A student compared the amylase activity in barley seeds that had been left to germinate for different numbers of days. She placed a dish of starch agar on t...
Question 1 Report
A student compared the amylase activity in barley seeds that had been left to germinate for different numbers of days. She placed a dish of starch agar on the bench and put four paper discs on its surface, as shown in Fig. 8.1. Each disc had been soaked in amylase squeezed from seeds germinated for a different time. The dish was left in a warm place so that amylase could soak out of each disc and digest the surrounding starch. The dish was then flooded with iodine solution. The starch that was left turned blue-black, but a clear zone appeared around each disc where the starch had been digested. The student measured the diameter of each clear zone. Her results are shown in Table 8.1.
Fig. 8.1
| Disc | 1 | 2 | 3 | 4 |
|---|---|---|---|---|
| Days of germination | 1 | 3 | 5 | 7 |
| Diameter of clear zone / mm | 6 | 14 | 22 | 30 |
(a) Measure, from Table 8.1, the diameter of the clear zone around disc 3. [1]
(b) Record which disc produced the largest clear zone. [1]
(c) Describe how a clear zone forms around a disc on the starch agar. [2]
(d) Explain why the iodine solution is added to the dish at the end. [2]
(e) Describe how the diameter of a clear zone is measured with a ruler. [2]
(f) Describe the relationship between the number of days of germination and the size of the clear zone. [2]
(g) Suggest one variable that should be kept constant. [1]
Answer Details
This question tests the starch-agar (clear-zone) method for comparing how much amylase germinating barley seeds produce.
(a) From Table 8.1, the clear zone around disc 3 has a diameter of 22 mm. [1]
(b) The largest clear zone was produced by disc 4 (30 mm). [1]
(c) A clear zone forms because amylase soaks out of the disc into the surrounding agar [1] and digests (breaks down) the starch there, so no starch is left to stain, leaving a clear zone [1]. [2]
(d) Iodine is added at the end because it stains the remaining starch blue-black [1]; this makes the clear zone (where the starch has been digested) stand out so its diameter can be seen and measured [1]. [2]
(e) To measure a clear zone, lay a ruler across the centre of the zone [1] and read the distance in millimetres from one edge of the zone to the opposite edge (the diameter) [1]. [2]
(f) The relationship is that as the number of days of germination increases, the clear zone gets larger [1]; this shows that more amylase is produced as the seeds germinate for longer (a positive correlation) [1]. [2] The plotted data show a straight-line rise:
(g) One variable to keep constant, any one: volume of amylase on each disc, size of disc, temperature, time left, or thickness of the agar. [1]
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