An investigation into the breakdown of Hydrogen Peroxide by Catalase.

Luke la Hausse 4 alpha

An investigation into the breakdown of Hydrogen Peroxide by Catalase

Background Theory

An enzyme is a protein. It is a globular protein – its molecules are round in shape and they usually have an area shaped like a pocket. This ‘pocket’ is called the active site. This is the area where a substrate can join the enzyme and be divided into two or more products. It can be compared to a ‘lock and key’ system, the lock being the substrate, and the key the enzyme. This is demonstrated in the diagram below:

ENZYME

ENZYME and SUBSTRATE ENZYME

We could think of this process as similar to a person catching a ball. As the ball enters the person’s hands they close around it. In the case of the enzyme, the ‘ball’ is the substrate and the ‘hands’ the enzyme. The enzyme ‘catches’ the substrate and then it puts pressure on the bonds within it. This pressure weakens the bonds between the two or more products and they therefore break away from each other. An enzyme is a biological catalyst. It will speed up a reaction or just facilitate the reaction on its own. The more enzyme that is present in the reaction the faster the reaction will take place, and the more products will be formed. An enzyme is never used up in a reaction, but can denature due to excessive heat, exposure to toxins or change in pH. When this happens the active site becomes deformed and the enzyme will no longer work. Catalase is an enzyme that breaks down hydrogen peroxide into its two products: oxygen and water. The reaction is represented by this symbol equation:

2H2O2 O2 + 2H2O

Enzymes

Catalase is one of the most efficient enzymes known to man. It is present in abundance in liver and the cells of potatoes. As potato is easily available, we will use potatoes to obtain our catalase. By varying the length of the potato sections used we can vary the amount of catalase that is present in the reaction.

Hypotheses

Based on the above, two predictions can be made with confidence:

As the length of the potato is increased, exposing more catalase, the rate of reaction will increase.
Due to the increased rate of reaction more water (H2O) and oxygen (O2) will be produced.

Method

To prove the hypotheses above, a simple experiment will be carried out. In this test O2 will be collected in a measuring cylinder released from the substrate, hydrogen peroxide by the action of the enzyme, catalase. To carry out this experiment the following equipment must be used:

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Hypotheses

Based on the above, two predictions can be made with confidence:

As the length of the potato is increased, exposing more catalase, the rate of reaction will increase.
Due to the increased rate of reaction more water (H2O) and oxygen (O2) will be produced.

Method

Beaker x 1
Boiling tube with delivery tube attachment x 1
Bung x 1
Glass rod x 1
Measuring cylinder x 1
Potato borer (diameter: 1cm) x 1
Ruler x 1
Scalpel x 1
Stop clock x 1
Syringe x 1
Water container x 1
White tile x 1

When carrying out this experiment it is essential that safety goggles be worn, and that any clutter, around benches is cleared away.

Procedure

Fill the water container up to 3cm from the brim. Immerse the measuring cylinder in the water and expel all the air from it. Place it upside down in the container (see diagram).

Collect 30cm³ of hydrogen peroxide and place it in the boiling tube with delivery tube attachment. Place the end of the delivery tube into the bottom of the measuring cylinder, under the water (see diagram).

Take the potato borer and cut out a cylinder of potato. Remove the skin from the ends using a scalpel. Then place the potato alongside the ruler and cut it to the length that is required.

Place the measured piece of potato into the boiling tube and insert the bung in the end. Only once the bung is in place should the end of the delivery tube be placed into the bottom of the measuring cylinder, to prevent air being pushed into the measuring cylinder by the insertion of the bung.

Once the delivery tube end has been placed in the measuring cylinder, start the stop clock. Leave the reaction to take place for two minutes. At the end of two minutes the delivery tube must be removed from the measuring cylinder and a reading of how much oxygen has been produced must be taken.

This is how the apparatus should be set up.

