Effect of Substrate Concentration on the Activity of Catalase.

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Effect of Substrate Concentration on the Activity of Catalase

Aim: To examine how the concentration of the substrate Hydrogen Peroxide affects the rate of reaction of the enzyme catalase.

Introduction:

Enzymes such as catalase are protein molecules which are found in living cells.  They are used to speed up specific reactions in the cells.  They are all very specific as each enzyme just performs one particular reaction.

Catalase is an enzyme found in food such as potato, apples and liver.  It is used for removing Hydrogen Peroxide from the cells.  Hydrogen Peroxide is a poisonous by-product of metabolism.  Catalase speeds up the decomposition of Hydrogen Peroxide into water and the oxygen as shown in the equation below:

Hydrogen Peroxide                Catalase        Water   +   Oxygen

         2H2O2                                        2H2O  +O2        

Prediction:

It was predicted that as the substrate concentration increases, the rate of reaction will increase at a directly proportional rate until the solution becomes saturated with the substrate Hydrogen Peroxide.  When this saturation point is reached, then adding extra substrate will make no difference.

The rate steadily increases when more substrate is added because more of the active sites of the enzyme are being used which results in more reactions so the required amount of oxygen is made more quickly.

Once the amount of substrate molecules added exceeds the number of active sites available then the rate of reaction will no longer increase.  This is because the maximum number of reaction are being done at once so any extra substrate molecules have to wait until some of the active sites becomes available.

Variables:

To ensure that this experiment is completed as fairly as possible, all the variables except for the concentration of Hydrogen Peroxide must be kept the same for all experiments.

Variables that must not be altered include:

  • 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 variables the temperature was maintained at a fairly constant level that allowed the enzyme to work effectively (room temperature, approximately 23°C.  This was achieved by using a test tube rack and tongs to handle the apparatus so that the heat from my hands did not affect the Catalase.
  • 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.
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In this experiment, the pH was 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).

  • 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 ...

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