An enzyme is a biological catalyst found within cells, and catalyse is an enzyme.
Enzymes:
- are proteins
- are produced by cells
- are ‘specific’ which means that each enzyme will only work on one substance
- work best at a particular temperature (around 30-40 °C for digestive enzymes) called their ‘optimum temperature’
- work best at a particular ph called their ‘optimum pH’
At a temperature that is too high the structure of an enzyme ill be changed so that it will not work. Once the enzyme has passed this temperature the process is irreversible and the enzyme is said to be ‘denatured.’ Extremes of pH can have the same effect.
Every cell contains many enzymes that control many chemical reactions occurring within it. Digestive enzymes are only one kind. They are created in cells lining the digestive system and are secreted to mix with the food. There are different groups of enzymes such as:
- proteases which break down proteins
- lipases which break down fats
- amylase which breaks down starch
- maltase, sucrase and lactase which break down the different sugars, maltose, sucrose and lactose
Increasing temperature will usually raise the rate of reaction as the more energy a reaction has the more collisions, therefore the more reactions, however this is not so in enzymes. The structure of an enzyme is affected by temperature, and when an enzyme is heated above a certain temperature the protein becomes denatured and it will cease to function as a catalyst. The illustration shows the rapid denaturing of an enzyme. Enzymes are also sensitive to pH in a cell there is a neutral level of pH (7) and this is usually when they work best. However enzymes in the stomach work best at a pH of 2.
As the temperature increases:
- increased collisions between reactant and enzymes.
- increased enzyme activity.
Until the optimum temperature:
-
Continues to cause increased collision but the enzyme molecules become denatured resulting in decreased enzyme activity or none at all.
To understand how an enzyme works it is easiest to picture it. The enzyme acts as surface to hold the reaction. The molecule has a small cavity known as the ‘active site.’ Molecules from the reactant become trapped in the active site and, as they are not moving, get hit more often and therefore react more frequently. This results in more effective collisions and a greater rate of reaction. The following diagram shows the process visually.
1 – The reactant is reacting however not all collisions with reactant II are successful.
2 - The reactant finds the enzyme specific for it and locks into place. Reactant II collides more often and reactions are more successful.
- The reactant is broken down and leaves the enzyme, broken down, leaving the enzyme (without being used up) to be used again.
When the enzyme is heated too much, and it denatures, the structure changes irreversibly so that the reactant will no longer fit and the enzyme is rendered useless.
Hydrogen Peroxide + Catalase ------> Water + Oxygen
2H̶̶̶̶̶̶̶̶₂O₂ ------->H₂O + O₂ + heat
Preliminary experiments
In order to determine the best way to do the experiment, we felt it necessary to carry out preliminary experiments.
Liver reacting with hydrogen peroxide
The aim of the experiment was to find out how much hydrogen peroxide and liver I should use in my experiment.
The equipment was setup as in the diagram
The experiment was started with 4ml of hydrogen peroxide and reacted it with 2g of liver. It proved to be much too fast so immediately it was apparent that the measurements needed to be lowered to decrease the reaction time. The reaction time needs to be lowered so I can read the measurements, and therefore get accurate results. For good results I would need the reaction rate at room temperature to be fairly slow as looking at my research the reaction time should decrease with a rise in temperature, with the fastest reaction time being around 37°c. After the first trail I changed the amounts of both liver and hydrogen peroxide to try and get a suitable time, however this proved difficult, as even with 0.5 ml of hydrogen peroxide the experiment only lasted 5 seconds.
We have learnt from this experiment that reacting with liver is not sensible as we will not be able to get accurate results due to its incredibly fast reaction rate. I therefore suggest we use something that will react at a lesser speed than liver, potato. Potato contains the same enzyme as liver – catalyse, however it will not react as fast
WHY?
Fair Test
It is essential to make sure that any experiment we do is fair and accurate and to make sure that this is the same, I shall be considering the following things during my experiment:
- Temperature – I will monitor the temperature of the water bath with a thermometer and adjust it accordingly to get the most accurate temperature I can.
