Catalyse is a type of enzyme that is very good at speeding up the rate of reactions. It is different in that it has different substrates to amylase:
AMYLASE
Starch glucose (C6 H12 O6)n C6 H12 O6
CATALASE
Hydrogen peroxide water + oxygen 2H2O2 2H2O +O2
Catalase is found in all tissues (cells) e.g. potatoes, yeast and liver. Catalase uses the lock and key theory (as do all enzymes).
Hydrogen Peroxide
Hydrogen Peroxide is a chemical compound of hydrogen and oxygen. As explained in ‘My Investigation’, its scientific formula is H2O2. If this solution has not been altered and is pure and anhydrous, it is colourless, slightly thick and has a gravity of 1.44. Although I do not think anyone would taste it, hydrogen peroxide has a metallic taste and will blister the skin. Concentrated solutions are unstable, and, if the liquid is still pure, it can explode when heated above a temperature over 100°C. Concentrations of hydrogen peroxide higher than 50 per cent can cause serious burns. Such factors as light, heat, chemical catalysts, dirt, and rust may cause hydrogen peroxide to decompose into water, oxygen, and heat. It is soluble in water. To keep hydrogen peroxide from decomposing, it is kept in dark bottles and at low temperatures.
It is manufactured in huge quantities by the electrolysis of aqueous solutions of sulphuric acid or of potassium bisulphate or ammonium bisulphate. Solutions containing 3 to 6 per cent hydrogen peroxide are used as antiseptics and germicides and as a skin cleanser. Higher concentrations are used in the manufacture of many chemical compounds. They also serve as bleaching agents for textiles and paper pulp, and as rocket propellants.
Hydrogen peroxide acts as both an oxidizing and reducing agent, because the oxygen freed in decomposition readily combines with other substances. Its oxidizing properties are used in bleaching thing such as hair, feathers, ivory and delicate fabrics e.g. wool, silk, nylon etc. that would be destroyed by other agents. In a different form, it can be used as an antiseptic or a throat wash. It restores the original colour in paintings, by oxidizing the black lead sulphide to white lead sulphate. It can also be used as a source of oxygen in the fuel mixture for many rockets and torpedoes. So on the whole, Hydrogen peroxide is a very useful substance.
VARIABLES:
In this investigation, the variables that affect the activity of the enzyme, Catalase, were considered and controlled so that they would not disrupt the success of the experiment.
i)Temperature – As temperature increases, molecules move faster (kinetic theory). In an enzyme catalysed reaction, such as the decomposition of hydrogen peroxide, this increases the rate at which the enzyme and substrate molecules meet and therefore the rate at which the products are formed. As the temperature continues to rise, however, the hydrogen and ionic bonds, which hold the enzyme molecules in shape, are broken. If the molecular structure is disrupted, the enzyme ceases to function as the active site no longer accommodates the substrate. The enzyme is denatured.
To control this variable, the temperature will be maintained at a fairly constant level that will allow the enzyme to work effectively.
ii) pH – Any change in pH affects the ionic and hydrogen bonding in an enzyme and so alters it shape. Each enzyme has an optimum pH at which its active site best fits the substrate. Variation either side of pH results in denaturation of the enzyme and a slower rate of reaction.
In this experiment, the pH will be kept constant using a pH 7 buffer, selected to maintain a pH level suited to the enzyme by being equal to the natural environment of the enzyme (potato tissue).
iii) Substrate Concentration – When there is an excess of enzyme molecules, an increase in the substrate concentration, produces a corresponding increase in the rate of reaction. If there are sufficient substrate molecules to occupy all of the enzymes’ active sites, the rate of reaction is unaffected by further increases in substrate concentration as the enzymes are unable to break down the greater quantity of substrate.
vi) Enzyme Concentration – Provided there is an excess substrate, an increase in enzyme concentration will lead to a corresponding increase in rate of reaction. Where the substrate is in short supply (i.e. it is limiting) an increase in enzyme concentration has no effect.
My dependant variable is going to be concentration of H202 and I will change this by combining the H202 with water (different amounts of each) to keep the total volume the same. This is important to insure a fair test.
PREDICTION:
I predict that the Hydrogen peroxide will breakdown to oxygen and water in the presence of Catalase. The reaction will increase with increasing concentration when molecules of hydrogen peroxide are freely available. The collision theory states simply that the more H202, the more collisions and therefore a faster rate of reaction. However, when the substrate molecules begin to out-number those of the enzyme, the collisions will decrease and so will the rate of reaction. Therefore I predict the rate of reaction will increase steadily with the higher percentage concentration of H202, although the rate will decrease slightly towards the top end of the graph due to the substrate molecules beginning to out-number the enzyme molecules. (see predicted graph)
APPARATUS:
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YEAST SOLUTION (10%)- This will be my source of enzyme.
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H2O2 (20%)- This will be my substrate for the experiment
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VARIETY OF BEACONS- to contain the water
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TEST TUBES- To contain the solution and collect the end result.
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SYRINGES- to insert the substrate to the yeast solution
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TEST TUBE RACK- To keep the test tubes in.
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STOPWATCH To time the reaction.
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WATER- To collect the oxygen in in.
