Enzymes Coursework (GCSE)

Here's the graph to show how the reaction rate changes according to the amount of substrate added.  

You can alter this experiment so that you add more enzymes to a fixed amount of substrate.

Here is the graph showing this reaction:-

Prediction

I predict that the more potato disks present the faster the reaction will go. This is because the more potato disks present the more enzymes there are to catalyse the reaction causing it to go faster. I predict that the experiment with 9 discs will go half as fast again as the experiment with 6 discs because I am adding half the amount of potato discs again. This increases the amount of enzymes present and so increases the number of collisions between the enzymes and substrate (which fits the enzyme exactly). This increase in collisions increases the reaction speed. Similarly I expect the experiment with 12 discs to go double the speed than the one with 6 discs and a third faster than the one with 9 discs.

Plan

Before I do anything I have to regard the safety aspects of the experiment, here are some simple safety rules to help avoid accidents.

• Don’t run in the Laboratory
• Don’t throw things
• Stand up when doing an experiment
• Clear the work area before starting
• Clear any bags and stools away from where you will be standing
• Tie Back long hair
• Tuck ties into your shirt
• Wear safety glasses
• If you spill anything mop it up immediately
• Be aware of the people around you

Here is a list of all the equipment I plan to use in the experiment:-

• Cork Borer
• Scalpel
• Forceps
• Boiling Tube
• Delivery Tube and Bung
• 75cm³ Beaker
• 250cm³ Beaker
• Stopwatch
• 2x 10cm³ Measuring cylinders
• Water Trough
• Ruler
• Potato
• 2x Paper Towels

Before writing my final method I carried out a preliminary experiment to find the amounts I need. I found that 5cm³ of Hydrogen peroxide wasn’t enough to keep the experiment reacting at a good pace for 5 minutes.

Here is how I plan to carry out my experiment. After clearing my work area I will fill my water trough with cold water and put one of my 10cm³ Measuring cylinders in it upside down and full of water, to do this I will fill my 10cm³ measuring cylinder to the brim with water and put my thumb over the top making sure that there is no air bubble in it, I will then turn it upside down, submerse the top in the water trough and remove my thumb, now I will put the end of the delivery tube into the upside down measuring cylinder. Now I will take a paper towel and clearly label it Hydrogen Peroxide, I will place my 50cm³ beaker full of Hydrogen Peroxide this. I mark the paper towel `Hydrogen Peroxide’ so that if anyone knocks the beaker over they will know it is not water and mop it up quickly. Using my second 10cm³ Measuring cylinder I will measure 10cm³ of Hydrogen Peroxide from the 50cm³ beaker. I will fill my 250cm³ Beaker with hot water from the tap and add cold water to it until the temperature goes down to 37ºc (this being the body temperature so the optimum temperature for the enzymes to work at), I will now pour the 10cm³ of Hydrogen Peroxide out of the Measuring cylinder and into the boiling tube which I will place in the beaker full of water at 37ºc to warm up. I will now take a core out of the potato using the corer, using the ruler and scalpel I will cut this core into 1mm thick discs. Using the forceps I will drop the required amount of discs (6 for the first three experiments, 9 for the next three experiments, 12 for the last three experiments) into the boiling tube containing the hydrogen Peroxide, I will give this a quick swirl then place it back in the beaker full of water at 37ºc and push the bung on the end of the delivery tube into the top of the boiling tube, at the same time starting the stopwatch. Every 30 seconds for 5 minutes I will record how much gas has collected in the upside down measuring cylinder. I will repeat this experiment as many times as I need to.

To make sure I am performing a fair test I will make sure that all the measurements I carry out are as accurate as possible – the water will be at exactly 37ºc in every experiment, every potato core will have been cut with the same corer and every disc will be 1mm thick, I will use 10cm³ of Hydrogen Peroxide in each experiment. Doing this will make sure that all the variables remain constant apart from the amount of potato disks used – which is what I am investigating – making this a fair test.

Here is my equipment Layout

In this experiment I will be very careful in my measuring of the Hydrogen Peroxide, the size of the potato discs and the temperature of the water as these are variables I will be keeping constant. The only variable I will be changing is the amount of potato discs in the Hydrogen Peroxide. The potato disks that I use will all be from the same potato to ensure that they each contain the same amount of enzymes.

