6: Nutrition in Humans - Practical

Question 1

6.2: Investigation of the action of pepsin

Three wells are made in a milk-agar. Pepsin solution and dilute hydrochloric acid is put in the well A, distilled water and dilute hydrochloric acid in well B, boiled pepsin solution and dilute hydrochloric acid in well C.
Why is dilute hydrochloric acid added into the three test tubes?

Answer 1

To provide an acidic medium for the action of pepsin.

Question 2

6.2: Investigation of the action of pepsin

Three wells are made in a milk-agar using a cork borer which is heated at the end.
Why does the cork borer need to be heated at the end in a Bunsen flame? (2)

Answer 2

This is to sterilise the cork borer so that no bacteria on the cork borer digests the proteins in the milk-agar, and the digestion of the proteins in the milk-agar can be attributed to the action of pepsin only.
This increases the validity of the experiment.

Question 3

6.2: Investigation of the action of pepsin

Three wells are made in a milk-agar and different solutions are added into the wells.
Why should the lid of the milk-agar plate be replaced as quickly as possible?

Answer 3

To minimize the exposure of the milk-agar plate to air and reduce the chance of contamination due to microorganisms in air.

Question 4

6.2: Investigation of the action of pepsin

Three wells are made in a milk-agar and different solutions are added into the wells. Pepsin solution and dilute hydrochloric acid is put in the well A, distilled water and dilute hydrochloric acid in well B, boiled pepsin solution and dilute hydrochloric acid in well C.
What are the purposes of setting up wells B and C?

Answer 4

Well B is a control to show that any clear zone formed around well A is due to the presence of pepsin.
Well C is a control to show that any clear zone formed around well A is due to the enzymatic action of pepsin.

Question 5

6.2: Investigation of the action of pepsin

Three wells are made in a milk-agar and different solutions are added into the wells. Pepsin solution and dilute hydrochloric acid is put in the well A, distilled water and dilute hydrochloric acid in well B, boiled pepsin solution and dilute hydrochloric acid in well C.
Explain the results of this experiment. (3)

Answer 5

A clear zone is formed around well A. Pepsin in well A diffuses to the milk-agar and catalyses the digestion of the milk protein into soluble substances, hence forming a clear zone in the milk-agar.
Well B contains no pepsin, so the milk proteins around well B have not been digested and no clear zone is formed around the well.
Boiling denatures pepsin, and the pepsin in well C is no longer able to catalyse the breakdown of milk proteins due to a permanent conformational change in its active site, so no clear zone is formed around well C.

Question 6

6.3: Demonstration of the effect of bile salts on oil

Explain the differences in the results when oil is mixed with distilled water instead of bile salt solution.

Answer 6

Water cannot break down oil into tiny droplets as the bile salt solution does. Therefore, no emulsion is formed and two layers of liquids can be seen.

Question 7

6.4: Investigation of the action of lipase

An equal amount of full-cream milk, sodium carbonate solution, and phenolphthalein indicator are added to three boiling tubes A to C. In addition, 1mL lipase solution is added to tube A, 1mL distilled water in tube B, and 1mL boiled lipase solution in tube C.
What are the purposes of setting up tubes B and C?

Answer 7

Tube B is a control to show that any colour change in the solution in tube A is due to the presence of lipase.
Tube C is a control to show that any colour change in the solution in tube A is due to the enzymatic action of lipase.

Question 8

6.4: Investigation of the action of lipase

An equal amount of full-cream milk, sodium carbonate solution, and phenolphthalein indicator are added to three boiling tubes A to C. In addition, 1mL lipase solution is added to tube A, 1mL distilled water in tube B, and 1mL boiled lipase solution in tube C.
What is the independent variable of this experiment?

Answer 8

The presence of active lipase.

Question 9

6.4: Investigation of the action of lipase

An equal amount of full-cream milk, sodium carbonate solution, and phenolphthalein indicator are added to three boiling tubes A to C. In addition, 1mL lipase solution is added to tube A, 1mL distilled water in tube B, and 1mL boiled lipase solution in tube C.
What is the dependent variable of this experiment, and how is it measured? (2)

Answer 9

The action of lipase,
which is indicated by the production of fatty acids due to the hydrolysis of lipids, and measured by the change in the pH shown by the phenolphthalein indicator.

