PAG 4.2

Question 1

Equipment

Answer 1

• Milk powder solution
• 2 % Trypsin solution
• Distilled water
• 5 cm3 syringes
• 4 boiling tubes for the serial dilution
• 5 boiling tubes for the main practical
• Stopwatch/timer
• Marker pen
• PAG4.2 OCR Observation Sheet

Question 2

method

Answer 2

1. Serial Dilution: First you need to prepare the following concentrations of enzyme solution: 1.00%, 0.50%, 0.25% and 0.13% by using the 2.00% Trypsin solution and distilled water. Use the following diagram to help you:
2. Label five boiling tubes with the Trypsin concentrations you have prepared and place 2 cm3 milk powder solution into each.
3. Hold one of the tubes up against the Observation Sheet. You should not be able to see the text through the milk solution.
4. Add 2 cm3 of 2.00% Trypsin solution to the boiling tube labelled 2.00% and immediately start the stopwatch.
5. Hold the tube up against the Observation Sheet and stop the timer once the writing becomes visible.
6. Record your result in a suitable table. Include a column for rate of reaction.
7. Repeat steps 3 - 5 for the other four concentrations, 1.00%, 0.50%, 0.25% and 0.13%.
8. Plot a graph of time versus enzyme concentration.
9. Finally, plot a graph of rate of reaction versus enzyme concentration.

Question 3

1. What variables were controlled in this practical activity?

Answer 3

1. What variables were controlled in this practical activity? Students might consider substrate concentration, the volumes of enzyme and substrate used, pH and temperature.

Question 4

2. What could you have done to improve the validity of your results?

Answer 4

2. What could you have done to improve the validity of your results? Ideally each concentration would have been replicated at least three times and a mean time calculated. Students may also suggestion a way to standardise mixing the solutions on addition of the enzyme.

Question 5

3. Discuss this activity with the rest of the class. Did everybody get a similar result?

Answer 5

3. Discuss this activity with the rest of the class. Did everybody get a similar result? This will depend on the class size and results achieved. It could be an opportunity to pool the class data.

Question 6

4. What conclusions would you draw from this practical activity?

Answer 6

The expected result is that as enzyme concentration is increased the (initial) rate of reaction increases. This question could be used as an extension into discussing why beyond a certain concentration the rate will not increase further.