WJEC Biology for AS Level Student Book: 2nd Edition

Method Setting up the apparatus 1. Place 10 cm 3 buffer pH 7 in the conical flask. 2. Ensure the rubber bung fits the flask neck tightly and place the delivery tube so that it opens under water in the water trough. 3. Fill a 10 cm 3 measuring cylinder with water so that there is a convex meniscus visible at the top. 4. Cover the top completely with your thumb and invert the measuring cylinder under the water in the trough, ensuring that no air enters the measuring cylinder. This may take some practice. 5. Rest the measuring cylinder against the edge of the water trough. Preparing the potato 1. Cut a cylinder of potato with the cork borers and remove the skin. 2. Using forceps, align the potato with the ruler and cut 30 discs of potato, each 1 mm thick. 3. Transfer the discs to the buffer and swirl the flask so the discs are separated. Running the experiment 1. Draw 5 cm 3 25 vol hydrogen peroxide into a syringe barrel and affix to the port on the needle which runs through the rubber bung. 2. Fill a second 10 cm 3 measuring cylinder as before, in case the volume of gas collected exceeds 10 cm 3 . 3. Simultaneously inject the hydrogen peroxide into the buffer, mix the flask contents by swirling and start the stop clock. 4. Bubbles appear as the syringe plunger displaces air in the apparatus. Immediately after this, place the upturned measuring cylinder over the end of the delivery tube. 5. Swirl the flask every minute to release gas bubbles that accumulate on the potato discs. 6. Measure the volume of gas collected after 10 minutes. If the volume reaches 10 cm 3 , replace the measuring cylinder with another full one that you have prepared. 7. Repeat all steps for 20, 15, 10 and 5 vol hydrogen peroxide and for distilled water. Results table Concentration of hydrogen peroxide / vol Volume of gas collected after 10 minutes / cm 3 1 2 3 Mean 0 0 0 0 0 5 3.3 2.9 3.4 10 6.9 6.1 7.1 15 11.1 11.7 10.8 20 13.8 12.2 13.2 13.1 25 13.9 13.1 12.0 13.0 1. Complete the table. 2. Plot the mean volume of gas collected after 10 minutes against the concentration of hydrogen peroxide. 3. Describe the change in gradient of the line. 4. From the graph, read the volume of gas that is produced when the enzyme is saturated. Study point The units of concentration of hydrogen peroxide are ‘vol’. It refers to the volume of oxygen released on breakdown. For a solution of concentration 10 vol, 1 dm 3 hydrogen peroxide produces 10 dm 3 oxygen. Study point Several readings for each value of the independent variable allow range bars to be plotted. – The range bar length indicates the consistency of readings, i.e. how similar the readings are. Shorter range bars result from more consistent readings and therefore produce a more reliable mean than longer range bars, which are produced by less consistent readings. – If range bars overlap, it may be that the means are not in fact different. Overlapping range bars reduce your confidence that the difference between data points is real. Overlap of range bars for several adjacent data points may represent a plateau on the graph. 89  1.4 Enzymes and biological reactions