Human Diet & Digestion (Cambridge (CIE) IGCSE Biology)

A group of students used the experimental setup shown in Fig. 1 to model the effects of the digestion of starch in the human digestive system. 

The aim is to determine the optimum pH for the action of amylase on starch.

Fig. 1

The students set up 3 beakers the same as the one shown in Fig. 1. They used buffer solutions to keep the pH of each experimental setup at the following three pH levels: pH4, pH8 and pH10.

Visking tubing (sometimes called dialysis tubing) is a non-living, partially permeable membrane made from cellulose. Pores in the membrane are small enough to prevent the passage of large molecules (such as starch and sucrose) but allow smaller molecules (such as glucose) to pass through by diffusion.

(i)

State the independent variable in this investigation.

[1]

(ii)

Suggest two tests that can be carried out to determine the progress of the enzyme-catalysed reaction.

[2]

(iii)

Suggest and explain a way to determine whether the buffer solution is performing its role effectively.

[2]

The students wrote out their experimental method as follows:

1. Set up each reaction mixture with identical volumes of amylase, starch solution and buffer solution. Each reaction mixture totals 20cm³ in volume. Start the stopwatch

2. Immediately withdraw a small volume (< 0.5 cm³) of the reaction mixture into one pipette and the same volume of the surrounding water into another pipette

3. Place 1 drop of the reaction mixture into the well of a spotting tile and add a drop of iodine solution. Note any colour changes

4. Every 5 minutes withdraw a small volume (< 0.5 cm³) of each reaction mixture using the dropping pipette. Also test a sample of the water from around the outside of the Visking tubing. 

5. Add the remaining contents of the dropping pipette to 2 cm of Benedict's solution in a test tube. Note any colour changes. 

6. Repeat steps 2-5 at 5 minute intervals until no further colour changes are observed

7. Collate the results in a results table

(i)     

An important action has been missed out from the students' method above, during Step 5. Suggest what this action should be.

[1]

(ii)

Draw a blank results table that the students could fill in to record the progress of their investigation. Use suitable column headings and units. 

[4]

Before beginning the investigation, the students decided on observations to record for each of the tests performed, as described in the method.  These observations would be written in the table they constructed in part (b) (ii). 

(i)

Suggest the observations the students could record to show the results of the iodine test.

[1]

(ii)

Suggest the observations the students could record to show the results of the Benedict's test.

[1]

(iii)

Describe a way for the results that the students observed to be made more reliable.

[2]

Fig. 2 shows the appearance of the spotting tile that the students used for their iodine tests. The colours they observed are shown in the key at the bottom of Fig. 2

Fig. 2

(i)

Explain the results at each pH level.

[3]

(ii)

Use the results in Fig. 2 to estimate the fastest time taken to fully digest the starch 

[1]

(iii)

Suggest a further experiment to give a more accurate answer to the answer you gave in part (d) (ii)

[2]

The students noticed that the results of the Benedict's test on the water outside the Visking tubing showed only very weak colour changes, when positive.

Suggest and explain a change to the experimental equipment that could increase the chances of a more definitive positive result being produced.