Eduqas Biology for A Level Book 1: 2nd Edition

Disaccharides Disaccharides are composed of two monosaccharide units bonded together with the formation of a glycosidic bond and the elimination of water. This is an example of a condensation reaction. The diagram shows water being removed between C4 of one glucose molecule and C1 of the other. The bond formed between glucose molecules is a glycosidic bond. It is between C1 and C4, so it is called a 1,4-glycosidic bond. Because the disaccharide molecule is straight and not twisted, the bond is an α -1,4-glycosidic bond. Formation of a glycosidic bond between two glucose molecules, making maltose Hydrolysis of the glycosidic bond in maltose The table summarises information about disaccharides: Disaccharide Component monosaccharides Biological role maltose glucose + glucose in germinating seeds sucrose glucose + fructose transport in phloem of flowering plants lactose glucose + galactose in mammalian milk Testing for the presence of sugars Reducing sugars are sugars that can donate an electron. The Benedict’s test detects reducing sugars in a solution. The reducing sugar donates an electron to reduce copper (II) ions in copper sulphate solution, which is blue. The Cu(II) ions are reduced to Cu(I) ions in red copper (I) oxide. Cu 2+ + e – ------> Cu + blue red The test is carried out as follows: Equal volumes of Benedict’s reagent and the solution being tested are heated to at least 70ºC. If a reducing sugar, such as glucose, is present, the solution will change colour from blue through green, yellow and orange until finally a brick–red precipitate forms. This test does not tell you the actual concentration of reducing sugar, so it is described as a qualitative test. Fig 02 O 1 4 1 4 OH H O HO CH 2 OH CH 2 OH CH 2 OH CH 2 OH glycosidic bond α-glucose O OH HO α-glucose O HO + H O maltose condensation O OH O O 1 4 1 4 HO O OH O HO OH HO hydrolysis + H 2 O O OH + O CH 2 OH CH 2 OH CH 2 OH CH 2 OH Study point When writing a chemical name, a word and a number are separated by a hyphen; numbers are separated by commas. Study point The names of sugars are abbreviated by using their first three letters: glu = glucose, fru = fructose, gal = galactose, mal = maltose, suc = sucrose, lac = lactose. Study point Reducing sugars include all monosaccharides and some disaccharides, e.g. maltose. Link An experiment using Benedict’s solution in a quantitative test is on p28. 17  1.1 Chemical elements and biological compounds