Variables
In my main experiment, I shall use the time taken for the yeast and glucose solution to respire 1 ml as my dependant variable and the temperature as my independent variable
Here is a list of variables that can have an affect on my experiment and also how I will control them if possible:
- Temperature
- Amount of yeast
- Amount of glucose
- Volume of water
TEMPERATURE
Temperature of the experiment will have a great affect on the results as explained by kinetic theory. Temperature will affect the rate of yeast respiration. I shall keep the temperature of the mixture and water bath under control by using a thermometer and checking it constantly. Also, as it will take longer for the temperature inside the test tube to get to the same as the water bath, I shall leave the apparatus for two minutes, keeping the temperature constant.
AMOUNT OF YEAST
The amount of yeast is crucial, more yeast means more glucose will be respired and more products created. An imbalance will upset the results. The amount of yeast will be measured out with a pipette and measuring cylinder.
AMOUNT OF GLUCOSE
The amount of glucose will affect the results also; as more glucose means that there are potentially more products, which would make the results inaccurate or the experiment unfair. The glucose will be measured out like the yeast with a pipette into a measuring cylinder.
Fair test
To produce a fair test I will need to repeat each part of the experiment three times to eliminate anonymous results. The indicator I am using is Bubbles of Carbon Dioxide that appear. The varying factor of this experiment will be the temperature. The controlled factors (always kept the same) are; the concentration of the yeast solution, amount of bubbles measured, amount of glucose and amount of water. After collecting the results three times for each temperature, to make it fair take the mean of each set of three results.
Prediction
With reference to my theory, I predict that the rate and speed of respiration of glucose by yeast will increase with temperature rise up until a certain point where the enzyme used and secreted by the yeast will become denatured and cease to function, reducing the rate significantly. This is explained through Kinetic theory, yeast respiration and the nature of enzymes and I predict that my graph will be like the one shown here.
Hypothesis and Theory
There are many ideas to suggest that the change in temperature will cause an increase of respiration in yeast. Yeast is a single celled fungus made up mostly of protein which has been used for its applications in fermentation. Yeast, after activation creates carbon dioxide and ethyl alcohol by secreting the enzyme zymase in the yeast which acts on simple sugars such as glucose. The alcohol produced has been used in making wines and beers and the carbon dioxide produced has been used in baking as it gets trapped in the dough and causes it to rise.
The Enzyme molecule acts as the lock.
The Substrate acts like the key.
The basic mechanism by which enzymes catalyze chemical reactions begins with the binding of the substrate (or substrates) to the active site on the enzyme. The active site is the specific region of the enzyme which combines with the substrate. The binding of the substrate to the enzyme causes changes in the distribution of electrons in the chemical bonds of the substrate and ultimately causes the reactions that lead to the formation of products. The products are released from the enzyme surface to regenerate the enzyme for another reaction.
The specific action of an enzyme with a single substrate can be explained using a Lock and Key theory. In this theory, the lock is the enzyme and the key is the substrate. Only the correctly sized key (substrate) fits into the key hole (active site) of the lock (enzyme). Only the correctly shaped key opens a particular lock.
The graph shows that as temperature is increased, the reaction rate of an enzyme increases. However, the graph shows that there is an optimum temperature where the reaction proceeds at its maximum. Above that optimal temperature, the reaction rate decreases.
What happens can be explained in terms of kinetics, which essentially means molecular motion. All molecules are in motion. As the temperature increases, their motion increases too. In the case of enzyme catalyzed reactions, as the speed of enzyme and substrate molecules increases, the chance for collisions so they can form enzyme-substrate complexes increases. Thus as the temperature rises, the reaction rate increases too. Above the optimal temperature however, this does not apply. The reaction rate begins to decrease again because some of the enzyme molecules are now warm enough so that their shape becomes altered (denaturing the enzymes). As the temperature rises above the optimal then, an increasing number of enzymes become denatured. Fewer and fewer enzymes are able to fit with their substrates at the active site. The reaction rate decreases until at some high temperature, all the enzymes are denatured, and reactions stop.
Yeast cells use glucose and oxygen to produce energy, this is aerobic respiration. They also produce water and carbon dioxide. If no oxygen is available, yeast will switch over to a process called anaerobic respiration - in this process; glucose is fermented to produce energy, carbon dioxide, and ethanol. Since ethanol is a type of alcohol, which is toxic for yeast cells, anaerobic respiration is a poor second choice to aerobic respiration.
The word equation of aerobic respiration is:
Glucose + Oxygen = Carbon Dioxide + Water + Energy
The word and symbol equation of anaerobic respiration is:
Glucose = Carbon Dioxide + Ethanol (alcohol) + Energy
C6H12O6 = CO2 +CH3CH2OH + H2O
Bibliography / References
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Oxford Complete Biology by W.R Pickering.
