Fair Test
To ensure that my procedure is a fair test I will make sure that the whole procedure is done at a constant temperature. I will use heated water baths to achieve this. I will also ensure that the pH of the solutions in the test tubes is constant. To do this I will use buffer solution. I must also keep the light levels constant and to do this I will do the experiment in the same part of the lab and do it all on one day, to avoid changing light levels. I will ensure that every disc of potato has the same volume of catalase in it (to as much accuracy as is humanly possible) by cutting the discs to exactly the same width and length. I will also have to keep the hydrogen peroxide levels exactly equal in all of my test tubes. To do this I will use a pipette when adding the hydrogen peroxide. I will vary the volume of catalase in the solution and keep every other possible variable constant, especially temperature, as my preliminary work showed me that temperature has a massive effect on how well the enzyme works.
Prediction
I predict that the more enzyme added to the hydrogen peroxide, the faster the reaction and therefore more oxygen produced in a given time. This will mean that the more potato discs (containing catalase), the higher its height of oxygen froth will be after 10 minutes. Thus that the heights of the froth in the test tubes with the most potato discs (9) will be higher than the test tubes with 3 potato discs in them. I predict that if the amount of catalase is doubled, the amount of oxygen froth will also double.
This is a graph of my predicted results.
To understand why the enzyme catalyses the reaction we need to understand more about how an enzyme works:
An enzyme is a highly specific catalyst that greatly speeds up reactions that would otherwise be too slow to allow a living organism to function. The reason why an enzyme is so specific is because it is a globular protein with a distinct three-dimensional shape which corresponds to the molecule or molecules it reacts with (substrate(s)). When an enzyme catalyses the reaction of its substrate it forms an association with it called an enzyme-substrate complex. This is possible because quite a large part of enzyme called the active centre matches the shape of its substrate. So when an enzyme and its substrate collide at the right orientation they become attached at the active centre. The enzyme and substrate can now interact in a way that allows the substrate to react in a much more efficient way and the products of the reaction are formed. When this happens they detach from the active centre and the enzyme is left unchanged to go and react with another substrate molecule.
This can be repeated extremely quickly especially in the case of catalase. Each molecule has the potential to combine with up to 10000 molecules of hydrogen peroxide per second. The mechanism of enzyme action I have just described is known as the Lock and Key Hypothesis with the enzyme being the lock and the substrate being the key. However this concept is not fully helpful, as the ‘lock’ and ‘key’ are not static structures. They actually interact with each other so that their shapes are both changed. A better way of looking at this is the concept of induced fit. This means that the enzyme’s configuration changes as the substrate joins at the active centre. As this happens, the binding process forces the chemical bonds of the substrate molecule to stretch or compress so that a particular chemical reaction is more likely. When the products are unbound, the enzyme returns to its normal shape.
It is important in my experiment to keep the temperature constant, as I found out in my preliminary work:
I did the same experiment as I am about to do, except I did not vary the amount of hydrogen peroxide or the number of potato discs. Instead I put the test tubes in water baths of varying temperatures, each with 3 potato discs and 5ml of 20vol hydrogen peroxide. The test tube that had been in the bath set to 40°C had the highest column of oxygen froth, showing me that catalase works most efficiently at a temperature of 40°C. This makes sense, as the human body is roughly 40°C and there is catalase inside our body tissues.
RESULTS
This is a table of my results.
This is a graph of my results.
Here is a table of some class results. Different sets of people did the same experiment as me and got these results. I have worked out the mean for the class results and plotted a graph
ANALYSIS AND CONCLUSIONS
There is a clear downward slope which gets less steep towards the end of the graph. I am only concentrating on the part before the slope decreases in gradient.
The graph shows us that when the number of potato discs is doubled, the time taken for 2ml of oxygen to be collected roughly halves. This is because the amount of the enzyme has increased, allowing more reactions to be catalysed by it, producing oxygen at a higher rate than previously. If we look at the Lock and Key Theory it is clear how this works. As said previously, an enzyme is a highly specific catalyst that greatly speeds up reactions. Enzymes are specific because they are proteins with a distinctive three dimensional shape that corresponds to the molecule (substrate) it reacts with. When an enzyme catalyses the reaction of its substrate it forms an association with it called an enzyme-substrate complex. This is happens because quite a large part of enzyme called the active centre matches the shape of its substrate similar to a puzzle fitting together. So when an enzyme and its substrate collide at the right angle, they join at the active centre. The enzyme and substrate can now interact in a way that allows the substrate to react in a much more efficient way than previously and the products of the reaction are formed. When this happens they detach from the active centre and the enzyme is left unchanged to go and react with another substrate molecule.
In the terms of my results, roughly twice the number of enzyme ‘locks’ have been introduced to the same amount of hydrogen peroxide ‘keys’, meaning the locks and keys can join at twice the rate previously. This means that double the amount of reactions take place in half the time previously. For example in my results when I used 4 potato discs, it took 188.6 seconds for 2ml of oxygen to be collected, but when I doubled the number of discs (and therefore the amount of the enzyme) it only took 92.3 seconds – that’s roughly half the time.
I can back up this statement by concentrating on the class results above:
When group 3 used 4 potato discs, it took 187 seconds for them to collect 2ml of oxygen, but when they doubled the number of potato discs, and therefore the amount of enzyme, it only took 94 seconds – roughly half the time.
I therefore conclude that by doubling the amount of catalase to a solution of hydrogen peroxide, the rate of the reaction will double, therefore meaning that the time taken to collect a given amount of oxygen will halve.