- Repetition – To make sure any discrepancies do not affect the result I shall be repeating each experiment 3 times over. This should ensure all the results are accurate
- Measurements – We will be using the most accurate measuring devices available to us. We will try and ensure that equal measures are given out and all experiments follow exactly the same process.
Safety
To ensure safety, I must follow several rules when conducting my experiment. All long hair must be tied back to stop it interfering with the experiment, and getting caught in any apparatus. For the same reason laboratory coats, or tucking in any loose clothing, must be worn at all times during the experiment. This also stops clothes and skin from getting damaged. Safety goggles must be worn as Hydrogen peroxide will be used, and the goggles will protect the eyes. All bags and stools must be kept under the desks to stop people falling over with acid and injuring themselves. For the same reason, running is never permitted in the laboratory. When pouring the solutions, care must be taken to avoid spills, which could damage or irritate the skin.
Range of temperatures
I have decided to use 10 °C as the difference in temperatures as this should give a good range of results, for the time we have to do the experiment. This should provide enough results to give an accurate picture of how reaction rates are affected, whilst allowing enough time to give a broad range of temperatures.
Key factors
As with any experiment there are several factors that could affect the experiment. We will be changing only one and keeping the rest constant.
- Temperature
- Pressure
- Potato chip size
- Amount of Hydrogen peroxide
- Concentration of Hydrogen peroxide
Prediction
I predict that the catalyse will perform better and better until an optimum temperature (around 37 centigrade) as this is the temperature of the body. After this point it will rapidly slow down and even denature as in the natural environment, the body would be dead, therefore the enzyme would not be needed. The colder the catalyse gets the slower it will work.
Apparatus
- Boiling tube
- 10ml measuring cylinder
- Potato chips (x15 for each test)
- Clamp stand
- Gas syringe
- Stop clock
- Water bath
Method
Heat (or cool) the potato chips in a water bath to the correct temperature in a boiling tube. Prepare the gas syringe and timer. Once you have checked the potato has reached the correct temperature with the thermometer add the Hydrogen peroxide to the test tube, insert the bung connected to the gas syringe and start the timer. Wait until 10cm of gas has been produced and then stop the timer. Record this number and repeat 2 more times. The experiment must be repeated for all temperatures. The gas produced will be Oxygen, and the enzyme (catalyse) will be contained in the potato.
Hydrogen Peroxide + Catalyse ------> Water + Oxygen
We will be measuring gas as if the reaction is more successful more oxygen will be produced, as the reaction will be occurring quicker.
Obtaining Results
Analysis
The graph shows quite clearly the enzyme works better until it reaches an optimum temperature, at which point it starts slowing down quickly until it denatures. The optimum temperature was around 40 °C after this point the experiment began to take longer until at 60°C the enzyme no longer worked (effectively) at all.
Conclusion
As I have stated previously the enzyme worked better and better as the reaction was heated. This was due to the fact that the molecules were receiving more energy, and therefore the particles were moving faster, resulting in more (successful) collisions and therefore more reactions.
However when the temperature increased too much the enzyme denatured and was no longer able to ‘hold’ the reactant. This meant that the enzyme was totally useless and had no effect on the reaction. The molecules were not colliding and reactions were unsuccessful. The activation energy of the experiment had been drastically increased and therefore the experiment was not working in the conditions.
Evaluation
The experiment had some unexpected results. I was expecting anything after 50°C to not react at all as the enzyme should have denatured, however in one of the three results the experiment got a reading. I think this was an error with the experiment. Aside from that the other results were as expected, and the experiment went well.
Extension
I think we could extend the experiment by reducing the increments of °C that we went up in. This would give us a more accurate picture of the time the enzyme denatured. We could experiment with other concentrations and temperatures (into freezing.) We could use other enzymes, or compare them to some industrial catalysts.
