How does changing the percentage of sucrose added to yeast affect the rate of respiration of yeast as measured by the height of gas bubbles (froth) ?

Conclusion:

In my experiment I investigated whether changing the percentage of sucrose added to yeast affects the rate of respiration of yeast as measured by the height of gas bubbles (froth) achieved above the yeast-sucrose suspension in 360 seconds.

From my results I can infer that changing the percentage of sucrose does affect the rate of respiration of yeast. At first as there is a steep increase in the amount of CO2 produced and thus the rate of the froth produced, when we increased the concentration of sucrose from 0% to 5%. However after this point there is a negative correlation and increasing the sucrose percentage decreases the volume of CO2 produced and thus the rate of the froth being produced. There is a gradual decrease from the highest value of 0.062mm/s at 5% sucrose eventually decreasing to 0.037 mm/s at 20% sucrose.

We were able to do a total of 5 repeats for each concentration which meant we were able to ensure greater reliability of our results, during our trials for 20% sucrose we recorded a result which did not correlate to the rest of the results for that trial (3mm in comparison to approximately 17mm), we were able to do a sixth trial and replace the outlier with a more accurate value of 16mm and thus we were able to preserve more of the integrity of our data and reduce the size of our error bars for that trial. There was another abnormality in our results which we kept in our calculation, this outlier occurred during the third 5% sucrose trial where the froth achieved height of 38mm and thus a rate of 0.11mm/s. As this result is much greater than the other results it would have greatly increased the average. For example if we exclude that value for that experiment we get an average rate of 0.0505mm/s opposed to the calculated average for 5%sucrose of 0.062mm/s. Thus causing the error bar at 5% sucrose to be very large (1SD is 0.028mm). The size of the error bar indicates that the values recorded for this percentage was less valid and some other factor may have been affecting the results. This error bar also overlaps the three other data points (10%, 15% and 20%) this indicates that there may be no clear trend in our results due to this overlap. The error bar for 20% is also large, that there were considerable differences in the data points, indicating that our results for 20% may not have all been very valid.

The calculated standard deviation and error bars are for the other three of my results (0%, 10% and 15%) were small, which indicates that most of my data is quite close to the mean, however in biological molecules there is always going to be some variation in rates.

This result is explained through the science behind aerobic respiration, cell membranes, enzyme theory and the Crabtree effect. Firstly we assume that the height of gas produced indicates its respiration rate because in aerobic respiration carbon dioxide gas is produced. As carbon dioxide is produced by the yeast cell the gas will rise as gas is lighter and less dense than the water and yeast solution, the foam we see is bubbles of gas the are trapped in the solution, this is only possible due to the slow dispersion of the gas in the solution by the yeast cells which allows the foam to be formed and then measured. 

The word and chemical equation for aerobic respiration is below.

Glucose + Oxygen → Carbon Dioxide + Water +Energy
C6H12O6 + 6O2 → 6CO2 + 6H2O + 2900kJmol -1

As seen from the equation above, it is glucose, which is a reactant for respiration. However it is important to note that we used sucrose instead of glucose in our experiment. Sucrose is a disaccharide sugar formed in a condensation reaction between glucose and fructose. Nevertheless yeast is a fungus, and thus like other living organisms it utilises enzymes to facilitate/catalyse and thus increase the rate of biological processes, without being used up themselves. In this case yeast contains the sucrase enzyme which catalysis the hydrolysis reaction of sucrose breaking it down into its constituent molecules glucose and fructose and thus respiration can take place.

Enzymes are particularly important in respiration, as without enzymes, respiration would occur too slowly to sustain life. Enzymes are proteins with unique shapes that are capable of binding with substrates to form enzyme substrate complexes. Enzymes have specific binding sites that cause enzymes to be substrate specific. The rate that enzymes and substrates collide is explained by principles of chemical kinetics, a branch of science that is concerned with the dynamics of chemical reactions: the way reactions take place and the rate of the process. ‘Collision theory’ states that for the rate of reaction to increase there must be more frequent collisions caused by an increase in speed or an increase in number of molecules. In our experiment the concentration of substrate/glucose is being increased and thus the number of molecules of substrate is also increasing. Therefore the frequency of collisions between the enzyme and the substrate will increase causing the reaction to speed up, and thus rate of respiration will increase and more gas will be produced. This explains our initial rise in respiration when we increased the sucrose concentration from 0% to 5%. Therefore at low concentrations, there will be few substrate molecules to collide causing the reaction to progress extremely slowly, this explains our result of 0mm/s at 0% as there was no glucose available for respiration to occur and thus no gas would be produced.

However this does not completely explain our results, although at f3irst we saw that increasing concentration increased rate of respiration, which fits with the scientific theory, subsequently rate decreased. This change is explained by the structure of membranes and the Crabtree effect.

Sucrose is very polar due to the presence of multiple hydroxyl groups. Therefore sucrose it is only soluble in polar substances such as water. Cellular membranes are made of a phospholipid bilayer, this bilayer is “permeable, meaning that some molecules are allowed to pass freely (diffuse) through the membrane.  The bilayer is virtually impermeable to large molecules, relatively impermeable to small polar molecules and charged ions, and quite permeable to lipid soluble low molecular weight molecules.”  Thus sugars are unable to diffuse directly though the membrane into the cell and must utilise membrane protein channels in the membranes that allow facilitated diffusion to occur.  “These channels are the way in which medium-sized polar molecules like glucose may pass into the cell.  The number of protein channels available limits the rate of facilitated transport” 1 this causes an increase in sucrose concentration to have less of an effect at higher sucrose concentrations. 

This explains why the rate did not continue to increase exponentially but still does not explain the decrease. The decrease is explained by a process called catabolite repression (Carlile and Watkinson, 1994, 66). “Catabolite repression, also known as the Crabtree Effect, happens when the amount of glucose in the environment of the cell is great enough to suppress the synthesis of the enzymes responsible for breaking down the glucose, and can also increase the decomposition of those enzymes and the proteins that form them. Therefore, higher levels of glucose would be detrimental to the viability of the yeast cell.”2 This would cause the rate of respiration to decrease, as the enzymes, which facilitate respiration, would break down. This explains the decline as the concentration of glucose increases so does the Crabtree effect.

Our results are reflected by other scientific findings, such as Julie Wynstra who investigated the effect of glucose concentration on the rate of respiration. As you can see from the graph included below the “rate peaked when the glucose concentration was at 5% and decreased substantially at the 10%, 20%, and 40%”. This experiment shows what we would expect to occur at extremely high glucose concentrations, namely that the excess glucose would have such an adverse effect on the cell and its enzymes that eventually it would cause respiration to stop completely. Wynstra’s results combined with our results indicate that there is an optimum sucrose concentration, at which yeast can respire quickly due to the number of sugar molecules yet is not adversely effected by the crabtree effect, both of our results indicate that this is at 5% but further testing around this percentage would enable us to further investigate the concept of an optimum concentration.

It is important to understand this science because of its importance in the food industry, as the yeast plays an important role in the creation of bread, as respiration causes carbon dioxide to be produced which aerates the bread.

Evaluation:

Bibliography

Carlile MJ, Watkinson SC. 1994. The fungi. New York (NY): Academic Press; 482p.