WJEC Biology for AS Level Student Book: 2nd Edition

Results pH of pectinase Volume of apple juice collected after 30 minutes / cm 3 1 2 3 Mean 1 18.8 20.9 19.7 19.8 3 19.2 20.8 20.3 5 24.4 26.2 25.6 7 20.6 22.3 22.8 9 19.0 19.2 18.2 18.8 1. Complete the table. 2. Plot a graph of mean volume against pH. 3. Determine the optimum pH of those tested. Further work ▪ To find the optimum pH, the experiment could be repeated using more pH values around the value that produced the highest volume of apple juice. ▪ The effect of pectinase concentration on juice volume could be tested by diluting the commercial pectinase solution to 25%, 50% and 75% and using the range 0–100% in the experiment. To determine the effect of the concentration of hydrogen peroxide on catalase activity Rationale Catalase is an intracellular enzyme that catalyses the breakdown of hydrogen peroxide formed as a by-product of metabolic reactions. Hydrogen peroxide is a very strong oxidising agent and unless broken down, risks oxidising the double bonds of membrane phospholipids and DNA. Catalase has a very high turn-over number, reflecting the significance of breaking down hydrogen peroxide. It has a very high activity in many animal tissues, such as heart muscle and, in particular, the liver, reflecting the liver’s role in detoxification. It is also present in fungi, bacteria and plant material, including potato tubers, which are used in this experiment. Design Experimental factor Description Value Independent variable concentration of hydrogen peroxide 0, 5, 10, 15, 20 and 25 vol Dependent variable volume of gas produced in 10 minutes cm 3 Controlled variables volume of hydrogen peroxide 5 cm 3 number of potato discs 30 Control use potato discs treated in boiling water for 10 minutes then cooled, but keep all other factors identical; no gas will be produced, showing that it was the active catalase in the potato that was responsible for the change in the gas production when unboiled potato wasused; see Study point on p83 Reliability calculate the mean gas volume of 3 replicates for each concentration of hydrogen peroxide; see Study point on p90 and Key term on p65 Hazard hydrogen peroxide is a strong oxidising agent and irritates eyes and skin, so skin should be covered and eye protection worn Study point If the fastest reaction is at pH 7 in this experiment, the optimum pH is between pH 5 and pH 9. Study point If one of your three readings is much higher or lower than the other two, it may be anomalous. Discard it and take another, but explain in your written report what you have done and why.  1.4 Enzymes and biological reactions 87 Theory check 1. What is an oxidising agent? 2. What is the advantage of cutting a potato into flat discs when you use it as a source of catalase? 3. Why is it preferable to use one large potato to make discs, rather than several small potatoes? 4. Why might an inverted burette give a more accurate reading of the volume of gas than a 25 cm 3 measuring cylinder? 8