Average time graph:
From this graph you can see that as the surface area/number of potatoes increases the average time taken to produce 5 ml O2 decreases. For example the average time taken for 1 potato is 98 seconds but for 2 potatoes it is 39.33 seconds. This shows a significant decline in the average time taken from 1 to 2 potato pieces. The longest average time taken is for 1 potato and the quickest is for 5 potatoes which is 10.67 (sec).
Conclusion: -
From my results you can see that as the surface area of the potato increases so does the rate of reaction. With what I know about enzymes I can see why I obtained a pattern out of these results. This is because as the number of potato pieces increase/surface area so does the number of collisions between the enzymes and substrate to form enzyme – substrate – complexes and to lead on to the formation of product. The surface area is a measure of the amount of enzyme that is in contact with hydrogen peroxide. So as there is an increase in the number of potatoes so there is an increase in surface area of the potato. So this means there is an increase in enzyme concentration and so there is an increase in the rate of reaction.
Another similar reason is that there are more free active sites in the enzyme to combine with the substrate and form product. This is when there is an increase in the amount of enzymes resent. The shape of the active site is specific to the shape of the substrate molecule (s). The substrate (s) fit perfectly into the enzymes active site and is held in place by temporary bonds between the substrate and the R groups of the enzymes amino acids at its active site.
Eventually the rate of reaction will reach its maximum as all the substrate molecules are converted to product by the enzymes as quickly as possible. The rate of reaction remains constant even if enzyme concentration is increased because the amount of substrate is limiting the rate of the reaction. The substrate is the limiting factor but this did not occur in my experiment. The substrate was not a limiting factor.
Another important trend that I analysed is that as the surface area increases so does the rate. This is due to both the rate proportional and the surface area being proportional to each other. 5 potato pieces had the fastest rate of reaction due to there being more catalase to react with the substrate and form product. 1 potato piece had the slowest reaction due to their being less enzyme to react with the substrate and form product.
My analysis of the average time taken graph has led me to conclude that as the surface area increases the average time taken to produce oxygen decreases. This is because there is more enzyme to react with substrate and form product in less time. 5 potato pieces has the shortest average time taken due to its overall surface area being the biggest and therefore containing much more enzyme so it means it has much more chance to react with the substrate and form product much more quickly. 1 potato piece has the longest average time taken because it has a small surface area and therefore has less chance of combining with the substrate and forming product.
So in conclusion as the surface area increases the rate of reaction increases and the average time taken decreases. The rate increases due to there being more enzymes and a greater chance of collisions. The average time decreases due there also being more chance of collisions with substrate and forming product more quickly.
Evaluating the procedure: -
In my opinion the experiment was very good because the results were as expected and match my prediction. I predicted that as the surface area increases the rate of reaction would increase. I also predicted that as the surface area increases the average time taken would decrease. These predictions match my results, which shows the experiment was good.
Though our experiment was good there are still some main sources of errors in the experiment, which didn’t allow us to get perfect results. I am going to discuss them all and give reasons for how they might affect the results and ways of improving the error.
Controlling temperature is a main source of error. As temperature increases, enzyme and substrate molecules gain more kinetic energy and move faster. In an enzyme catalysed reaction, such as the decomposition of hydrogen peroxide, temperature increases the rate at which the enzyme and substrate molecules meet and form enzyme – substrate - complexes and therefore the rate at which the products are formed. As the temperature continues to rise it will eventually reach its optimum temperature, however if it goes above its optimum temperature, the hydrogen and ionic bonds, which hold the enzymes tertiary structure will brake and the enzyme will unfold and the active site will be lost. This means molecular structure is disrupted, the enzyme ceases to function, as the active site no longer is able to accommodate the substrate. The enzyme is said to be denatured but this did not occur in my experiment. The rate and time could have been affected because the room could have heated up as we did our experiment meaning the rate could have increased and the time could have decreased.
To control this variable, the temperature should be maintained at a fairly constant level that allowed the enzyme to work effectively (room temperature, approximately 23ºC). This can be achieved by using a test tube rack and tongs to handle the apparatus so that the heat from my hands does not affect the Catalase or substrate. This is because my body heat could raise the temperature, increasing the rate of reaction. You can also monitor the temperature by using a thermometer to ensure that it remains constant and does not disrupt the results of the experiment by affecting the activity of catalase. There may have been fluctuations in the temperature inside the room that we did the experiment in. You could use special temperature chambers, which keep the temperature constant so the rate of reaction, enzyme and substrate activity is not affected. You could use a water bath to make sure that all the substances are at the same temperature.
The pH is a main source of error in the experiment and any change in pH affects an enzymes rate of reaction. Each enzyme has an optimum pH at which its active site best fits the substrate. Variation either side of pH’s optimum temperature results in the enzyme having a slower rate of reaction. This is because the change in pH affects the charges on the enzymes active site making it harder for the enzyme and substrate to form enzyme-substrate-complexes. Extreme changes in pH away from optimum causes the enzyme activity to stop and the enzyme is said to be denatured. This is due to the bonds, which hold the enzymes tertiary structure unfolding and distorting the active site but this did not occur in my experiment. The pH might have been affected by me handling the potato, equipment and also by rinsing out the equipment. This could have caused contamination and the pH may have affected the rate by either increasing it or decreasing it.
