effect of enzymes on a reaction

Key Factors:

To make this a fair test, I shall:

• Keep original concentration of hydrogen peroxide the same before dilution
• Keep size and length of potato the same
• Use the same potato for every experiment
• Use the same amount of solution every time
• Use a stopclock to time my experiment rather than a watch.

I need to find the vol of my solutions:

Safety:

Prediction

I predict that as the substrate concentration decreases, the rate of reaction will go down and less oxygen will be given off and as the substrate concentration increases, the rate of reaction will go up and more oxygen will be given off.  The most oxygen will be given off when there is 20vol hydrogen peroxide. The rate steadily increases when the substrate is more concentrated because more of the active sites of the enzyme are being occupied which results in more reactions so the oxygen is given off more quickly.  Enzymes have an active site, which is shaped so that only a molecule, known as the substrate, with the correct shape can link into the enzyme.  Once the enzyme and the substrate become associated, the enzyme can increase the probability of a chemical reaction occurring by a number of mechanisms all explicable in terms of the principles of mass action and activation energy.  When the substrate is bound at the active site, random movements of the substrate are prevented and the molecule becomes orientated in a fixed position on the enzyme thereby increasing the probability of reaction at specific sites on the substrate. Positioning the substrate on the active site may also distort the structure thereby producing a strain in one or more chemical bonds.  As the substrate molecule comes into contact with the enzyme's active site it forms a temporary union with the enzyme. Thus, instead of a one-step reaction taking place, enzyme mediated reactions occur via an intermediate step in which the substrate first forms an enzyme-substrate complex which in turn undergoes a second reaction to form the final product and return the enzyme to its original status. By converting a one step reaction with a large activation energy into a 2-step process, each step of which has a smaller activation energy, the overall rate of product formation is increased.  If the amount of substrate molecules in the solution exceeds the number of active sites available then the rate of reaction will no longer go up.  This is because the maximum number of reactions are being done at once so any extra substrate molecules have to wait until some of the active sites become available.  The least amount of oxygen will be given off by the 10vol solution this will be because there is not enough substrate to fill the active sites of the enzymes.  It works in a similar way as if there were not enough enzymes and therefore active sites for the substrate to fill.

Equipment

Method

1. Bore 18, 25mm long, 5mm round tubes from the Potato, on the tile and cut to correct length using the scalpel and ruler.
2. Fill ice-cream tubs with water, fill large measuring cylinders with water and place into ice-cream tubs without letting air in.  Set up boiling tube rack next to the full ice-cream tubs and place end of delivery tubes into large measuring cylinders.
3. Measure out 2.5 cm3 of solution into boiling tubes.  Start with 2.5cm3 or 10ml of 20vol hydrogen peroxide and no water, 20vol solution.
4. Add potato to the boiling tube.
5. Place the bungs into the boiling tubes and start the timer.
6. Measure the amount of oxygen given off into the large measuring cylinders.
7. Repeat for every concentration of solution, 18vol, 16vol, 14vol, 12vol and 10vol.

Results:

Results from my experiment showing the amount of gas given off when certain solutions of hydrogen peroxide where decomposed with the aid of catalase.

Analysis and Conclusion

I conclude from my experiment that as the substrate concentration decreased, the rate of reaction went down and less oxygen was given off and as the substrate concentration increased, the rate of reaction went up and more oxygen was given off.  The 20vol hydrogen peroxide gave off the most oxygen and the 10vol solution gave off the least oxygen.  The rate steadily increased when the substrate was more concentrated because more of the active sites of the enzyme were being used, which results in more reactions so the oxygen was given off more quickly. The least amount of oxygen was given off by the 10vol solution because there was not enough substrate to fill the active sites of the enzymes.  My graph supports my conclusion, although I do have one major anomaly on my graph.  As the solution became more concentrated, the amount of oxygen given off went up.

Amount of oxygen produced / time produced in = rate of reaction

Need way more detail, link to scientific knowledge.

Evaluation

My results were not entirely accurate, I had a few small anomalies in my results which showed in my average, however there was only one major anomaly and that was caused by faulty equipment and I didn’t actually get any results for that particular experiment.  The delivery tubes I used for my experiment were not at all reliable, the tubing was cracked and had holes in places, so this may have affected my results.  The displacement method is also not entirely as accurate as using a gas syringe to collect the gas given off.  On a gas syringe it is easy to get a direct reading, whereas, in my experiment I had to use measuring cylinders which were actually too big, as my reaction did not give off a large amount of gas, so it was hard to get a correct reading.  I did use only one potato to get my results from which made my experiment a fairer test, however as I had to do my experiment over 3 days with limited time periods in which to do my experiment, I kept my potato in a beaker with distilled water just covering them to keep them fresh.  They may have absorbed a little water in this process but my results in my repeats were all similar so I assume that this did not affect my results too much. Also, although only slowly, hydrogen peroxide still decays in the absence of a catalyst, this too could affect the results.  It would also have been a more of a fair experiment if I had left all of my equipment set up and used the same equipment every time.  In general for a lot of factors, it would have been better if the experiment had taken place all on the same day.

Limitations repets overall

References

1.  ( Wikipedia, 15.02.07)
2. Glenn Toole, Susan Toole, (2004), Essential AS Biology for OCR, Cheltenham, Nelson Thornes Ltd
3.