution depends on which carbon atoms form the link.

Scoring tip: Always name the carbon atoms in the link (C1 to C2, C1 to C4). That detail is the mark.