Ib Bio Experiment on Effect of Substrate Conc on Enzyme Activity

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Practical Assessment 2 – The effect of substrate concentration on the rate of enzyme activity of Catalase

To investigate the effect of substrate concentration (manipulated by increasing concentration of hydrogen peroxide) on the rate of enzyme activity of catalase, produced by liver cells, on the decomposition of hydrogen peroxide.

Enzymes are biological catalysts that increase the rates of reactions. In an enzyme-catalyzed reaction, the substrate binds to the active site and forms enzyme-substrate complex with the enzyme through the lock and key method (where the lock represents the enzyme and the key represents the substrate). The enzyme then breaks the bonds in the substrate. The product of the reaction then leaves the enzyme, which remains unchanged after the reaction. Without enzymes, many essential processes, such as digestion, would occur too slowly for life to continue.

Catalase is an enzyme produced by our liver cells to break down hydrogen peroxide – a common end product of metabolism, but highly toxic to tissues if accumulated in the body – into water and oxygen.

The equation of the reaction is as follows:

2 H2O2 O2 + 2 H2O

In this experiment, we obtain 6% hydrogen peroxide solution from a pharmacy and extract equal concentrations of catalase from liver cells. Filter paper discs are dipped into the catalase solution before they are submerged in hydrogen peroxide solution. The oxygen produced from the enzyme reaction will form on the discs and cause the disc to be buoyant enough to float upwards. We can investigate the effects of substrate concentration on the rate of reaction by catalase by using different concentrations of hydrogen peroxide solution, and measuring the rate of reaction by measuring time taken for the disc to float to the surface when sufficient oxygen is produced.


The hypothesis for this experiment is that the rate of reaction will increase with the increase of hydrogen peroxide concentration, if the other factors of enzyme activity (such as temperature, pH and enzyme concentration) are kept constant. However, the rate of reaction will stop increasing with hydrogen peroxide concentration at a point where the enzyme concentration becomes a limiting factor. At high substrate concentrations, most of the active sites available are occupied since they are saturated with substrate molecules at any given time. Hence, a further increase in substrate concentration will not cause the rate of decomposition of hydrogen peroxide to increase.

As such, the expected graph from this experiment is as follows:

Graph 1: Expected graph of rate of reaction against concentration of substance


Dependent variable: Rate of enzyme activity of catalase in terms of time taken for the disk to float to the surface of the hydrogen peroxide solution when sufficient oxygen is produced.

* Once the filter paper disc has reached the bottom of the test tube, the stopwatch is started. The stopwatch is stopped once the disc has reached the surface of the hydrogen peroxide solution. The recorded timing indicates the amount of time taken for the disk to float to the surface of the hydrogen peroxide solution.

Independent variable: Concentration of hydrogen peroxide solution.

* Add different volumes of water to the different volumes of 6 % hydrogen peroxide solution.

Controlled variable(s):

1. Enzyme concentration
* We are provided with homogenous liquid liver solution. Hence, the concentration of catalase is constant throughout the liquid liver solution.

2. Volume of hydrogen peroxide solution
* The volume of hydrogen peroxide solution in each test tube is 5cm3. Controlling the volume of hydrogen peroxide solutions ensures that the same amount of hydrogen peroxide molecules (substrates) is available for reaction in the test tube.

3. Size of test tubes...
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