WJEC Biology for AS Level Student Book: 2nd Edition

Design Experimental factor Description Value Independent variable pH 1, 3, 5, 7, 9 Dependent variable volume of apple juice cm 3 Controlled variables temperature 40°C volume of pectinase 5 cm 3 concentration of pectinase 5% dilution of stock solution mass of apple 100 g Control boil and cool pectinase before use; see Study point on p83 Reliability calculate the mean volume of apple juice from 3 replicates at each en- zyme concentration; see Study Point on p90 and Key term on p65 Hazard pectinase is a protein and is potentially allergenic so skin should be covered and eye protection worn; the proximity of water and electricity is a potential hazard and water above 60°C scalds Apparatus ▪ Juicy apples, e.g. Gala ▪ Chopping board ▪ Sharp knife ▪ Balance ▪ 250 cm 3 beakers ▪ 5% pectinase made by making 1 cm 3 commercial pectinase up to 20 cm 3 with buffers at pH 1, 3, 5, 7, and 9 ▪ Water bath at 40ºC ▪ 5 × 10 cm 3 measuring cylinders ▪ 5 × 20 cm 3 measuring cylinders ▪ 5 × filter funnels ▪ Filter paper ▪ Timer Method 1. Measure 5 cm 3 of each enzyme preparation into a separate test tube and place in the 40ºC water bath for 10 minutes, to equilibrate to temperature. 2. Peel the apples and chop the flesh into cubes of about 3 mm on each side. 3. Weigh 100 g apple cubes into each of 5 beakers. 4. Tip one enzyme preparation into each beaker, stir and replace in the water bath. The beakers may need weighting down, for example, with a glass rod, to prevent them from overturning in the water. 5. After 1 hour, filter the contents of each beaker into a 20 cm 3 measuring cylinder, allowing the samples to filter for 30 minutes. Study point The control is an experiment that shows that it is the independent variable that is the cause of changes in the dependent variable. In a control experiment, the independent variable is inactivated but the experiment is otherwise identical with the experimental tests. No change in the dependent variable shows that it is changes in the independent variable that generate the different readings. WJEC Biology for AS Level 86 Theory check 1. Name the bonds that maintain the shape of an enzyme’s active site. 2. Why might a low pH decrease the rate of an enzyme-controlled reaction? 3. Why might a high pH decrease the rate of an enzyme-controlled reaction? 4. As pectinase digests pectin, how might it affect a fragment of plant tissue? 7