This outlines the basic procedure. During the actual experiment this basic procedure was repeated several times. For each length of potato the experiment should be carried out three times. This ensures that no anomalies will distort the results, and provides more data which can contribute to a more accurate graphical display than just one result. When carrying out any experiment it is necessary to recognize the variables, and whether they are fixed or not, within the procedure. To decide what was going to be kept constant we carried out preliminaries. What was found is shown here:

From these results it was decided that the best combination of variables was as follows:

Time: Fixed: 2 minutes
Volume of H2O2 needed: Fixed: 30cm³
Length of potato: Variable: 1 – 5cm
Concentration of the H2O2: 2M

The decision was taken that the time would be fixed, otherwise the results would not be comparable to each other. To give a good spread of data it was decided to use 30cm³ of H2O2, for the more reactant available the more likely that the reaction will last out the two minutes. If the reaction stopped before the two minutes was up then once again the results would not be comparable. In any experiment there must be at least one variable, in this case it is the length of the potato cylinder. The lengths will be 1 – 5cm, with the procedure for each length being repeated three times. The concentration of the hydrogen peroxide is fixed because it would firstly: be a difficult variable to change, and secondly, produce a minimal change in results. All of these decisions will be carried through into the practical.

Analysis

The method was followed precisely. These were the results that were attained:

From these results a graph was plotted:

The data displays a very good pattern in terms of a steadily increasing rate of reaction. It shows, in the graph, that as the length of potato increases the oxygen produced increases. The results are proportional to each other and the method that was used has returned a reliable spread of data. There seems to be a slight dip in the results for the 3cm length that may be due to the slight inaccuracy of the results. It seems also that as the pieces of potato got bigger the rate of reaction also got much faster. There is a significant jump from the 3cm value to the 4cm value. The difference is 1.5cm³, of oxygen, between 3cm and 4cm; this seems to show that this batch of 3cm lengths produced less oxygen than could be expected.

Conclusion

In conclusion it can be said that as the amount of catalase present in the reaction increases, so does the rate of reaction and therefore the volume of oxygen produced from the reaction. It was predicted that:

As the length of the potato is increased, exposing more catalase, the rate of reaction will increase.
As a result of the rate of reaction increasing more water (H2O) and oxygen (O2) will be produced.

In both instances this has been demonstrated. Although the amount of water that was produced from the reaction could not be measured directly, it can be said that it would increase proportionally to the amount of oxygen produced. This is because in any reaction there is a proportional amount of each product produced: at each end of a reaction the number of molecules and atoms must balance. In theory the more catalase added, the faster the reaction would take place, and the more oxygen and water would be produced.

Evaluation

The method that was used to obtain the results was fairly simple and easy to carry out. The results were easy to collect and to convert into a graphical display. The results themselves were close together indicating that the method of collection was both reliable and accurate. They were only accurate in the sense that they were close together. The actual equipment that was used to measure the amount of oxygen produced, was relatively unreliable. The markings to measure volume, were hazy and hard to distinguish. Working alone, it was difficult to place a bung in the boiling tube, place the delivery tube into the base of the measuring cylinder and start the stop clock at the same time. Therefore the stop clock was started after the reaction had begun. In all fairness this did not jeopardise the results because this was true of every attempt that was made. Care was taken that each time the stop clock was started after the bung was inserted. Overall this was quite suitable for what was required.

It would have been ideal if there were two people carrying out the experiment together to ensure that all of the procedure was done at the correct time. Ideally four or five attempts should have been carried out at each length rather than three. This would give far more accurate results and avoid the type of anomalies such as that of the 3cm measurements.

In summary, if the experiment were to be carried out again or simply extended then the following changes could have been made:

Rather than just 3 repeats four of five should be carried out.
Due to the limited lengths of potato the graph provided a small snapshot of the relationship. It would therefore be useful to increase the number of lengths to extend from 1 – 10cm.
A second person should be present to help carry out the experiment.

In accounting for the anomaly in the set of results evidenced by the 3cm length, perhaps this was due to human error. On the whole this did not really upset the conclusion drawn from the results, but introduced the possibility of an exponential relationship between the amount of oxygen produced and the amount of catalase present. This was because in the next result there was a dramatic increase in the production of oxygen. This conclusion cannot be made without the provision, and examination of a greater number of results.