A student investigated how pH affects the enzyme pectinase, which releases juice from crushed fruit. She added the same volume of pectinase to equal masses ...
Question 1 Report
A student investigated how pH affects the enzyme pectinase, which releases juice from crushed fruit. She added the same volume of pectinase to equal masses of crushed apple in five beakers, buffering each mixture at a different pH, and left them for the same time at the same temperature. She then poured each mixture into a filter funnel lined with filter paper so that the clear juice ran through into a beaker while the solid pulp stayed behind, as shown in Fig. 25.1. She measured the volume of juice collected, repeating the whole method twice at each pH. Her results are shown in Table 25.1.
Fig. 25.1
| pH | 3 | 4 | 5 | 6 | 7 |
|---|---|---|---|---|---|
| Volume of juice, repeat 1 / cm3 | 10 | 24 | 40 | 30 | 14 |
| Volume of juice, repeat 2 / cm3 | 12 | 26 | 44 | 34 | 16 |
| Mean volume of juice / cm3 | 11 | 42 | 32 | 15 |
(a) Measure, from Table 25.1, the mean volume of juice collected at pH 5. [1]
(b) Measure the pH at which the greatest mean volume of juice was collected. [1]
(c) Complete Table 25.1 by calculating the mean volume of juice at pH 4. Show your working. [2]
(d) Record, from Table 25.1, the two repeat volumes measured at pH 7. [1]
(e) Describe how the funnel and filter paper separate the juice from the pulp. [2]
(f) Describe the effect of pH on the volume of juice collected. [3]
(g) Explain, in terms of the enzyme, why little juice was collected at pH 3 and pH 7. [2]
(h) Suggest why a buffer solution was added to each beaker. [1]
(i) Name the apparatus used to measure the volume of juice collected. [1]
(j) State three variables that must be kept constant. [3]
(k) Describe two ways the student could make the results more reliable. [2]
(l) Suggest why each mixture was left for the same length of time. [1]
(m) Describe how the student could measure the mass of crushed apple accurately. [2]
(n) State one safety precaution for this investigation. [1]
Answer Details
This tests enzyme activity against pH measured by juice volume, plus filtration, means and fair-test design.
(a) The mean volume of juice at pH 5 is 42 cm3. [1]
(b) The greatest mean volume was collected at pH 5. [1]
(c) Mean at pH 4 \( =\dfrac{24+26}{2}=\mathbf{25\ cm^3} \); working [1], answer [1] [2]. Completed row of means: 11, 25, 42, 32, 15 cm3.
(d) The two repeat volumes at pH 7 are 14 cm3 and 16 cm3. [1]
(e) Separation works because the mixture is poured into the funnel [1], and the liquid juice passes through the pores of the filter paper while the solid pulp is trapped on the paper [1] [2].
(f) Effect of pH on juice volume: as the pH rises towards 5 the volume of juice increases [1]; the greatest volume / optimum is at pH 5 [1]; above pH 5 the volume falls again [1] [3].
(g) Little juice is collected at pH 3 and pH 7 because the pectinase is denatured at these pH values [1]; the active site changes shape so it can no longer break down the fruit to release juice [1] [2].
(h) A buffer is added to keep the pH of each mixture constant during the reaction [1].
(i) The volume of juice is measured with a measuring cylinder. [1]
(j) Any three controlled variables: mass of crushed apple; volume or concentration of pectinase; temperature; time left; same variety of apple. [3]
(k) To make results more reliable: repeat each pH more times and take a mean [1], and discard anomalous results / use the same apple variety throughout [1] [2].
(l) Each mixture was left for the same time so the enzyme has the same time to act, keeping the comparison fair [1].
(m) To measure the mass of crushed apple accurately: place a beaker on the balance and set it to zero (tare) [1], then add crushed apple until the required mass is shown [1] [2].
(n) One safety precaution: wipe up spills (accept handle glassware carefully, or wear eye protection). [1]
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