Experiment 12. The action of lipase

There are three or four liquids to be added to each test-tube, in various combinations, so look at the table at the foot of this page to get an idea of the final contents of each tube.

(a) Label three test-tubes 1-3.

(b) Using a graduated pipette or syringe, place 5 cm3 milk in each tube.

(c) Rinse the pipette or syringe and use it to place 7 cm3 dilute (M/I0) sodium carbonate solution in each tube. This solution is to make the mixture alkaline.

(d) Rinse the pipette or syringe and use it to place 1 cm3 3% bile salts solution in tubes 2 and 3 only.

(e) Use a dropping pipette to add phenolphthalein solution to each tube until the contents
are bright pink. About six drops will be sufficient and equal quantities should be added to each tube.
Phenolphthalein is a pH indicator. In alkaline solutions (above pH10) it is pink; in 'acid' solutions (below pH 8.3) it is colourless.

(f) In a spare test-tube, place about 15 mm of 5% lipase solution and, using a test-tube holder, heat the liquid over a small Bunsen flame until it boils for a few seconds.
Cool the tube under the tap and, using the graduated pipette or syringe, transfer 1 cm3 of the
boiled liquid to tube 2.

(g) With the graduated pipette or syringe, place 1 cm3 unboiled lipase solution in tubes 1 and 3.

(h) Note the time. Shake the tubes to mix the contents, return them to the rack and copy
the table below into your notebook, observing the tubes from time to time.

(i) Note the time required for the contents of each tube to go white and then complete
the table of results.

Action of lipase on milk
Tube All three tubes contain milk, sodium carbonate and phenolphthalein plus: Time taken to change from pink to white
1 lipase only
2 boiled lipase and bile salts
3 lipase and bile salts

Experiment 12. Discussion

1 What food substances are present in milk?

2 If phenolphthalein changes from pink to colourless, what kind of chemical change must have taken place in the tube?

3 Recall (or look up) the final products of digestion of the principal classes of food and
write down which of these products could be formed by the digestion of milk.

4 Which of the final products of digestion of milk could be responsible for the change of
conditions in the test-tube?

5 Which part of the experiment suggests that lipase acts as an enzyme?

6 What chemical change could the lipase be producing which would account for the colour change in the test-tubes?

7 Which part of the experiment indicates that bile salts do not contain an enzyme which affects milk (at least in the way being investigated here)? Explain your reasoning.

8 From the results, assuming that lipase is an enzyme, what part do the bile salts appear to be playing in the reaction (in general terms)?

9 Do the results tell you whether lipase is acting on the fat or the protein in milk? Explain.

Experiment 12. The action of lipase - preparation

Outline Lipase hydrolyses the fat in milk to fatty acids which react with sodium carbonate to
lower the pH of the mixture. This pH change is observed by using phenolphthalein.

Prior knowledge The use of indicators to observe pH changes; the final digestion products of
proteins, carbohydrates and fats.

Advance preparation and materials - per group

milk 20 cm3 phenolphthalein 5 cm3 [1]
3% bile salts 10 cm3 [2] 5% lipase solution 10 cm3 [3]
0.05M sodium carbonate 40 cm3

Apparatus - per group

test-tube rack and 4 test-tubes Bunsen burner
3 labels or spirit marker test-tube holder,
dropping pipette beaker or jar (for rinsing pipette or syringe)
graduated pipette or syringe 10 cm3

- per class

Results Tubes 1 and 3 will probably change from pink to white in about 4 minutes. However, since the efficacy of lipase varies, it is advisable to try out the experiment (for tube 3 only) beforehand and, if the reaction is too slow, reduce the volume of sodium carbonate solution or place the tubes in a water bath at 35C.

1. 1 g dissolved in 200 cm3 ethanol.
2. Available as sodium tauroglycocholate from Philip Harris.
3. Preferably freshly prepared but will keep for