I know, the question is vague, but we were doing a lab that involved the use of catalase and hydrogen peroxide and we measured the amount of oxygen gas that was produced over 30 second intervals for 5 minutes. The amount of oxygen gas produced gradually increased to the point where it remained relatively constant (it was slowly increasing). So why do rates change over the course of the experiment?
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The rate of any enzyme-catalyzed reaction depends on the concentration of the substrate, in this case, hydrogen peroxide. At the beginning of the reaction the concentration of H2O2 is very high, so it reacts quickly. As the reaction proceeds, the concentration of H2O2 decreases, so the reaction occurs more slowly. The graph showing the rate of the reaction as a function of the concentration of substrate is called a Michaelis-Menten plot.