WJEC Biology for AS Level Student Book: 2nd Edition

Results table Temperature / ºC Time until acetate film cleared / seconds time 1 / seconds –1 1 2 3 1 2 3 Mean 10 1170 1356 1126 0.0009 0.0007 0.0009 0.0008 30 402 378 342 50 287 378 244 70 ∞ ∞ ∞ 90 ∞ ∞ ∞ 0 0 0 0 1. Complete the table, using the correct number of decimal places. 2. Plot a graph of mean time 1 against temperature. For a chemical reaction, the value Q 10 can be calculated and used to assess accuracy. For a standard reaction, Q 10 ≈ 2. The part of the graph with temperatures lower than the optimum represents a normal chemical reaction, as the enzyme is not denatured. A line of best fit is drawn through those points and rates of reaction at temperatures 10Cº apart are read. They are substituted into the equation Q 10 = rate of reaction at t ºC rate of reaction at ( t + 10)ºC . The closer this value is to 2, the more accurate the readings. 3. Draw a line of best fit through the data points that show increasing values of time 1 . 4. Read the values of time 1 for two temperatures 10Cº apart and calculate Q 10 . 5. Assess the accuracy of these results. Further work ▪ This method can be used to determine the effect of enzyme concentration on the rate of reaction, using trypsin at concentrations 1, 2, 3, 4 and 5 g / 100 cm 3 . ▪ Using the same technique, the effect of pH on trypsin activity could be determined by testing trypsin at 1 g/100 cm 3 in buffers at pH 1, 3, 5, 7, 9 and 11. To determine the effect of pH on the activity of pectinase Rationale Commercially bought pectinase is a mixture of enzymes that digest the middle lamella of the plant cell wall, which is largely calcium and magnesium pectate. The pectinase separates the cells, which lyse and release the cell sap from their vacuoles. The cell sap is the apple juice in this experiment. Maths tip It is likely that at the highest temperatures, the trypsin will be denatured and so no digestion will occur. The film will not clear and the time for digestion would be infinite. Infinity cannot be plotted. To give a meaningful graph, instead of plotting time, time 1 can be plotted. ∞ 1 = 0 and time 1 can be considered proportional to the rate of reaction.  1.4 Enzymes and biological reactions 85