In this experiment, the pH can be kept constant using a pH buffer solution, selected to maintain a pH level suited to the enzyme by being equal to the natural environment of the enzyme (potato tissue). Also so the rate of reaction is not affected.
Enzyme Concentration is a main source of error. Provided there is an excess substrate, an increase in enzyme concentration will lead to a corresponding increase in rate of reaction. Where the substrate is in short supply (i.e. it is limiting) an increase in enzyme concentration has no effect. In the experiment there may be an uneven distribution of catalase in each potato and different potatoes may have different concentrations of enzymes.
You can vary the enzyme concentration by altering the number of equal sized discs of potato that contain the Catalase, in the reaction. Another way of solving this problem is to try and find a very big potato so you are able to bore out of it 45 potato pieces. Though it is very hard to find a potato that big it will reduce the problem of getting different concentrations of enzymes in different potatoes and different potato sticks taken from different potatoes. Another improvement is to put the potato in a blender and mash up the potato so this will make sure that there is an equal distribution of catalase in the potato mash and so the rate of reaction will not be affected. Adding increase vols. of the mash instead of increase potato sticks as a measure of enzyme concentration. Also it will ensure a fairer test. Maybe having a source of catalase from yeast would have been much easier to use and handle. Specific amounts could be measured much more precisely. All these things could cause a faster reaction due to an increase in the rate and also a decrease in the time taken.
There may have been a small error in the cutting of the potato to 4cm. The potato piece may not have been straight when cutting the ends so this means some potato pieces are not exactly 4cm and this could lead to inaccurate results due to there being variations in the amount of enzyme in the potato. A way of improving this error is to put a protractor or any thing with 90o angle and cut straight down to ensure the ends are straight.
Another error is in the inconsistency of the collection of the bubbles i.e. sometimes the bubbles collect up in the tube and come up out of the tube all at one time. This will cause inaccurate results because there will be a very fast reaction at the beginning. Which means patterns and trends could be affected. This is because if the bubbles come up all at once then this will cause a faster rate of reaction. If the bubbles come up very slow then this will cause a slower rate of reaction. A way of improving this is to make sure that the collecting tube is clean and there are no restrictions in the tube to allow the bubbles to pass through easier. Also the tube has a wider diameter so the bubbles pass through easier. You can also collect O2 in a gas syringe instead.
Putting the potato pieces under distilled water in a petri dish prevents the potato from being contaminated or dehydrated. Handling potato pieces with a pair of tweezers to prevent contamination. The conical flask must be clean after each trial so to prevent any unwanted contamination. Preferably you should use completely new equipment each time to prevent contamination and unfair results. It is impossible to precisely measure out the amount of hydrogen peroxide each time. As the scale on the measuring cylinder shows the measurement to the nearest 1ml. This should be changed to the nearest 0.1ml.
Evaluating evidence: -
Form doing my experiment I can conclude that I have no anomalous results.
I think that the overall accuracy of the measurements was quite accurate. The volume of hydrogen peroxide was measured using a measuring cylinder to the nearest 1ml. This is quite accurate but it would be better to have it to the nearest 0.1ml for the most accurate results. The volume of oxygen was measured using a measuring cylinder to the nearest 0.2ml. This is quite accurate but just as before it would be better to have it to the nearest 0.1ml for the most accurate results.
Timing the amount of time it took to produce 5ml of oxygen was very accurate and does not need to be changed. We measures the (min: sec: 1/100th sec) and this is accurate enough it is dependant on your own reaction times. The potato piece size was measured using a ruler in mm. I think this is good enough as long as you’re precise in getting the right size.
I have analysed all the main errors in the section above this and I don’t think they are great enough to have affected my results to an extent that makes my results unreliable. Some of the main errors are temperature, pH and enzyme concentration and I don’t think these were big enough to have affected my results by that much. My prediction matches my results so they have to be good.
Range for the varying potato pieces:
We are working out the range because it is a good measure of the average. It reduces the problem of extreme values and can be used to work out the uncertainty.
1 potato piece – 105, 85, 104 therefore range = 105 - 85
= 20
This number is big but not that big to consider this result to be an anomaly.
2 potato piece – 47, 33, 38 therefore range = 47 - 33
= 14
This number is not that big and shows the result not to be an anomaly. It also shows that the values are quite accurate.
3 potato piece – 28, 28, 25 therefore range = 28 - 25
= 3
This number clearly shows that the results are very accurate.
4 potato piece – 16, 15, 13 therefore range = 16 - 13
= 3
This number clearly shows that the results are very accurate.
5 potato piece – 10, 11, 11 therefore range = 11 - 10
= 1
This number clearly shows that the results are extremely accurate.
The range shows that were possible anomalies with 1 and 2 potato pieces but with the results that were calculated this anomaly is not enough to affect pattern or results.
Percentage uncertainty:
We are working out the percentage uncertainty because it tells us the amount of error in the experiment and to check how valid our results were.
A table to show the range, uncertainty and percentage uncertainty of the time taken to collect 5 ml of O2: -
At the end of my experiment my % uncertainty was 10. This is low enough to accept my results are valid and to say my conclusion is valid. So the errors discussed above are not significant enough to have affected my results and made my conclusion invalid.