pH of buffer solution Time for iodine test to remain brown / s 4 270 5 180 6 90 7 45 8 75 9 210 Table 1 shows results from a teacher's investigation of amyl...
Question 1 Report
| pH of buffer solution | Time for iodine test to remain brown / s |
|---|---|
| 4 | 270 |
| 5 | 180 |
| 6 | 90 |
| 7 | 45 |
| 8 | 75 |
| 9 | 210 |
Table 1 shows results from a teacher's investigation of amylase. Equal volumes of starch solution and amylase solution were mixed in tubes at different pH values. Samples were removed every 30 s and tested with iodine solution. The time shown is when the iodine no longer turned blue-black, showing that starch had been digested.
(a) Complete the statement: the pH at which this enzyme worked fastest was ________. [2]
(b) Explain why the rate of starch digestion was lower at pH 4 than at pH 7. [3]
(c) Describe three variables the teacher should control to make this a fair test. [3]
Answer Details
(a) The fastest rate was at pH 7, because iodine remained brown after the shortest time, 45 s. A shorter time for all starch to be digested means a greater rate. [2]
(b) pH 4 is far from amylase’s optimum pH. The acidic conditions disrupt bonds maintaining the enzyme’s shape, changing the active site. Fewer starch molecules fit the active site, so fewer enzyme-substrate complexes form and starch digestion is slower. [3]
(c) Keep temperature constant; use the same volume or concentration of amylase; use the same volume or concentration of starch. Other suitable controls are total reaction volume, sample volume and interval, and iodine volume or concentration. Any three. [3]
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