Question 10

6.4: Investigation of the action of lipase

An equal amount of full-cream milk, sodium carbonate solution, and phenolphthalein indicator are added to three boiling tubes A to C. In addition, 1mL lipase solution is added to tube A, 1mL distilled water in tube B, and 1mL boiled lipase solution in tube C.
What is the significance of adding sodium carbonate solution to all test tubes? (2)

Answer 10

The alkaline nature of the sodium carbonate solution makes the phenolphthalein indicator turn pink.
This ensures all the reacting solutions are alkaline, and the contents of the test tube are pink in colour at the start of the experiment, to observe for any colour change at the end of the experiment.

Question 11

6.4: Investigation of the action of lipase

An equal amount of full-cream milk, sodium carbonate solution, and phenolphthalein indicator are added to three boiling tubes A to C. In addition, 1mL lipase solution is added to tube A, 1mL distilled water in tube B, and 1mL boiled lipase solution in tube C.
The milk and lipase solutions should be shaken well before use. Explain the significance of this step. (2)

Answer 11

This ensures the validity of the experiment;
if these viscous solutions are not well shaken before the experiment, there might be lipids or proteins settled at the bottom of the solution.

Question 12

6.4: Investigation of the action of lipase

An equal amount of full-cream milk, sodium carbonate solution, and phenolphthalein indicator are added to three boiling tubes A to C. In addition, 1mL lipase solution is added to tube A, 1mL distilled water in tube B, and 1mL boiled lipase solution in tube C.
Explain a significant assumption made in this investigation.

Answer 12

Fatty acids produced by the hydrolysis of lipids in milk is the only substance that will cause the pH to decrease in the reaction set-up.

Question 13

6.4: Investigation of the action of lipase

An equal amount of full-cream milk, sodium carbonate solution, and phenolphthalein indicator are added to three boiling tubes A to C. In addition, 1mL lipase solution is added to tube A, 1mL distilled water in tube B, and 1mL boiled lipase solution in tube C.
Explain the results of the investigation. (4)

Answer 13

The mixture changes from pink to white in tube A. This is because lipase catalyses the breakdown of lipids in the full-cream milk into fatty acids and glycerol. The fatty acids cause the pH of the solution to drop to a value below the pH for colour change of phenolphthalein, and the phenolphthalein turns from pink to colourless.
Tube B contains no lipase, so the lipids in milk in tube B have not been digested.
Boiling denatures lipase, and the lipase in well C is no longer able to catalyse the breakdown of lipids in milk due to a permanent conformational change in its active site.
No fatty acids are formed to lower the pH in both tubes B and C. The pH remains at the level where phenolphthalein indicator is pink, so the tubes both remain pink.

Question 14

6.4: Investigation of the action of lipase

An equal amount of full-cream milk, sodium carbonate solution, and phenolphthalein indicator are added to three boiling tubes A to C. In addition, 1mL lipase solution is added to tube A, 1mL distilled water in tube B, and 1mL boiled lipase solution in tube C.
Explain a source of error in this investigation and suggest a modification. (2)

Answer 14

The colour change of the phenolphthalein indicator is subjective.
It can be modified by using a pH sensor to measure pH, instead of using phenolphthalein indicator.

Question 15

6.5: Simulation of digestion and absorption using dialysis tubing

A dialysis tubing is filled with starch and amylase solution, rinsed on the outside, and put into a boiling tube of distilled water at 37ºC.
Why is a boiling tube instead of a beaker used to bath the dialysis tubing?

Answer 15

Less water allows a higher concentration of starch or reducing sugar molecules for easy detection.

Question 16

6.5: Simulation of digestion and absorption using dialysis tubing

A dialysis tubing is filled with starch and amylase solution, rinsed on the outside, and put into a boiling tube of distilled water at 37ºC.
State 5 limitations of this experimental set-up in simulating the proces of digestion and absorption in the small intestine. (5)

Answer 16

Maltose molecules are not small enough to pass through the epithelium of the small intestine.
The small intestine can absorb digested food by active transport but the dialysis tubing cannot.
The small intestine shows peristalsis but the set-up does not.
There are many types of food molecules in the small intestine apart from starch.
The food molecules have to pass through more than one layer of cells instead of only one layer of tubing.