Here are the results I recorded during the experiment:-

I will now calculate the average results for each number of potato discs:-

 

Analysis

I have found out that the more potato discs present the faster the reaction. After 5 minutes 12 potato discs gave me 7.7cm³ of gas in 5 minutes, this is because when the  potato disks were added the enzymes in them acted as a catalyst to the hydrogen peroxide, providing active sites to convert the hydrogen peroxide to water. Using 6 discs only gave me 4.6cm³ of gas in 5 minutes. This is because I added fewer potato disks, so fewer enzymes, there were less collisions between the enzymes and the substrate making the reaction go slower. The graph for 12 discs is also a lot steeper than the graph for 6 potato discs showing that the reaction went faster due to there being more active sites open to catalyze the reaction and consequently more chance of collisions between the enzymes and the substrate. This allows more individual reactions to take place at the same time. These results prove the first part of my prediction that the more potato discs the faster the reaction would go, it does not follow the second half of my prediction at all, that the experiment with 12 potato disks would go twice as fast as the experiment with 6 potato disks.  I cannot tell however whether this is due to the inaccuracy of my results or a wrong prediction.

 

Evaluation

My results are as accurate as our equipment and method allowed me to be. I think that my method was suitable and the results it gave me are reliable because all of my repeats showed very similar results. My results are reliable enough to conclude that the more potato discs, and consequently enzymes present, the faster the reaction. I have circled the anomalies on the graph, I believe that the first anomaly at 150 seconds and 9 potato disks should show 3.8cm³ of gas collected rather than 3.6 and the second anomaly should show 4.4cm³ of gas collected rather than 4.3cm³. There are many possible reasons for these anomalies. Firstly the experiment was kept at 37ºc by keeping the boiling tube in a beaker of water, at the start of the experiment the water was at 37ºc but as the experiment proceeded the water may have cooled, lowering the temperature of the reaction, if this happened the particles in the reaction would have less kinetic energy, meaning less collisions between the enzymes and the substrate. This would slow down the reaction. This could be prevented by using an electronic water bath which would keep the temperature at 37ºc constantly.

 Another possible cause for these anomalies is that cutting each potato disc to exactly 1mm by hand is impossible, leaving lots of room for error in the experiment. If one potato disc was thicker than the rest, it would have more surface area, the extra surface area would increase the amount of enzymes, providing more active sites and so speeding up the reaction. Conversely, if the disk was thinner than the rest it would have a smaller surface area, reducing the number of enzymes, reducing the number of active sites and consequently slowing down the reaction. To make sure the measurements were exact I would have had to use equipment such as a microtome.

I measured the gas released by the reaction in an upside down measuring cylinder and stopwatch, when a bubble of gas arrived just as I was taking a reading was I supposed to count it in or not? To make this accurate I could use a computer to measure for me. This could cause the anomalies in my results as I could have recorded a measurement a little after or before another bubble of gas entered the measuring cylinder.  I did not have the required equipment for any of these improvements.

Other causes for my anomalies could be that if the Hydrogen Peroxide (H2O2) had been left out for a while it could have degraded. This would dilute it, reducing the chance of a Hydrogen Peroxide molecule colliding with the enzyme, slowing down the reaction. Also the potato I used could affect the reaction rates. In all my experiments I used the same potato but I did each set of experiments (6 discs, 9 discs and 12 discs) on separate days so the potato was ageing. The age of the potato affects the amount of enzymes it contains, the older it is the fewer enzymes. Therefore it is likely that there were slightly fewer enzymes in the potato disks I used in the nine discs experiment than the 6 discs experiment and slightly fewer enzymes in my 12 discs experiment than in my 9 and 6 discs experiments. The fewer the enzymes the fewer active sites available and the less chance of collision between enzyme and substrate. This produces a slower reaction.

To extend my experiment I could find the temperature at which the enzymes stop catalysing the reaction either due to denaturing or lack of energy due to insufficient heat. To do this experiment I would use a set amount of potato disks and change the temperature of the water bath, recording the amount of gas collected in 5 minutes for a range of temperatures between 10˚c to 40˚c.