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The points with a concentration of 0%, 1%, 3% and 4% can be connected by nearly a straigt line. But 2% is above this line. Between 0% and 1% the gradient is 6.8. Between 1% and 2% the gradient is 10. Between 2% and 3% the gradient is 4.8. Between 3% and 4% the gradient is 6.9. Between 4% and 5% the gradient is 11.4.
For molecules to react they have to be very close together. That’s normally just the case when they collide. And then there still must be certain circumstances for them to react. One of these circumstances is that the molecules collide with the right angle. Another one is that they collide with enough energy so that they can break some of their bonds to form new bonds. This minimum amount of energy is called activation energy. If the molecules collide with less energy they will just bounce back without any reaction. So the activation energy is needed to break bonds in the molecules.
The molecules collide randomly. That means that the chance that they collide with enough energy and with the correct angle is low. Some reactions are slow under “normal” circumstances or don’t even occur. To increase the rate of reaction one can increase the number of collisions and the energy the molecules collide. Or one can bring the molecules in position so that they are close together and that it is more likely that they collide with the correct angle. This is done by catalysts. Very good for that are biological catalysts, which are known as enzymes.
Enzymes consist of a unique sequence of amino acids. This sequence is known as primary structure. The enzymes don’t lie flat but they fold themselves in a three-dimensional shape which is hold together by bonds formed by the side groups of enzymes. Obviously the unique primary structure gives the enzyme a unique tertiary structure.
Enzymes have a part called active site. A small number of molecules with a certain shape called substrates can bind to this part. Reactions between these molecules are catalysed by this enzyme. When substrates bind to the active site they form together with the enzyme an Enzyme-Substrate-Complex (ESC). In the ESC the enzyme wraps itself around the subtrates, so the enzyme changes its shape.
This causes the substrates to come into a position where it is likely that they collide with the correct angle and with enough energy. So it is much more likely that the substrates react. When the substrates reacted they change their shape and so they stop bonding to the active site. So they leave the enzyme after the reaction. The enzyme is flexible and changes its shape back to its original shape. So it can again form an ESC and catalyse reactions.
The more enzymes there are the more ESC can be formed and so more products can be formed in a certain time. That means that the rate of reaction increases.
Doubling the number of enzymes doubles the number of ESC and so also doubles the number of products. And finally the rate of reaction doubles. That means that the rate of reaction is proportional to the number of enzymes and/ or to the concentration of enzyme.
I expected that the rate of reaction is proportional to the enzyme concentration. In the experiment we got for 3% twice a faster rate of reaction than for 4%. So we repeated 3% twice and 4% once. The result we got then were better. The graph shows nearly a straight line but 2% and 5% seem to be too high. That is caused by errors.
The errors come from 3 sources: The procedure, the equipment and human error.
Errors in the procedure were:
-         We found our endpoint of the reaction by comparing the colour of the mixture to a colour table. But comparing colours by eye isn’t very accurate. So we stopped the time at different stages of the reaction.
-         We found our endpoint of the reaction with the help of pH. During the reaction the pH was changing. The problem is that the pH affects the rate of reaction.
An enzyme has an optimum pH (for urease this optimum pH is 7.4). That means with this pH it works better than with every other pH. That is because the pH affects the shape of an enzyme and the shape is crucial for the work of an enzyme. The lower the concentration of the enzyme is the bigger is the effect of the pH on the rate of reaction.
That means that the pH affected the reaction in the mixture with a urease concentration of 1% more than the reaction in the mixture with a urease concentration of 5%.
For me this seems to be the biggest error in the experiment.
-         The water temperature wasn’t constant during the experiment.
The temperature affects the rate of reaction. So it has to be kept constant.
But the temperature didn’t vary a lot and so it just slightly affected the results.
-      We did the experiment just for 5 concentrations and repeated each concentration just 3 times. By using more concentrations and repeating them more often we would get better and more accurate results.
Errors from the equipment were:
-         The urease didn’t dissolve completely in the water. At the bottom of the beaker was a lot urease. So the concentration varied from the concentrations we wanted to have. That could explain that twice the reaction with an enzyme concentration of 3% was faster than the reaction with an enzyme concentration of 4%

7.12.06 16:09

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