Biology 9700/52 — February/March 2018

The students used the information from the website and the recommended dilutions to calculate the concentrations of lipase that could be present in ‘working’ concentrations of washing liquid.

The students calculated the maximum possible concentration of lipase in washing liquid A when diluted, as shown below.

The maximum possible concentration of lipase in washing liquid A before dilution is:
$155\ \text{mg cm}^{-3}$
Therefore, the maximum possible concentration of lipase in diluted washing liquid A is:
$\frac{155\ \text{mg cm}^{-3} \times 20\ \text{cm}^3}{5000\ \text{cm}^3} = 0.62\ \text{mg cm}^{-3}$

Complete the calculation below to show the minimum possible concentration of lipase in diluted washing liquid B.

The minimum possible concentration of lipase in washing liquid B before dilution is:
______ $\text{mg cm}^{-3}$
Therefore, the minimum possible concentration of lipase in diluted washing liquid B is:
$\frac{\_\_\_\_\_\_\ \text{mg cm}^{-3} \times \_\_\_\_\_\_\ \text{cm}^3}{\_\_\_\_\_\_\ \text{cm}^3} = \_\_\_\_\_\_\ \text{mg cm}^{-3}$

The students simulated fat-stained fabric by cutting $20\ \text{mm} \times 20\ \text{mm}$ pieces of cotton fabric and soaking them in cooking oil. The fabric pieces were then allowed to dry.

The students diluted both washing liquids according to the recommended dilutions, added a piece of fat-stained cotton fabric to each and measured the activity of lipase using a pH probe and stop-clock. The students recorded the time taken for the pH to decrease from 7.9 to 6.5. The pH decreases because fatty acids are produced from the hydrolysis of fats by lipase.

Fig. 1.1 shows the apparatus the students used.

Table 1.1 shows the results of two trials using each of the diluted washing liquids at the recommended temperature of $30\,^{\circ}\text{C}$. This is lower than the optimum temperature of lipase, which is $60\,^{\circ}\text{C}$.

Table 1.1

To find the ‘working’ concentrations of lipase in diluted washing liquids A and B, the students decided to make a range of lipase concentrations from $700\ \mu\text{g cm}^{-3}$ to $200\ \mu\text{g cm}^{-3}$. They were provided with a stock solution of lipase with a concentration of $1000\ \mu\text{g cm}^{-3}$.

Describe how the students could dilute the stock solution to make solutions of lipase with a range of concentrations from $700\ \mu\text{g cm}^{-3}$ to $200\ \mu\text{g cm}^{-3}$. $100\ \text{cm}^3$ of each solution is required.

The students used the apparatus shown in Fig. 1.1 to investigate the effect of lipase concentration on the activity of lipase. They used the different concentrations of lipase that they had decided to prepare.

Identify the independent and dependent variables in this investigation.

independent variable = ______
dependent variable = ______

Describe a method the students could use to find the effect of the different concentrations of lipase on the activity of lipase, using the apparatus shown in Fig. 1.1.

Your method should be set out in a logical order and be detailed enough for another person to follow. It should not repeat the details from (b) of how to dilute the stock lipase solution.

The students used the results of their investigation to calculate the activity of lipase as the rate of release of fatty acids from fats. They then plotted a graph of their results.

The students’ results suggested that diluted washing liquid A had a higher concentration of lipase than diluted washing liquid B.

Suggest one reason why this conclusion may not be valid.