Question

Pre-Lab Questions (PLQ) for Exp. 9 Carbohydrates (sugars) Use the information in Exp. 9 to answer the following questions 1. (2 pts) What is produced when polysaccharides and disaccharides are hydrolyzed? 2. (2 pts) A monosaccharides can exist as an open chain or as a cyclic compound. Which form is more stable? 3. (4 pts) Sucrose is the only carbohydrate for which it is necessary to make a fresh solution (all of the other carbohydrates will already be prepared in solution for you). When performing the Benedict's test (and the others), why is it necessary for YOU to make a fresh sucrose solution? 4. (4 pts) For the Seliwanoff's test, what do each of the possible color changes indicate? 5. (4 pts) Which of the three tests will be positive for a polysaccharide (starch) and what will a positive test look like? 6. (4 pts) After conducting the hydrolysis of sucrose (a disaccharide) and starch (a polysaccharide), which two tests will you perform to determine if hydrolysis took place?

Answer 1

1. Hydrolysis of polysaccharides involve the cleavage of the glycosidic bond, resulting in the formation of two, three, or several or many monosaccharides. Hydrolysis of disaccharides yield two molecules of either the same or two different monosaccharides.

2. Any ring formation generally tends to be energetically more stable than a open chain conformation. Thus monosaccharides in their cyclic form are more stable than in open form.

3. It is essential that the sucrose solution for Benedict's test and other tests is freshly prepared. Benedict's test is used to distinguish a non- reducing sugar from a reducing sugar. Now, sucrose is disaccharide and by itself a non-reducing sugar. But its constituent two monosaccharide sugars, are glucose and fructose; both of which are reducing sugars. Now if a solution of sucrose is prepared and left for long time, there is a slight hydrolysis which occurs and little amount of glucose and fructose would be produced. This would give us a positive Benedict's test, whereas for Sucrose we would be expecting a negative result.

4. Seliwanoff’s test is a chemical test which is used to distinguish between an aldose and a ketose sugar. The principle of this test is that when heated, ketose sugars are dehydrated more rapidly than aldoses. For a ketose sugar, a red color is formed rapidly indicating a positive test while for an aldose sugar, a slower forming light pink is observed.