Biology Essay

1107 words - 5 pages

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 ...view middle of the document...

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. What part would an enzyme play in
this reaction?

8 If you tried experiment 8, state in what ways the conditions for the reaction between starch and hydrochloric acid differed from those in the starch/saliva reaction.

Experiment 9. The action of saliva on starch - preparation

NOTE The use of saliva in school experiments is not banned but certain precautions should be taken:
Students should work with only their own saliva
They should wash out their own glassware
The tubes should be sterilised in 1% sodium hypochlorite (sodium chlorate (1)) (See the ASE’s ‘Safeguards in the School Laboratory’ 11e p.95

Outline This is similar to ‘Enzymes’ experiment 1 but includes a control. It is shown that normal saliva will act on starch to produce sugar while boiled saliva will not..

Prior knowledge Starch/iodide reaction, Benedict's reaction.

Advance preparation and materials-per group

2% starch solution * (freshly prepared)
iodine solution 5 cm3
Benedict's solution 15 cm3

Apparatus-per group

test-tube rack and 8 test-tubes dropping pipette...

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