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DELIVERY TUBE- to transport the oxygen.
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GOGGLES- For safety.
The apparatus will be set up as follows:
PILOT STUDY:
For my preliminary experiment my dependent variable was concentration of H2O2.
FAIR TEST:
To make it a fair test I will use the same apparatus every time, but I will wash the apparatus out thoroughly each time in case they are contaminated. I will do this so one experiment will not have any advantages or disadvantages than the other. I will stir the yeast suspension solution to make the suspension even, so the denser and the less dense molecules are mixed. I will use different Hydrogen Peroxide and different yeast solutions for every experiment, because most or all of the solutions will be used during reaction.
The following things must be kept constant in this experiment (CONTROLLED).
Yeast concentration: the rate of reaction is proportional to the concentration, so changing it can alter the rate of reaction. We only want the rate to be affected by the hydrogen peroxide concentration.
Temperature: The temperature can alter the speed of the reaction because the molecules move faster with a higher temperature (see background knowledge).
pH: If changed this can alter the shape of the enzyme, so that the enzyme works less efficiently.
Amount of substrate: Increasing this can alter the speed of the reaction, as more substrate molecules will react with the enzyme quicker. There could also be more oxygen produced, making the test unfair and inaccurate.
Amount of enzyme: The rate would be changed, as with a higher amount there are more active sites to link with the substrate.
Also, to make the experiment a fair test, you would need to wash the tube and dry it between solutions. Otherwise, there could be more substrate or enzyme than stated in the method, altering the rate of reaction and making the test unfair.
SAFETY:
When carrying out the experiment I will wear safety glasses in case of spillage or if the substances spit during reaction. Hydrogen Peroxide is very toxic and if it gets in my eyes it may cause blindness.
STOPWATCH:
I will start the stopwatch at the point of input of the yeast suspension solution. I will only be timing the first 1-minute of the experiment, because if I did this for longer I would need more time to do the experiment but I am pushed for time. The oxygen bubbles will be counted and written into a table in the results.
METHOD:
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Arrange the apparatus as seen on a previous page. Then add 1cm³ of yeast, 2.5cm³ of water and 3cm³ of H2O2 to the first test tube. The overall volume should always be the same at 6.5cm³. It is important to make sure that the variable being inserted into the test tube last, is the least in volume to decrease any air being pushed through the system and ruining the results.
- After 1 minute of the reaction taking place, isolate the test tube that has been collecting gas to stop any more being collected and record results in a table.
- Repeat this experiment again 4 times to ensure reliable results.
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Now repeat the experiment with different concentrations of H2O2 as shown in the table below:
I have chosen this particular range because I felt it worked well in my preliminary results as any higher conc. H2O2 resulted in too much gas to be collected in the available tubes and a less conc. H2O2 would mean a too insignificant range to get good results.
I will make my test valid by testing the apparatus in the preliminary but also doing the experiment more than once to isolate any systematic errors which I have collected previously.
To make my results precise I will have to get someone else to time the experiment, as I could easily get distracted from the watch.
The measurements or an observation I will make is the gas collected in the measuring cylinder after the H2O2 and yeast are mixed. You can measure the gas by filling the beaker and cylinder with water and when the gas enters it pushes down the water giving you the mark on the cylinder from which to measure off the result from that part of the experiment.
The one thing I will change to be different to my preliminary results will be to make sure the tube collecting the gas is constantly upright, this is because if it was not it would give uneven results due to the fact the gas line will be horizontal while the tube is not.
PRELIMINARY RESULTS:
* YELLOW cells are where the reaction was too violent and the gas given off was too much to collect in the available test tubes.
RESULTS:
CONCLUSION:
From my results it is clear that my prediction was correct. The Hydrogen peroxide broke down into oxygen and water in the presence of the Catalase. The reaction increased with the increasing concentration because molecules of hydrogen peroxide were freely available. This is shown because the graph shows that there is a steady incline and strong positive correlation until it peaks at a concentration of about 4% H2O2 where the graph begins to level out (this is where the number of H2O2 molecules are beginning to out number the yeast molecules and so the collisions in the reaction begin to decrease, thus creating the graph to slow the steep incline).
EVALUATION:
I experienced no anomalous results in my experiment which shows that it was done accurately and I think that all in all my experiment was fair and that my results were reliable. If I was to do the experiment again there are a few things I would do differently if the circumstances (i.e. better facilities and equipment) were different. Firstly, I would do it all in one go rather than over a period of two lessons as this may have slightly altered my results.
To make the experiment more accurate I could have used a more precise way of measuring the amount of gas collected instead of just using a ruler, this would have made my results more reliable.
If I could do the experiment again I would take more readings so that my results would be more reliable.
My results are definitely enough to draw a firm conclusion from, that hydrogen peroxide broke down into oxygen and water in the presence of the Catalase. The reaction increased with the increasing concentration because molecules of hydrogen peroxide were freely available.
To find out more about how concentration can affect a catalase reaction you could investigate the effect of changing conc. of enzyme such as yeast or potato.
BIBLIOGRAPHY:
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- Lock and Key theory Diagrams.
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- Lock and key theory.
- Class Notes