The aim of this experiment is to demonstrate that the substrate Hydrogen Peroxide will break down to oxygen and water, in the presence of the enzyme catalase.

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BIOLOGY AS LEVEL

KATHERINE WYATT

ANALYSIS AND EVALUATION

OF COURSEWORK EXPERIMENT

5 DECEMBER 2002

COURSEWORK EXPERIMENT

Further to the 'Coursework Plan' which was submitted to my tutor on 5 December, outlining a plan for conducting an experiment 'to establish that varying the surface area of catalase exposed to the substrate Hydrogen Peroxide will affect the rate of the reaction', all students were given a 'Student Worksheet', which detailed the prescribed apparatus and methodology for conducting the experiment.  The following information will outline the evaluation and analysis of this experiment, under the following headings:

  • Introduction;
  • Prediction;
  • Variables;
  • Preliminary Work;
  • Method;
  • Results;
  • Conclusion;
  • Interpretation;
  • Evaluation.

INTRODUCTION

The aim of this experiment is to demonstrate that the substrate Hydrogen Peroxide will break down to oxygen and water, in the presence of the enzyme catalase.  The catalase will be obtained from potatoes.  This experiment also aims to investigate and demonstrate that an increase in the surface area of potato (catalase), placed in equal volumes of Hydrogen Peroxide, will increase the activity of catalase and hence increase the level of oxygen production.  This will demonstrate an increased rate of catalase reaction.

Specifically, with regard to this experiment, Catalase is a biological enzyme, which helps to speed up the reaction between catalase (found in potato) and the breakdown of Hydrogen Peroxide.  Most intracellular enzymes work only on one particular substrate.  Catalase, for example, acts only on Hydrogen Peroxide, and is ineffective on any other natural substrate.  The active site is contained within the enzyme (Lock), and once the substrate (Key) is locked into the active site, the reaction takes place.  The products (oxygen and water) will then be released, and the enzyme will move on to another substrate (Hydrogen Peroxide).

PREDICTION

The experiment, therefore, aims to demonstrate firstly an enzyme reaction between catalase (found in potato) and Hydrogen Peroxide; and secondly, the speed of the reaction when the surface area of the potato is increased.  Once the experiment has been conducted, I predict that the greater the surface area of potato, and hence the higher the concentration of catalase, then the faster the reaction will occur due to the increased concentration of enzyme (catalase) in the Hydrogen Peroxide solution.  With the varying increases in surface area of potato, I should record an increase in the amount of oxygen that is produced, due to the catalase reacting with the Hydrogen Peroxide.  

KEY VARIABLES

In this experiment, there are several key variables that will affect the rate of the catalase (enzyme) reaction and they are listed below, with an explanation of how and why they will affect the reaction, and how they will be controlled/varied.

Temperature:

As temperature increases, molecules move faster (Kinetic Theory), thus increasing the chances of collision of the substrate with the enzyme's active site.  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.  The temperature, at which enzymes work most efficiently, is known as the Optimum Temperature.  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 (Figure 1).  The enzyme is denatured.  

To control this variable, the temperature was maintained at a fairly constant level that allowed the enzyme to work effectively (at room temperature, approximately 22oC).  This was achieved by utilising a clampstand and sidearm boiling tube and handling the boiling tube as quickly as possible, so that the heat from my hands should not affect the Catalase reaction.  Whilst the optimum temperature for catalase to react with Hydrogen Peroxide may be higher than room temperature (approximately 22°C), I conducted a pilot experiment, which demonstrated that oxygen was given off at room temperature, at a rate that was acceptable; however as outlined later, this was released so fast that the amount of oxygen produced was difficult to measure.

Figure 1

pH:

Any change in pH affects the ionic and hydrogen bonding in an enzyme and so alters its shape.  Each enzyme has an optimum pH at which its active site best fits the substrate.  Variation either side of the optimum pH, results in denaturation of the enzyme and a slower rate of reaction (Figure 2).  In this experiment, the pH was retained at the same low pH of the Hydrogen Peroxide, as the same quantity of Hydrogen Peroxide was utilised for each experiment.  This acid maintained a pH level which was suited to the enzyme, as it was similar to the natural environment of the enzyme (potato tissue).  

Figure 2

Substrate Concentration:

The concentration of substrate also affected the rate of the reaction.  When there is an excess of enzyme molecules, an increase in the substrate concentration (Hydrogen Peroxide) produces a corresponding increase in the rate of reaction, as more substrate molecules will occupy the available sides of the potato's catalase molecules.  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 great quantity of substrate, due to the 'saturation' of the enzyme.  To control the substrate concentration, identical quantities and concentration of the substrate Hydrogen Peroxide were used for each variation in increased surface area, however the quantity and concentration provoked a very quick reaction, which proved difficult to accurately record the volume of oxygen produced, as a result.  

Enzyme (Catalase) Concentration:

Provided that there is an excess supply of substrate, an increase in enzyme concentration lead to a corresponding increase in the rate of reaction.  This is due to the larger numbers of catalase molecules which increased the frequency of successful collisions between reacting particles (Hydrogen Peroxide) at the increased numbers of active sites.  Lowering the enzyme concentration decreased the number of active sites available and this slowed down the reaction.  Eventually, increasing the concentration of enzyme had no further effect on the rate of the reaction (see point x on Figure 3) due to the limiting factor of the number of substrate molecules.  The enzyme concentration was not be altered in this experiment, as the mass of the potato remained equivalent, however, the surface area of potato (and hence catalase) was increased, in this reaction.

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Figure 3

Inhibition:

Certain substances (Inhibitors) compete with the substrate for the active sites of the enzyme (competitive inhibitors).  Alternatively, other inhibitors attach themselves to the enzyme, altering the shape of the active site so that the substrate is unable to occupy it, and the enzyme cannot function (non-competitive inhibitors).  Inhibitors therefore slow down the rate of the reaction.  There were no Inhibitors that affected this investigation, however, as none were added.

Enzyme Cofactors:

Enzyme Cofactors are non-protein substances, which influence the functioning of enzymes, as some enzymes only work in ...

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