Catalyst:
Catalysts enable substances to react more quickly with out it being used itself in the reaction. They do this by helping the bonds to break and form easily. A catalyst does this by giving the particles something to stick to where they can pump into one another. This obviously increases the number collisions and also increasing the number of successful collisions therefore increasing the rate of reaction. The catalyst also lowers the activation energy, so the reaction proceeds quickly. The catalyst does this by lowering the amount energy needed for successful collisions so more collisions will be successful collisions and the rate of reaction will increase.
An enzyme is a biological catalyst. Enzymes catalyse production of sugars by photosynthesis and the breakdown of our food. They also catalyse the synthesis of important chemicals like proteins in our muscles and DNA in our genes. Enzymes are crucial to life. Without enzymes, the reaction in our bodies and in all living things would go so slowly that we would die. Enzymes are proteins. They are synthesised in living things using other enzymes as catalysts. Enzymes are very specific in the reactions they catalase. So, catalase, the enzyme that breaks down hydrogen peroxide does not catalase the break down of any other molecule similar to hydrogen peroxide, like water.
Most enzymes work best when the pH is close to neutral (i.e. pH=7). If pH is changed by adding acid or alkali, enzymes work less well. H+ ions from an acid or OH- ions from an alkali react with enzymes. This changes the composition and structure of an enzyme and it works less efficiently. Different enzymes work best at different pH values.
On every enzyme there is an active site where the substrate molecules are joined to the enzyme. When the substrate is temporarily bonded to the enzyme at the active site, the two molecules form an enzyme substrate complex. The enzyme then breaks down the substrate into molecules of its product. Every enzyme has a specific active site for the substrate with the correct shape to fit into. This is the ‘lock and key’ mechanism. Here is a diagram to illustrate this:
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An enzyme is a biological catalyst. Enzymes are very large and complex organic molecules that are synthesized by the cell to perform very specific functions. These biological catalysts are important because they speed up the rate of the reaction they catalyze that would otherwise be too slow to support life. It promotes the conversion of hydrogen peroxide, a powerful and potentially harmful oxidizing agent, to water and molecular oxygen.
Hydrogen peroxide Catalyst oxygen + water
2H2O2 O2 + 2H2O
Most enzymes are proteins. Enzymes control the rate of reactions in living tissues. The enzyme catalase can be found in many biological tissues like liver potatoes and apples. Catalase is an enzyme present in the cells of plants, animals and aerobic (oxygen requiring) bacteria in the human body enzymes control the breakdown of food and reactions, which make chemicals. Catalase is located in a cell organelle called the peroxisome. Peroxisomes in animal cells are involved in the oxidation of fatty acids, and the synthesis of cholesterol and bile acids. Hydrogen peroxide is a by-product of fatty acid oxidation. White blood cells produce hydrogen peroxide to kill bacteria. In both cases catalase prevents the hydrogen peroxide from harming the cell itself. Peroxisomes in plant cells are involved in photorespiration (the use of oxygen and production of carbon dioxide) and symbiotic nitrogen fixation (the breaking apart of the nitrogen molecule N2 to reactive nitrogen atoms). Hydrogen peroxide is produced as an intermediate during these chemical processes and must be removed to prevent damage to cellular machinery. Aerobic (oxygen requiring) bacteria produce hydrogen peroxide as a by-product of metabolism. This fact is used when identifying bacteria. If hydrogen peroxide is added to a bacterial colony and bubbles are produced, this is evidence of oxygen production and confirms that the colony is aerobic.
Prediction
Small amounts of enzyme can catalyse large amounts of substrate. However this would take longer. If the concentration of the enzyme was to be increased then the time taken for the reaction to reach completion would decrease. Therefore, I predict that as the enzyme concentration (number of potato discs) increases, the time taken for the catalase to break down the hydrogen peroxide (the rate of which catalase enzyme breaks down the hydrogen peroxide) will also increase. Varying the enzyme concentration, will consecutively allow me to measure the rate at which 5ml of oxygen will be produced.
