Experiment 9. The action of saliva on starch
Study the flow chart on p. 9.02 for a few minutes to gain an idea of the outline of the experiment.
(a) Prepare a water bath by using a Bunsen burner to heat some water in a beaker on a tripod and gauze till it boils; then turn the flame down to keep the water just boiling. While waiting for the water to boil, carry on from (b).
(b) Label eight test-tubes 1 - 8 and in tube 1 collect saliva as follows:
(i) Thoroughly rinse the mouth with water to remove food residues (ii) Collect about 50 mm saliva.
(c) Pour half the saliva into tube 2 and place the tube in the boiling water bath for 3 minutes.
(d) Using a graduated pipette or syringe, add 5 cm3 2% starch solution to tubes 3,4 and 7.
(e) Rinse the pipette or syringe and use it to transfer 5 cm3 boiled saliva from tube 2 to tube 3. Shake the tube sideways to mix the contents.
(f) Use the graduated pipette or syringe to transfer 5 cm3 unboiled saliva from tube 1 to tube 4. Shake the tube to mix the contents.
(g) Leave tubes 3 and 4 to stand for five minutes and copy the table below into your notebook.
(h) After five minutes, pour half the contents of tube 3 (the boiled saliva and starch) into tube 5 and add three drops of iodine solution to tube 5.
(i) To the remaining liquid in tube 3, add about 20 mm Benedict's solution and place the tube in the boiling water bath for 5 minutes.
(j) Pour half the contents of tube 4 (starch and saliva) into tube 6 and then add three drops of iodine to tube 6.
(k) Test the remaining liquid in tube 4 with Benedict's solution as you did in (i).
(l) Pour half the contents of tube 7 (starch solution) into tube 8 and test the two samples respectively with iodine as in (h) and Benedict's solution as in (i). Record the results in your table.
|Tube |Contents |Tested with |Result |Interpretation | |3 |starch and boiled saliva |Benedict’s solution | | | |4 |starch and saliva |Benedict’s solution | | | |5 |starch and boiled saliva |iodine | | | |6 |starch and saliva |iodine | | | |7 |starch solution (control) |iodine | | | |8 |starch solution (control) |Benedict’s solution | | |
Experiment 9. Discussion
1 What substances do iodine and Benedict's solution test for?
2 What change takes place when starch and saliva are mixed, according to the results in tubes 4 and 6?
3 Tubes 3 and 5 probably did not give the same results as tubes 4 and 6. In what way were the contents treated that could account for this difference?
4 (a) Are your results consistent with the hypothesis (theory) that an enzyme in saliva has changed starch to sugar?
(b) Do your results prove that an enzyme in saliva has changed starch to sugar?
5 In what way do the results with tubes 3 and 5 support the enzyme hypothesis?
6 Do your experimental results rule out the possibility that (a) starch converts unboiled saliva to sugar or (b) starch and unboiled saliva combine chemically to form sugar?
7 The starch molecule consists of a long chain of carbon atoms with oxygen and hydrogen atoms attached. A sugar, such as glucose, has molecules consisting of six carbon atoms with oxygen and hydrogen atoms attached (see p. 8.02). Using this information, suggest a way in which sugar could be formed from starch....
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