WJEC Biology for A2 Level: Revision Workbook

You might write: ‘I would make sure that the same mass of apples is added, and that they were the same age.’ This is a basic answer. A good answer needs to be both accurate and detailed. For example: ‘I would make sure that the same mass of apples is added, for example 100g, and that they were the same age, e.g. 1 week old. I would also control the temperature at an optimum for the pectinases involved, e.g. 30°C.’ Look at the following example: A student carried out an experiment to investigate the effect of temperature on respiration in yeast cells. 1g of dried yeast was added to 25 cm 3 of a 5% glucose solution and after 10 minutes incubation at 15°C, 1 cm 3 of 5% TTC solution was added. TTC is an artificial hydrogen acceptor which turns from colourless to red in the presence of hydrogen atoms which are released during respiration. The time taken for the yeast solution to turn red was recorded. The experiment was repeated at 30°C and 45°C and the time taken for the yeast suspension to turn red was recorded below. Temperature (°C) Time taken for the yeast suspension to turn red (s) Trial 1 Trial 2 Trial 3 Mean (nearest whole second) 15 450 427 466 448 30 322 299 367 329 45 170 99 215 161 Q: What conclusions could be drawn from this experiment regarding the effect of temperature on respiration in yeast? You might write: ‘Increasing temperature decreases the time taken for the yeast suspension to turn red, indicating that respiration is occurring more quickly.’ A good answer needs to be both accurate and detailed, for example, ‘Increasing the temperature increases the rate of respiration in the yeast, so dehydrogenase enzymes remove hydrogen atoms from triose phosphate more quickly. This is due to increased kinetic energy of the dehydrogenase enzymes and triose phosphate substrate molecules at higher temperatures. As more hydrogen atoms are released more quickly, so TTC is reduced more quickly, turning the yeast red in a shorter time.’ If asked to comment on the validity of your conclusion, you might write: ‘It was difficult to determine when the solutions turned red, making it difficult to know when to stop timing the reactions.’ A good answer would be more detailed. For example: ‘The results at 45°C are very variable and range from 99 to 215 seconds. It is difficult to reach a conclusion about the effect of temperature on respiration in yeast as only three temperatures were investigated. Another major difficulty would be in determining the endpoint of the reaction, as no standard red colour or colorimeter was used.’ As part of this skill, you could also be asked to identify the independent, dependent and controlled variables in an investigation. Remember: ⦁ The independent variable is the one I change. ⦁ The dependent variable is the one I measure. ⦁ Controlled variables are variables which affect the reaction being investigated and must be kept constant. Introduction | 9 Copyright: Sample material