By increasing the concentration I will be increasing the number of enzyme molecules present to react with the hydrogen peroxide. This means that there are more active sites available for the hydrogen peroxide molecules to bind to. This subsequently increases the chance of a successful collision happening. The collision theory states that “Concentrations of the reactants in the rate determining step. If we increase the concentration, we are, in essence, increasing the total number of collisions. This, in turn, will increase the number of effective collisions, and the rate will increase.”
So in increasing the concentration of the Catalase enzyme, the times taken for it to break down the hydrogen peroxide will disproportionately decrease.
By increasing the number of potato discs I will increase the no of Catalase molecules present. So it is fair to say that the speed at which oxygen is formed will increase at higher number of potato discs.
I predict that the graph of the rate of the reaction will look similar to this graph. It will increase steadily and will then reach a certain point after which the rate will stop increasing. The reason why the reaction rate stops rising is there will come a point where all substrate molecules will have joined with enzyme molecules and no more of these substrate-enzyme complexes can be made.
The lock and key theory suggests that the substrate must bind to the active site in order for a reaction to occur. The enzyme acts as a lock and the substrate a key. They form an enzyme substrate complex. The hydrogen peroxide will bind to the active site of the catalase and break it down to form oxygen and water. Eventually every substrate molecule will have bounded to the active site and the reaction will be complete.
List of Equipment
- 250cm³ of hydrogen peroxide (diluted to 20% vol.), so it isn’t that harmful or corrosive to skin if it comes into contact with it. The actual volume needed is 180cm³, but the experiment might need to be repeated as it might have gone wrong or the required amount been miscalculated
- 3 stop clocks to measure the time taken to break down hydrogen peroxide
- 2 knives to cut the potato tubes into discs in order to increase the surface area
- 1 white cutting tile to rest the potatoes on while cutting (and not the table)
- 1 potato borer (with width of 1 cm), to make sure that the smaller pieces of potato are the equal in size and surface area
- 3 large sized potatoes (where the enzyme will be obtained)
- 15 boiling tubes
- Distilled water in order to make dilutions
- 3 delivery tubes attached to bungs which fit delivery tubes and the gas syringes
- Boiling tube racks to place the boiling tubes into
- 3 clamp stands to hold the boiling tubes upright
- 10 pipettes or 2 measuring cylinders to add the hydrogen peroxide into the boiling tubes
- Safety goggles to avoid eyes coming into contact with hazardous chemicals
- Rubber gloves to prevent skin from coming into contact with hazardous chemicals
- Forceps (tweezers) to place and remove the potato discs from the boiling tubes
Method:
- Collect the apparatus listed and set up the experiment as shown.
- Using a potato borer with a diameter of 1cm bore the potato on a white tile. Making sure fingers are kept clear of the borer end, so you don’t hurt yourself.
- Using forceps (tweezers), remove the tube of potato from the borer. Forceps should be used to allow minimal exposure of the potato discs to the surrounding environment.
- Using a knife, cut the tube of potato into equal sized discs with a diameter of 2mm. Make certain that you are careful when cutting the potato and keep your fingers clear of the blade.
- There are 30 discs need as the first experiment is carried out3 times using 10 discs each time.
- Into a boiling tube, collect 10cm³ of hydrogen peroxide (diluted to 20%vol.) using a pipette or measuring cylinder.
- Next, collect the rubber bung with delivery tube attached to it. Make sure that the two are well connected to prevent any loss of the oxygen gas that will be produced.
- The other end of the delivery tube should be attached to the gas syringe.
- Then add 10 potato discs to a boiling tube containing hydrogen peroxide and close the bung straight away.
- Immediately start the stop clock.
- Observe the gas syringe to see the amount of oxygen gas being produced.
- When there seems to be no more change in the amount of gas being produced, stop the clock. Measure the amount of oxygen produced in the gas syringe and record the results obtained.
- Repeat the experiment 2 more times and record the results.
- You will also need to repeat these steps from 1 to 13 (shown above) varying the number of potato discs to 15, 20 and 25.
REMEMBER TO CARRY OUT EACH EXPERIMENT 3 TIMES
Safety
- Wear Goggles as the hydrogen peroxide is a strong corrosive and may burn and bleach any skin that it comes into contact with, even if dilutions have been made. If hydrogen peroxide does come into contact with any skin, immediately wash the area with water.
