WJEC Biology for AS Level Student Book: 2nd Edition

Sources of error and remedies Sources of error Remedies The reaction is exothermic so as the collection period passes, the temperature of the flask will increase, increasing the rate of reaction. The volume of oxygen collected is, therefore, an overestimate of the true volume. The flask should be maintained in a water bath at, e.g., 25ºC, to keep the temperature constant. Oxygen is water-soluble and so some will dissolve in the water as it is being collected, making the volume an underestimate. Gases are less soluble in hot water than in cold water so if the water trough contained hot water, e.g., at 50ºC, to avoid scalding, the volume read would be closer to the true volume of oxygen produced. Further work This technique may be used to determine the effect of pH on catalase activity. Buffers at pH 1, 3, 5, 7, 9 and 11 may be used with 20 vol hydrogen peroxide. To determine the effect of copper sulphate on the digestion of starch by amylase Rationale Copper ions bind irreversibly to amylase molecules and act as non-competitive inhibitors. Starch changes iodine solution from brown to blue–black. Amylase digests starch. If iodine solution is added to a starch–amylase mixture, the iodine solution changes colour to blue-black if undigested starch is present. When all the starch has been digested, the iodine solution does not change colour. The time taken for all the starch to be digested in the presence of copper ions can be measured by timing how long it is before the iodine solution remains brown when it is added to the starch–amylase mixture. Experimental factor Description Value Independent variable concentration of copper sulphate 0, 0.025, 0.050, 0.075 and 0.100 mol dm –3 Dependent variable time for starch to digest minutes Controlled variables pH of starch and amylase solutions pH 7 volume of copper sulphate 2 cm 3 volume of amylase 4 cm 3 volume of starch 4 cm 3 concentration of amylase 2 g / 100 cm 3 concentration of starch 2 g / 100 cm 3 temperature 40°C pH 7 Control boil and cool amylase before use; see Study point on p83 Reliability calculate the mean time for starch to digest, using 3 replicates for each concentration of copper sulphate; see Study point on p90 and Key term on p65 Hazard the proximity of water and electricity is a potential hazard, copper sulphate is a potential skin and eye irritant and can cause diarrhoea and vomiting if ingested Study point Do not confuse reliability with reproducibility. Reliability relates to the similarity in readings within an experiment. Reproducibility relates to the similarity of readings when an experiment is done somewhere else or by someone else. Key term Reproducibility: The closeness of readings produced in an experiment performed using the same method and apparatus on different occasions. Theory check 1. Why does a competitive inhibitor only bind at the active site? 2. How does a non-competitive inhibitor reduce the rate of a reaction? 3. Could a non-competitive inhibitor bind at an active site? 4. Describe the effect on the mass of product of increasing the concentration of a non- competitive inhibitor. 9 WJEC Biology for AS Level 90