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Enzymes - investigate how the substrate concentration (H2O2) affects the activity of catalase on hydrogen peroxide.

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Enzymes Aim To investigate how the substrate concentration (H2O2) affects the activity of catalase on hydrogen peroxide. Introduction Enzymes are protein molecules, which can be defined as biological catalysts. A catalyst is a molecule that speeds up a chemical reaction, but remains unchanged at the end of the reaction. An enzyme catalyses virtually every metabolic reaction, which takes place within a living organism. Enzymes have a globular protein structure. The enzyme molecules are coiled into a precise three-dimensional shape, with hydrophillic R groups (side chains) on the outside of the molecule ensuring that they are soluble. Enzyme molecules also have an active site, which is usually a cleft of depression, to which another molecule or molecules can bind. This molecule is the substrate of the enzyme. The shape of the active site allows the substrate to fit perfectly, and to be held in place by temporary bonds, which form between the substrate and some of the R groups of the enzyme's amino acids. The combined structure is termed the enzyme-substrate complex. Each type of enzyme will usually act on only one type of substrate molecule. This is because the shape of the active site will only allow one shape of molecule to fit. The enzyme is said to be specific for this substrate. The enzyme may catalyse one of two types of reactions. One in which the substrate molecule is split into two or more molecules; the other in which it may join two molecules together, as when making a dipeptide. Interaction between the R groups of the enzyme and the atoms of the substrate can break, or encourage formation of, bonds in the substrate molecule, forming one, two or more products. The product or products leave the active site when the reaction is complete. The enzyme is unchanged by this process, so it is now available to receive another substrate molecule. The rate at which substrate molecules can bind to the enzyme's active site, be formed into products and leave can be very rapid. ...read more.


Preliminary Results Results at 200C Time (seconds) 02 Gas accumulated (Cm3) 1 02 Gas accumulated (Cm3) 2 Average 02 Gas accumulated (Cm3) Rate of reaction (Cm3/s) 0 0 0 0 0 30 0.8 0.9 0.85 0.0616 60 1.5 1.7 1.6 0.0266 90 2.4 2.4 2.4 0.0266 120 3.2 3.1 3.15 0.0262 150 4.1 3.9 4 0.0266 180 4.9 4.4 4.65 0.0258 Result at 300C Time (seconds) 02 Gas accumulated (Cm3) 1 02 Gas accumulated (Cm3) 2 Average 02 Gas accumulated (Cm3) Rate of reaction (Cm3/s) 0 0 0 0 0 30 1.1 1.0 1.05 0.035 60 2.0 1.9 1.95 0.0325 90 2.9 2.8 2.85 0.0316 120 3.8 3.6 3.7 0.0308 150 4.5 4.2 4.35 0.029 180 5.6 5.3 5.45 0.0303 Results at 400C Time (seconds) 02 Gas accumulated (Cm3) 1 02 Gas accumulated (Cm3) 2 Average 02 Gas accumulated (Cm3) Rate of reaction (Cm3/s) 0 9.5 N/A 9.5 0 30 12 N/A 12 0.4 60 14.5 N/A 14.5 0.241 90 17 N/A 17 0.188 120 19 N/A 19 0.158 150 21 N/A 21 0.14 180 23.5 N/A 23.5 0.131 Yellow = 40 degrees. Blue = 30 degrees. Pink = 20 degrees. Prediction The aim of this investigation is to investigate how the substrate concentration affects the activity of catalase on hydrogen peroxide. I will do this by carrying out an experiment in which I will measure the volume of gas produced at when catalase kept at a constant concentration is used to catalyse different concentrations of hydrogen peroxide. I predict that as the concentration of hydrogen peroxide is increased so will the rate of reaction. I think this will happen until a point whereby all the active sites are in use. At this point the rate of reaction will plateau and become constant. When the concentration of an enzyme is kept constant, the substrate concentration is varied, the rate of reaction is affected. At low substrate concentration there are few substrate molecules around. ...read more.


I will cap the boiling tubes with a rubber bung once the dilution has been made. This will mean that the hydrogen cannot react and form water, or evaporate. As a result decreasing the concentration of the hydrogen peroxide and inaccurate results. The use of a bung to seal the boiling tubes until the solution is required will prevent this occurring and the results will be more accurate and reliable. To allow consistency I will use the same measuring cylinder throughout, one to measure distilled water and the other to measure hydrogen peroxide solution. This will mean that all the measurements are subject to the same inbuilt error, and not to different ones if more than one measuring instrument was to be used and both had different inbuilt errors. As a result I will be limiting the error when measuring the solutions. I will use a separate measuring cylinder for distilled water and hydrogen peroxide solution to avoid contamination. When using hydrogen peroxide solution I will start with the lowest concentration first. This will ensure that remainders of higher concentrations of hydrogen peroxide do not increase the concentration of the more diluted solution. Another measure is when using the measuring cylinder I will make sure that all measurements are read off at exactly eye level. I will also use a stopwatch to time the duration of the experiment. This will allow for the duration of 180 seconds to be timed exactly. Finally, I will carry out all the tests in the same place. This will mean that all the samples will experience the same conditions. Rather than an experiment whereby one set of tests being carried out in different conditions, which would result in unreliable and inaccurate results. These measures that I will maintain throughout the experiment will allow for an accurate and reliable set of results. Some of which have been introduced as improvements to the errors in the preliminary experiment. Overall, I think that this method will be carried out to an appropriate degree of precision, allowing for a set of accurate and reliable set of results to be obtained. ...read more.

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Here's what a teacher thought of this essay

3 star(s)

*** A well constructed report that contains usable data. There are some technical issues with regard to the recording of data and details of the methodology that could be improved.
To improve:
Research and rationale
The background research could be extended to include sources that are beyond those of the standard A level texts. The sources quoted should be clearly referenced within the text to illustrate where they have been used. It should be made clear to the reader how the hypothesis and methods used have been developed.
Whilst the use of a pilot experiment is always advisable, it needs to be explained to the reader why this was carried out varying temperature and not substrate concentration, as was the intended aim of the experiment. The pilot experiment should be used to inform the planning of the investigation.
The candidate needs to take care that key technical terms are used correctly. e.g dependent and independent variables. The preparation or source of the catalase was not explained in any way and this has an impact on the interpretation of the experiment. The control of the key variables could be improved by using a thermostatically controlled water bath and a buffer. The risk assessment would not be considered adequate under current guidelines.
The candidate seems too have used the apparatus competently to collect usable data but results were not recorded in suitable tables with clear headings. Some of the headings were incorrect. Units should be clearly indicated in the headings only and should follow the Institute of Biology guidelines.
Analysis and Evaluation.
The summary tables were presented in a format that did not present the results as clearly as possible. The inclusion of a table showing the initial rates of reaction for each substrate concentration would have been helpful. The graph was not included.
The limitations could be discussed in more depth as they are concerned mainly with basic errors. The candidate is reasonably aware of the lack of precision in the experiment and has made some sensible suggestions for improvement. Some proposals for extending the scope of the work would be helpful.

Marked by teacher Stevie Fleming 22/08/2013

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