Time started Time ended Time elapsed (min) Initial reading of syringe (cm) Final reading of syringe (cm) 3 Volume of carbon dioxide gas evolved (cm ) 3 -1 Rate of respiration of yeast (cm hr )
Glucose 09:25 09:55 30 3 4 1 2
Respiratory substrate Starch 09:30 10:00 30 3 0 0 0
Distilled water 09:35 10:05 30 3 0 0 0
Table showing the rate of respiration of yeast in different respiratory substrates Discussion Using glucose as the respiratory substrate, the volume of carbon dioxide evolved and the rate of respiration of 3 3 -1 yeast are, are, which are 1cm and 2 cm hr respectively, are greater than that of using starch, which are both 0. That is because glucose, which are one of the monosaccharides, are only small enough to pass through the cell membranes of yeast cells and then undergo respiration while starch cannot because it is a polysaccharide and it takes time for the conversion to simple sugars and to respiration. Therefore, the rate of respiration of yeast using glucose as the respiratory substrate is faster than that of using starch. Apart from it, in both aerobic (with oxygen) and anaerobic (without oxygen) respiration of yeast, carbon dioxide is produced. In the syringe, as its opening where needle is inserted is covered by blue tag, it is regarded as a close system. Therefore, as the respiration is in progress, yeast continues to produce carbon dioxide such that it will push the plunger upward. By measuring the displacement of plunger, the volume of carbon dioxide gas evolved and in turn the rate of respiration yeast can be found and compared with other respiratory substrates. For sources of errors of the investigation, first of all, the temperature of the reaction mixture is not kept constant because it is touched by human hand so as to take re ading, therefore, some heat is transferred. Thus, it may affect the displacement of plunger due to Pressure Law (P and T (in K) are directly proportional). Secondly, there may be gas leakage from the blue tag and making the system not closed and affect the displacement of the plunger. Therefore, the actual carbon dioxide evolution may be greater than the readings. For the first error, the corresponding improvement is to carry out this experiment in a thermostat or and water bath with 35°C to 40°C, because it can ensure uniform temperature or reduce temperature fluctuation and increase the rate of respiration of yeast without denaturing the enzyme which carry out respiration in yeast. For the second error, seal a layer of Vaseline and (保鮮紙?) above the blue tag to further reduce the leakage of gas so as to improve the accuracy of the investigation. Conclusion Yeast using glucose as the respiratory produces more carbon dioxide gas than that of using starch. Thus, it infer that the rate of respiration of yeast using glucose as the respiratory substrate is faster than that of using starch because carbon dioxide is a by-product of respiration of yeast.
