For reactions to take place molecules of the reactants must require a certain amount of energy to cause collisions to take place. Molecules often do not react spontaneously because of the stability of molecular covalent bonds. Chemical reactions require an amount of energy to start them off; this is known as activation energy. Enzymes act catalysts to lower the required activation energy. Enzymes do this by weakening the covalent bond with a substrate molecule or by holding the substrate in a particular position that increases the probability of a reaction occurring. Products can only be formed when effective collisions occur, this means that reactant molecules colliding is not enough to form effective collisions. Reactions can only occur when particles first collide. The collision that took place must have enough energy impact to overcome the activation energy; they must also have a proper positioning to be effective. Increase in concentration of the enzyme means that there will be more molecules of that enzyme in that solution. The more amylase molecules there are, the higher the probability of collisions occurring. An increase in total number of collisions consequently results in an increase in the number of effective collisions; this means that the rate of reaction will increase. Catalase is an enzyme found in food such as potato and liver. It is used for removing Hydrogen Peroxide from cells. Hydrogen Peroxide is the poisonous by-product of metabolism; this compound can kill cells. Catalase speeds up the decomposition of Hydrogen Peroxide into water and oxygen because the shape of its active site matches the shape of hydrogen peroxide molecule. This type of reaction where a molecule is broken down into smaller pieces is called a catabolic reaction. This reaction can be represented by the following equation;
H202 (l) H20 (l) + O2 (g)
Hydrogen Peroxide is a chemical compound of Hydrogen and Oxygen with the formula H2O2. Pure, anhydrous Hydrogen Peroxide is a colourless, syrupy liquid that is a strong oxidizing agent (which is harmful) and, in water solution, a weak acid. It has an exact gravity of 1.44. It blisters the skin. In this experiment, the source of enzymes is a cylinder of potato tissue.
DIAGRAM
Procedure
- The experiment is set up as shown in the diagram above.
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Obtain a beaker or a boiling tube and place 2cm3 of buffer solution plus 1 potato disk. Buffer solution was used since it does not cause a change in the pH of the medium allowing the enzyme catalase to work in an optimum environment.
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Add the H2O2 and quickly put the bung on which contains the tubing. The bung should be placed in such a way to prevent any air bubbles from entering to ensure that a fair experiment was carried out.
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Any oxygen given off will collect in the burette. The oxygen given off indicates that a reaction is taking place since H2O2 is decomposed to form water and oxygen gas. This gas can also be tested further by placing a glowing splint into the gas and it will catch fire.
- Once an observable volume of oxygen gas has be produced, read off how much oxygen was produced.
- This is repeated 3 times to ensure reliable results.
- The whole experiment is repeated for 2,3,4 and 5 disks
Changes to experiment.
- Time taken for 2cm3 of oxygen to be produced was recorded to make results fair.
- Only experiments for 1,2,3,4 disks were carried out since time to carry out experiment was limited.
- Each experiment was also carried out twice since time wasn’t enough.
Controlled Variable
The controlled variable is the factor that I will keep constant throughout the experiment. In this case the controlled variables are: the volume of Hydrogen Peroxide, temperature, timing, pH and substrate concentration.
In order to obtain a fair test, the following factors need to be controlled:
1. the pH – The pH can slow down the enzyme hydrolysing the substrate. Extremes of pH can denature the enzyme so the reaction would not proceed. This will lead to an unfair test if not controlled, and experiment would produce very strange results.
2. The substrate concentration – If the substrate concentration is increased each time, reactions will occur very fast. However if the substrate concentration is decreased each time, reactions will occur very slowly. This will lead to unreliable results. So controlling the substrate concentration is very important in order to gain a fair test and good results.
3. The volume of substrate (Hydrogen Peroxide) added – If the volume of the substrate is increased, this means more of it has been added so again, reactions will occur fast. However if the volume of the substrate is decreased, this means less of it has been added so reactions occur slowly. So the volume of the substrate needs to be controlled and must remain constant throughout the whole experiment.
4. Temperature- Temperature affects the rate of the reaction taking place. Its directly proportional to the rate of reaction. The temperatures must be kept constant (room temperature) to ensure accurate results.
5. Time – It is important that the volume of Oxygen produced is always noted after the 1 minute interval. This will assure a fair test.
Enzyme concentration on the rate of reaction so the other factors will have to remain the same, so that they do not affect my results.
Independent Variable
This is the factor which is changed in the experiment. In this case the independent variable is the number of potato disks.
Accuracy:
To get accurate measurements of the buffer and substrate I will use a 5ml syringe. To control the s
ubstrate and buffer I will use the same volume and concentration each time, so that it will not affect my results. I have decided to control the pH and keep it the same so that this will not affect the rate of reaction by using a buffer. The time will also be controlled by using a stop watch to assure a fair test. The discs have been cut with a knife using a ruler to ensure that sizes are as equal as possible. Hydrogen peroxide solution would be added by the use of a syringe to ensure very accurate results. Results obtained for the time taken for reaction to take place will me measured to 3 decimal places to ensure very accurate results. Thermometer was used to check the temperature of each solution before experiment was carried out to make sure the temperatures were the same.
Risk Assessment
Chemicals such as Hydrogen Peroxide and a buffer solution are used. Hydrogen Peroxide is a chemical compound of Hydrogen and Oxygen with the formula H2O2. Pure, anhydrous Hydrogen Peroxide is a colourless, syrupy liquid that is a strong irritant oxidizing agent (which is harmful) and, in water solution, a weak acid. It has an exact gravity of 1.44. It blisters the skin. Gloves will be worn along with goggles. If any of the chemical gets into the eye then it will be washed out with clean water.
Discussion
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It was observed that as the number of discs increased, the time taken for 2cm3 of oxygen to be produced increased.
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Rate of reaction was at its highest when 5 discs were placed in the solution, since it took only 8.047 minutes for H2O2 to decompose to form 2cm3 of O2(g)
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Rate of reaction was at its lowest when only 1 disc was placed in the H2O2 solution, as it took 47.971 minutes for H2O2 to decompose to form 2cm3 of O2(g)
- There was a big difference in rate of reaction when 4 and 5 discs were added respectively.
Explanation of results
The potato discs served as a source of Catalase enzyme. As the number of discs added to the hydrogen peroxide solution was increased, the concentration of the catalase enzyme also increased thus providing more activation sites to react with the substrate that was kept constant. This is evident as the time taken for amount of oxygen to be produced decreased as more potato discs were added. The big jump in reaction rates when 4 and 5 potato discs were added may have been due to little amounts f heat being given off which also further increased the reaction with its increased temperature. It can be predicted that by adding 6 potato discs the time taken to produce 2cm3 of would be less.
Sources of error
- Since knife was used in cutting up potato pieces, not all sizes were equal. The sizes of potato discs greatly affected the experiment since concentration of the enzyme catalase was directly proportional to the size and number of discs. This could be corrected by using more precise instruments to cut slices evenly to ensure fair results.
Reference
- A-level biology text book by Heinemann
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