To investigate how concentration of the enzyme catalase in celery extract affects the rate of reaction with hydrogen peroxide

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Aminul Hoque

Aim: To investigate how concentration of the enzyme catalase in celery extract affects the rate of reaction with hydrogen peroxide


  2 H2O2 (aq) 2 H2O (l) + O2 (g) 

Scientific Knowledge:

Enzymes are protein molecules which can be defined as biological catalysts. A catalyst is a molecule which speeds up a chemical reaction. Nearly every metabolic reaction which takes place within a living organism is catalysed by an enzyme.

Catalase is present in all living cells. Enzymes are globular proteins and like all globular proteins, are coiled into a three-dimensional shape with hydrophilic side chains, ensuring solubility.

Enzymes also contain an active site (usually a cleft or depression) to which a substrate molecule can bind to. The shape of the active site is complementary to the shape of the substrate molecule and each enzyme is substrate specific, meaning each type of enzyme will only act on only one type of substrate molecule.

Like all catalysts, enzymes work by lowering the activation energy (ΔG) for a reaction, thus dramatically accelerating the rate of the reaction.

Catalase breaks the chemical hydrogen peroxide down to water and oxygen. Catalase is found in all cells and protects them from this dangerous waste chemical. I will use the catalase found in celery extract for this investigation.

The substrate (hydrogen peroxide) and the catalase molecules are continuously on the move. Every so often they will collide so that the substrate molecule(s) fits into the enzyme’s active site.


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How the enzymes work can be explained using the “lock and key” hypothesis, substrate approaches the active site and fits exactly in to it. No other types of substrate have this ability. Products are formed as a result. The enzyme is ready to be used in another reaction.

Each type of enzyme has its own specific optimum conditions under which it works best.

Enzymes work best when they have a high enough substrate concentration for the reaction they catalyse. If there is too little substrate available, the rate of the reaction is slowed and as a result cannot increase any further.

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When the enzyme and substrate are first mixed, there is a large number of substrate molecules. At any moment, virtually every enzyme molecule has a substrate in its active site. The rate at which the reaction occurs will depend on how many enzyme molecules there are, and the speed at which the enzyme can convert the substrate in to product, release it, and then bind with another substrate molecule. As more substrate is converted in to product, there are fewer and fewer substrate molecules to bind with the enzymes. As less substrate molecules are left, the reaction gets slower and slower until it gradually stops as all the substrate has been converted in to product.

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The pH must be correct for each enzyme, because if the conditions are too alkaline or acidic then the activity of the enzyme is affected. This happens because the enzyme’s shape, especially the active site, is changed. It becomes  denatured, and cannot hold the substrate molecule.

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Temperature is a key factor too. If it is cold there will be little or no kinetic energy and so the enzyme molecules will move around too slowly to meet the substrate molecules, so the reaction rate is slowed. Likewise, if it is too warm they do not work properly either. This is because the extra heat energy shakes them around so much that the active sites change shape so, just like with pH, the enzyme molecules are denatured, and cannot hold the substrate.


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In order for particles to react, they need kinetic energy. Reactant must collide, but only a certain fraction of the total collisions, the effective collisions, cause the transformation of reactant molecules into products. This is due to the fact that only a fraction of the molecules have sufficient energy and the right orientation at the moment of impact to break the existing bonds and form new bonds. The minimal amount of energy needed so that the molecule is transformed is called activation energy. In a chemical reaction, catalysts lower this.


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Biology 1, authors Mary Jones, Richard Fosebery, Dennis Taylor;

Method Outline:

In this experiment I will measure the amount of oxygen released when hydrogen peroxide reacts with the enzyme catalase found in celery extract. I aim to see how concentration of enzyme affects rate of reaction. I shall use 15ml of Hydrogen Peroxide and vary the concentration of celery extract by making dilutions from a 10ml base. Concentration of the enzyme is the only variable. All other variables (temperature, pH) will be attempted to be kept constant.


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