9) Record the results.
10) Wash the water bath and syringe and refill both of them to their designated levels with their designated liquids. The syringe must always be filled with 5ml of one-part glucose and one-part yeast mixture, as the level of this mixture is not a variable and must be kept to the same quantity to ensure a fair test.
11) Next, heat the water to 55ºc and then place the syringe in (under the measuring cylinder) for one minute, maintaining a constant temperature of 55ºc.
12) Record the results.
HYPOTHESIS
It is predicted that up to 40ºc, the higher the heat, the quicker anaerobic respiration will occur between yeast and glucose (in this experiment this rate will be measured by the decrease of the volume of water as it is displaced by the carbon dioxide in the measuring cylinder). It is also predicted that after 40ºc, the rate of anaerobic respiration (the amount of carbon dioxide released) will go down.
SCIENTIFIC KNOWLEDGE
All substrates need an enzyme to break them down into simpler forms. In the case of this experiment, the substrate is glucose and the enzyme is zymase (it is carried by yeast).
When a substance is heated it is given more energy, and the molecules it is composed of are able to move around more freely due to the heat energy they are receiving, which they are converting into kinetic energy. In the case of this experiment, the glucose and yeast mixture is being heated. This means that the molecules it is made of can move around freer, and hence react with each other more easily (this is because the atoms in the mixture are more free to create bonds with other atoms).
The reaction between the yeast and glucose can be described by the lock and key hypothesis. The zymase molecules are a specific shape and only the glucose molecules can lock into that shape:
When heated, the active site (where the substrate fits into the enzyme) is altered. At first, the site is altered to a point where the substrate (glucose) can fit into the enzyme more easily. However beyond a certain temperature the active site becomes mutated to such a point that the substrate can no longer fit into the enzyme. It is at this point that the enzyme is said to be “denatured”. From that point onwards, the substrate and the enzyme can no longer react and henceforth the substrate will not be broken down.
The formula for anaerobic respiration is:
zymase ( enzyme)
glucose ethanol + carbon dioxide + energy
It can be seen that the enzyme, zymase is the catalyst in the reaction and when it has been denatured, it cannot function, and hence the anaerobic respiration will occur very slightly, or not at all.
All enzymes are complex biological molecules and are very precise in the area of temperature that they will function at. It is known for a fact that pepsin is denatured at 40ºc. Henceforth, it is believed that, like pepsin (another enzyme), zymase will be denatured at around 40ºc and hence will not be able to break down the glucose in such high temperatures. It is believed that the zymase will be denatured at 40ºc, as many enzymes have similar properties as they are molecularly constructed in similar fashions. This temperature is just above the body temperature of many mammals and is hence the temperature at which it is thought that ensures the enzyme will work most effectively, when it is needed, in the body of a mammal.
Because heat speeds up reactions, it is predicted that yeast will perform anaerobic respiration with glucose, the higher the temperature, until 40ºc, at which point the zymase will be denatured. Once the zymase has been denatured, the rate of anaerobic respiration will drop rapidly, as the active site is no longer usable. So it is also predicted that at roughly 55ºc the rate of anaerobic respiration will be much lower than at the optimum temperature of 40ºc.
When yeast reacts with glucose, it releases on average 1.2 kilojoules per gram of glucose compared to an average of 16.1 kj/g for aerobic respiration. According to the laws of conservation of energy, energy cannot be created or destroyed it is just transferred. Henceforth we can see that considering yeast produces 1.2kj/g of glucose, then little energy is put into reacting the yeast and glucose. Hence, it is also predicted that yeast should perform anaerobic respiration to quite a good rate, at lower temperatures like 20ºc. This is because, as shown above, little energy is needed to perform anaerobic respiration.
RESULTS:
ANALYSIS:
From the results which were collected it would seem that that the higher the temperature, the faster the rate of anaerobic respiration. We know this because more carbon dioxide was produced and carbon dioxide is one of the products of anaerobic respiration.
Here are the mean results from the repetitions of the experiment
From these it can be seen that the results are quite constant, considering the first amount of carbon dioxide shows a 0.2ml increase, and the second shows a 0.21ml increase. This shows a steady and regular ascension. The first three temperatures go up by 15°c each time but the last result goes up in temperature by 25°c and in carbon dioxide production by 0.72. As you can see from the graph, with its line of best fit the carbon dioxide produced at 70°c is around 0.79ml. This is an increase of 0.16 ml from the previous measure.
The reason temperature makes the production of carbon dioxide increase is because the heat made the active site of the zymase easier to fit its substrate (glucose) in it. Therefore the higher the heat the more efficient the enzyme’s (zymase) breakdown of its substrate (glucose) is.
From this we can conclude that the prediction was correct in that respect. This is because it was stated that, to an extent the higher the heat, the faster the rate of anaerobic respiration.
The incorrect aspect of the prediction was the fact that it was predicted that the temperature at which zymase was denatured (the temperature at which anaerobic respiration would stopped) would be roughly 40°c. It was thought that this was the case because 40°c is roughly human body temperature. However, the experiment clearly showed that zymase was not denatured at 80°c. Due to time constrictions, the point at which zymase was denatured was not found. However it is clear that the rate of anaerobic respiration continued its ascension long after 40°c.
Conclusion
The evidence that we obtained showed a consistent pattern. It showed an obvious rise in carbon dioxide production, relative to the rise in temperature. Although the carbon dioxide production wasn’t exactly regular in its ascent, it produced no outright anomalous results.
The results we recorded lead to the conclusion that: up to a point, the higher the temperature, the quicker the rate of the anaerobic respiration in the mixture. This conclusion can be drawn so clearly because the results obtained all created very similar sets of figures each time.
The procedure used was very efficient. We made sure that it was a fair test because we kept the temperature, the amount of mixture and the time span all the same and did several repetitions to ensure the results were constant and not random. The measuring procedure was also accurate as we used a measuring cylinder to measure the carbon dioxide produced, this was preferable to counting the bubbles produced, which could be inaccurate and more difficult to measure.
We encountered some problems of manual nature however; it was hard to keep the measuring cylinder stuck to the side of the water bath, as the cellotape became unsticky as soon as it became wet. Therefore if the experiment was to be done again a few changes could be made. For example, we could use a clamp stand to hold the cylinder instead of sticky tape. Also, due to time constrictions, the temperature at which zymase actually denatures was not found. It would be better if more time were available so that the temperature at which zymase was denatured could be found. Another experiment that could be done to further test this experiment would be to use the variable of the ratio of yeast to glucose, and what the results are when this is changed.
The combination of such subsequent experiments along with the one explored in this investigation, could be used to build up an accurate conclusion as to the best conditions for yeast and glucose mixture to perform anaerobic respiration.
