I predict that the rate of reaction will be fastest in a neutral pH for the simple reason that peas don’t make me ill so they can’t be very acidic or alkaline.
Method:
- Set up five test tubes labelled A, B, C, D and E:
To tube A add 5 ml buffer pH 12
To tube B add 5 ml buffer pH 9
To tube C add 5 ml buffer pH 7
To tube D add 5 ml buffer pH 4
To tube E add 5 ml buffer pH 1
- Grind up two soaked peas using a pestle and mortar. These contain the enzyme peroxidase. Add them to tube A using a clean spatula. Repeat this for the other four tubes also.
- Leave for 5 minutes
- Add 2ml of Hydrogen Peroxide (this is the substrate) to each test tube and assess the rate reaction on a scale of 0-4:
0 = no reaction
1 = very slight reaction
2 = slow reaction
3 = moderate reaction
4 = very rapid reaction
- Measure the amount of froth there is sitting on top of the solution with a ruler.
Results:
Conclusion:
My results show that enzymes are affected by pH. As I predicted, peroxidase works fastest at a neutral pH because pH 7 showed the most extreme reaction. Conversely, more extreme pHs have lower rates of reaction. I am surprised that pH 9 had such a high rate of reaction, but this may show that the enzyme peroxidase can work in slightly acidic environments also. However, the sudden drop in the rate of reaction at pH 12 may to contradict this theory so it maybe that either one or both of the results for pHs 9 and 12 are anomaly.
A change in pH breaks the ionic bonds that hold the tertiary structure of the enzyme in place. This causes the enzyme to lose its functional shape, meaning that the substrate (in this case hydrogen peroxide) is no longer able to fit in. This is called denature and means that the chances of successful collisions needed for a reaction to take place are decreased. Therefore, the rate of reaction is decreased – in some cases to a point where very little, if any reaction happens at all e.g. pH 12.
Evaluation:
Although I believe my experiment does illustrate the effect pH has on enzymes, I do not think my experiment was particularly accurate.
I used two peas for each test tube. This means that my results will be affected by the fact that some peas will have contained more perioxidase than others. If there was more perioxidase in one test tube that another, then the rate of reaction will be faster as there will be more collisions. Also, some peas may have been ground up more effectively than others so more of the enzyme was available. The experiment could have been improved if I could have used a certain amount of enzyme rather than the relatively inaccurate measurement of two peas.
One of the biggest flaws in my experiment is that my measurements for rate of reaction were merely based on perception rather than an objective measurement. The experiment would have been better had I measured something objective, such as heat produced by the reaction.
Either of the results for pH 9 or 12 could be anomaly. These could be explained firstly by the fact that some peas probably contained more peroxidase than others and secondly that my measurements were subjective.