Question 17

6.5: Simulation of digestion and absorption using dialysis tubing

A dialysis tubing is filled with starch and amylase solution, rinsed on the outside, and put into a boiling tube of distilled water at 37ºC.
Suggest two ways which enables the results to be obtained faster. (2)

Answer 17

Diffusion rate of the reducing sugar molecules can be increased by stirring the surrounding water and using a water bath at a higher temperature.
More concentrated solutions of starch and amylase can be used to increase the reaction rate by increasing the chance of successful collision of starch and amylase molecules to form enzyme-substrate complexes. As the rate of reaction increases, the rate of product formation increases and the steepness of concentration gradient of the reducing sugar molecules inside the dialysis tubing and outside the dialysis tubing increases, so the reducing sugars will diffuse from the inside of the dialysis tubing to the boiling tube at a higher rate, allowing results to be obtained faster.

Question 18

Extra: Action of lipase in different types of milk

An equal amount of sodium carbonate solution, phenolphthalein indicator, and 1mL lipase solution are added to two boiling tubes A and B. In addition, 5mL full cream milk is added to tube A, and 5mL skimmed milk is added to tube B. The time taken for the phenolphthalein indicator to change colour is recorded.
What is the independent variable of this investigation?

Answer 18

The type of milk used.

Question 19

Extra: Action of lipase in different types of milk

An equal amount of sodium carbonate solution, phenolphthalein indicator, and 1mL lipase solution are added to two boiling tubes A and B. In addition, 5mL full cream milk is added to tube A, and 5mL skimmed milk is added to tube B. The time taken for the phenolphthalein indicator to change colour is recorded.
What is the dependent variable of this investigation, and how is it measured? (2)

Answer 19

The activity of lipase,
indicated by the rate of fatty acid production upon the hydrolysis of lipids, and measured by the time it takes for the phenolphthalein indicator to turn from pink to colourless.

Question 20

Extra: Action of lipase in different types of milk

An equal amount of sodium carbonate solution, phenolphthalein indicator, and 1mL lipase solution are added to two boiling tubes A and B. In addition, 5mL full cream milk is added to tube A, and 5mL skimmed milk is added to tube B. The time taken for the phenolphthalein indicator to change colour is recorded.
Explain a significant assumption in this investigation.

Answer 20

Fatty acids produced by the hydrolysis of lipids in milk is the only substance that will cause the pH to decrease in the reaction set-up.

Question 21

Extra: Action of lipase in different types of milk

Wells are made in a full cream milk-agar and a skimmed milk-agar, and 1mL lipase solution is added to each well. After a set time of incubation, the diameter of the clear zones of the milk-agars are measured.
What is the dependent variable of this investigation, and how is it measured? (2)

Answer 21

The activity of lipase,
indicated by the rate of breakdown of lipids in the milk-agars, and measured by the diameter of clear zone in the milk-agars.

Question 22

Extra: Action of lipase in different types of milk

Wells are made in a full cream milk-agar and a skimmed milk-agar, and 1mL lipase solution is added to each well. After a set time of incubation, the diameter of the clear zones of the milk-agars are measured.
Explain whether sodium carbonate and phenolphthalein indicator is needed in this set-up. (2)

Answer 22

No, the use of the sodium carbonate is to turn the phenolphthalein indicator pink so pH changes can be observed through the colour change of the phenolphthalein indicator.
Since the investigation does not require pH change to be observed, no sodium carbonate nor pH indicators is needed.

Question 23

Extra: Action of lipase in different types of milk

Wells are made in a full cream milk-agar and a skimmed milk-agar, and 1mL lipase solution is added to each well. After a set time of incubation, the diameter of the clear zones of the milk-agars are measured.
Explain two significant assumptions made in this investigation. (2)

Answer 23

The diameter of the clear zone is directly proportional to the lipase activity, and that the lipase activity is the only factor for formation of clear zone in the of milk-agars.