- Rubber gloves to be worn at all times to prevent skin coming into contact with harmful chemicals such as hydrogen peroxide.
- Handle the knife with care and keep in a suitable place, where it doesn’t come into contact with anything or anyone.
- Take care when using the potato borer. Keep fingers out of the way to avoid harming yourself.
- Set up the experiment in a suitable environment where it doesn’t get in anyone’s way and ensure your workplace is safe
- Dispose used chemicals and solutions in a waste bucket to avoid blockages in sink.
IT IS VERY IMPORTANT TO MAKE SURE THAT YOU ALWAYS WEAR YOUR SAFETY GOGGLES AND RUBBER GLOVES AT ALL TIMES
Fair Test
It is very important that the experiment is carried out fairly. Taking fair testing into account will allow the results obtained to be accurate and reliable. I need to take into account:
- The fact that the potato discs are of equal diameter. As this may provide a greater surface area for the Catalase enzyme to work on.
- The thickness of the potato discs should be maintained throughout the experiment as this again can affect that rate of reaction.
- The substrate (hydrogen peroxide) concentration and volume must be kept the same, as in turn this will affect my results giving the enzymes more substrate to work on.
- Each test should be carried out on an average of three times, which allows a mean of the three to be calculated. This will not only show an overall result, but also indicate an increased amount of reliability.
- The same borer, knife, measuring cylinder or pipette and sample of hydrogen peroxide coming from the same bottle, should be sued when carrying out this experiment. This is to prevent any impurities or contaminations to experiments, which may allow results to be anomalous.
- The same type of potato needs to be used as using different types of potato may result in their being different amounts of Catalase enzyme, which would lead to the results becoming biased and unfair to the experiments with the other potato.
- The pH of the experiment will be kept constant, as varying the pH of the experiment will contaminate the experiment and cause peculiar results.
- All the experiments will be carried out at the same temperature, which will be room temperature (between 20-25°C), to ensure each experiment is given an equal chance to be as fair as possible.
Results Table
Analysis
From the results that I have obtained, I can see that the time taken (number of seconds) for 5ml of oxygen to be produced decreases, as the number of potato discs increases. This means that the activity of the Catalase enzyme on the hydrogen peroxide also increases. In addition, this allows me to suggest that the rate of reaction likewise increases.
This can be seen on the graph showing the average time taken to break down hydrogen peroxide into oxygen gas and water.
The relationship between the concentration of Catalase and the time taken for 5ml of oxygen to be produced is disproportional. At 15 discs the average time taken was 272 seconds, whereas, at 25 discs, when there was clearly more Catalase present, the average time was 130 seconds.
272 = 2.092
130
The average time taken approximately halves as the number of discs increases by just 10. More oxygen gas evolves at a larger number of potato discs. You can see the points plotted seen fairly accurate, as most points fit the trend line (line of best fit). There is one particular result which does not fit the line of best fit. This is at 10 discs. It took 376.33 seconds. Looking at the line of best fit, the time at 10 discs should have been 344 seconds. The time expected is much lower than the one I attained. This could be due to many experimental errors.
From my graph showing the average time taken for 5mlof oxygen gas to be produced, I can see that the time taken for the gas to evolve, decreases as the number of potato discs increases, i.e. the concentration of Catalase.
This related to the collision theory; “the time taken for a reaction to take place or for the set volume of gas be give off, becomes shorter for higher concentrations of enzyme. This is because higher concentrations of enzyme contain more enzyme molecules than the lower concentrations. If there are more molecules, then there are subsequently more collisions taking place over the period of a second. This means that more reactions between the enzyme and substrate molecules take place in a second and therefore the product is evolved more promptly.”
Therefore at higher concentrations of Catalase, the time taken for 5ml of oxygen to evolve will decrease.
Calculations:
Looking closer, I calculated that the time taken to produce 1ml of oxygen was:
10 discs: 376.33 = 75.266 seconds
5
This shows that when 10 discs were reacting with 10ml of hydrogen peroxide, it took around 75 seconds to produce 1ml of oxygen.