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Principles of anatomy and Physiology By Gerard J. Tortora and Sandra R. Grabowski.
- http://www.biotopics.co.uk/other/enzyme.html
- http://www.bbc.co.uk/schools/gcsebitesize/biology/
Science Biology: Analysis
I have found that as I increased the temperature of the yeast solution, the rate of respiration of the yeast increased to a certain point where, as the temperature rose to a certain level, the rate of respiration eventually cut off.
At 25 deg there were hardly any collisions as it took a mean of 160.97 seconds to respire just 1 ml of carbon dioxide.
At 30 deg there was quite a significant change in the amount of collisions as it took a mean of 105.93 seconds to respire 1 ml of carbon dioxide.
At 35 deg there was another significant change in the amount of collisions it took a mean of 58.24 seconds to respire 1 ml of carbon dioxide.
At 40 deg the amount of collisions are reaching the optimum high as it took a mean of 24.25 seconds to respire 1 ml of carbon dioxide.
At 45 deg the rate of respiration is at the optimum high as it took 21.40 seconds to respire 1 ml of carbon dioxide.
After this temperature the rate of respiration drops until at 60 deg it took a mean of 120.46 seconds to respire 1ml of carbon dioxide.
My hypothesis and prediction can be backed up with the findings; from looking at my results and graphs you can see the rise and fall of respiration. Thus my hypothesis and prediction are shown to be present. They are explained due to the theories of enzyme-substrate with lock and key and kinetics. Where these meet is when kinetic theory states that an increase in temperature means more particle collisions between reactants and so a faster rate of reaction; and in enzyme-substrate where the enzyme is sensitive to heat, and about a certain temperature, the active site will begin denaturing, so slowing and eventually stopping the reaction. This will give an area where the rate of respiration drops off and goes to nothing instead of a precise 'cut-off' point. These both apply to my experiment and were described in my planning.
Science Biology: Evaluation
I think that the method I used, whilst giving results, was also quite sensitive to changes and didn't allow the full potential of the experiment. I would suggest using equipment which would not allow any biased results.
I think that any other anomalous results where mostly due to a longer acclimatization or the fact that I did not allow all the solutions to acclimatize the same amount, which I would definitely, do if I repeated the experiment. If I repeated the experiment I would also take more readings for example at every 5°C because if I did this I would be able to plot a more accurate graph and it would be easier and more accurate to tell when the enzyme got to the optimum and denaturing temperatures. This could be explained by the spread of results at each interval and that the reaction could not be totally accurately controlled with the apparatus used. I know my results to be reliable because I conducted a fair test, also I used a digital stopwatch instead of an analogue and the results were accurate to two decimal places. By using a 10 ml measuring cylinder I was able to get the concentration right. The use of a thermometer helped by monitoring the temperature of the solution in order to know when to conduct the experiment.
To make sure that the results were as reliable as I could make them, I calculated the mean of three results at each interval.
I took all precautions to make the apparatus used to be reliable and give good values so I think the slight unreliability was caused by the preparation of the solution and the 'unpredictability' of how the reaction went. To obtain more reliable results I would want complete continuity with preparations, by preparing the solutions but not actually activating the yeast so as to prevent any getting a 'head start' over the others. This would ensure that all the preparations are the same and would give continuity. I would want to be more strict and thorough with preparing solutions and mixing them up. This would help give more reliable results throughout. Although I conducted the experiment as accurately as I could there were many sources of error in the method that I used. Firstly, some help from a friend was needed to start the experiment and this lead to a small delay in starting the stopwatch. I needed to put the yeast and glucose into the test tube, put the bung on to the test tube and start the stopwatch all at the same time. I also think it would have been better if I had used the same yeast and glucose from the whole experiment but was unable to due to the time restrictions. I had to conduct the experiment over a number of days and could not therefore use the same solutions. This is a source of error because the amount of time the yeast would have had already to respire the respiration of the yeast may have been different which may have produced an inconsistent rate of reaction. My results could have been more accurate because the results I obtained did not produce a good pattern on the graph. To remove this problem, I could repeat the experiment not only with three readings at each temperature, but also with three different solutions for each set of results, which would provide an even more accurate reading, as I could calculate an average.
I would perhaps use a gas syringe instead of a boiling tube to make the reading more accurate. Maybe for a further investigation see what affects radiation has on yeast respiration. But that is hard because of the restrictions on the use of radioactive substances. I could maybe use methylene blue. I would time how long it takes to revert to the original colour using a control. This would be done at various temperatures to obtain the best range of values.