15 discs: 272 = 54.4 seconds
5
This shows that when 15 discs were reacting with 10ml of hydrogen peroxide, it took around 54 seconds to produce 1ml of oxygen.
20 discs: 197.67 = 39.534 seconds
5
This shows that when 20 discs were reacting with 10ml of hydrogen peroxide, it took around 40 seconds to produce 1ml of oxygen.
25 discs: 130 = 26 seconds
5
This shows that when 25 discs were reacting with 10ml of hydrogen peroxide, it took exactly 26 seconds to produce 1ml of oxygen.
These values also show that the time taken for a set volume of oxygen gas to be produced decreases. The values prove that the trend is valid and that activity increases as the concentration of the catalase increases, therefore decreasing the time.
I worked out the rate by dividing 1 by the time taken to evolve 5ml of oxygen gas. (1/T)
My graph showing the rate corresponds with my prediction. The rate increases as the concentration increases. This therefore displays positive correlation. You can see on the summary table above that the rate increases as the no of potato discs added to the hydrogen peroxide increases. the increasing rate undoubtedly indicates increased levels of oxygen gas produced. My graph looks as though it is still increasing; indicating that substrate concentration has not yet become a limiting factor. The graph is consistent with my prediction. I predicted that as the concentration of the enzyme increases the rate of reaction will also increase.
Evaluation
The experiment was satisfactory and allowed me to produce valid results. The results showed clear trends and patterns. The experiment was sufficient enough to support my prediction and did not give any anomalous results that were extreme.
Although valid results were produced the degree of accuracy was not at its highest standard. Evidence of this can bee seen on the graph showing the average time taken. For example when 10 discs were added, the point plotted does not fit the line of best fit. According to the line of best fit the value should have been much lower. It should have taken 344 seconds but it took 376.33 seconds. This could be due to many experimental errors. These may have occurred because the affect of external variables such as temperature. The experiment was conducted under room temperature. The room temperature may have altered during the course of the experiment. This would have affected the results as enzyme activity is affected by the temperature. The temperature may have dropped or raised, this would have either resulted in a fall in activity or an increase.
Limitations on the apparatus and techniques meant that I couldn’t get the most reliable and accurate results possible from the experiment and investigation that I was caring out. One factor could have been that the tile used to cut the potato discs may have been contaminated from previous use. This could also account for the potato borer and the knife. To overcome this problem I could clear the tile with disinfectant and wash out other apparatuses with distilled water, to washout impurities.
I think the main reason that could have lead to inaccurate results was the fact that I used a boiling tube. I found that as the potato discs increased they seem to block the opening of the boiling tube. This prevented oxygen gas being collected in the syringe. This could be overcome by using a conical flask. Conical flask will give a bigger volume to react in would allow maximum oxygen gas to be collected.
A good point is that the syringe was held on the clamp stand. There was no contact with it. This would prevent body heat being transferred to the syringe. Having held the syringe at 180 degrees, meant that it was easy to see how much gas had been collected. It made reading of the meniscus straightforward. However there could have been a possibility that I mis-read the meniscus reading. This could have also been true when measuring the hydrogen peroxide. Another positive element was that no kinetic energy was applied to the boiling tube.
As you can see that there are many criticism of my method. Overall the method was reliable but may be the way things were used may have lead to inaccuracies. And of course there was human error too. If I was to do this experiment again then I would make sure that all apparatus was cleaned with a disinfectant. This time I would utilise a conical flask rather than a boiling tube. An additional thing I could do is that I can control some of the variable affecting the enzyme activity. For example temperature can be kept constant by carrying out the experiment in a water bath. pH can be controlled by adding an equivalent amount of buffer solution each time.
Investigations on enzymes are a very wild field. If I was to make further investigations, I would investigate how the activity of enzyme differs across different potatoes. So I could try a range of potatoes such as canned, frozen dried. I can see hoe the activity of the catalase differs across different potatoes. Also since the aim of my experiment was to research on the concentration of enzymes, I will conduct the further experiment with a much higher range of potato discs, to see the affect of the rate of reaction.
