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Wednesday, May 9, 2012

The Effect of Temperature on Enzymes

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Introduction An Enzyme is any one of many specialised organic substances, composed of polymers of amino acids, that act as catalysts to regulate the speed of the many chemical reactions involved in the metabolism of living organisms. Those enzymes identified now number more than 700.


Enzymes are classified into several broad categories, such as hydrolytic, oxidising, and reducing, depending on the type of reaction they control. Hydrolytic enzymes accelerate reactions in which a substance is broken down into simpler compounds through reaction with water molecules. Oxidising enzymes, known as oxidises, accelerate oxidation reactions; reducing enzymes speed up reduction reactions, in which oxygen is removed. Many other enzymes catalyse other types of reactions.


Individual enzymes are named by adding ASE to the name of the substrate with which they react. The enzyme that controls urea decomposition is called urease; those that control protein hydrolyses are known as proteinases. Some enzymes, such as the proteinases trypsin and pepsin, retain the names used before this nomenclature was adopted.


( Fig 1.0 on the following page )


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Structure and Function of an Enzyme


Enzymes are large proteins that speed up chemical reactions. In their globular structure, one or more polypeptide chains twist and fold, bringing together a small number of amino acids to form the active site, or the location on the enzyme where the substrate binds and the reaction takes place. Enzyme and substrate fail to bind if their shapes do not match exactly. This ensures that the enzyme does not participate in the wrong reaction. The enzyme itself is unaffected by the reaction. When the products have been released, the enzyme is ready to bind with a new substrate.


Properties of Enzymes


As the Swedish chemist Jöns Jakob Berzelius suggested in 18, enzymes are typical catalysts they are capable of increasing the rate of reaction without being consumed in the process.


Some enzymes, such as pepsin and trypsin, which bring about the digestion of meat, control many different reactions, whereas others, such as urease, are extremely specific and may accelerate only one reaction. Still others release energy to make the heart beat and the lungs expand and contract. Many facilitate the conversion of sugar and foods into the various substances the body requires for tissue-building, the replacement of blood cells, and the release of chemical energy to move muscles.


Pepsin, trypsin, and some other enzymes possess, in addition, the peculiar property known as autocatalysis, which permits them to cause their own formation from an inert precursor called zymogen. As a consequence, these enzymes may be reproduced in a test tube.


As a class, enzymes are extraordinarily efficient. Minute quantities of an enzyme can accomplish at low temperatures what would require violent reagents and high temperatures by ordinary chemical means. About 0g of pure crystalline pepsin, for example, would be capable of digesting nearly metric tons of egg white in a few hours.


The kinetics of enzyme reactions differ somewhat from those of simple inorganic reactions. Each enzyme is selectively specific for the substance in which it causes a reaction and is most effective at a temperature peculiar to it. Although an increase in temperature may accelerate a reaction, enzymes are unstable when heated. The catalytic activity of an enzyme is determined primarily by the enzymes amino-acid sequence and by the tertiary structure-that is, the three-dimensional folded structure of the macromolecule. Many enzymes require the presence of another ion or a molecule called a cofactor, in order to function.


As a rule, enzymes do not attack living cells. As soon as a cell dies, however, enzymes that break down protein rapidly digest it. The resistance of the living cell is due to the enzymes inability to pass through the membrane of the cell as long as the cell lives. When the cell dies, its membrane becomes permeable, and the enzyme can then enter the cell and destroy the protein within it. Some cells also contain enzyme inhibitors, known as antienzymes, which prevent the action of an enzyme upon a substrate.


Aim To find the effect of temperature on enzymes, using a potato as a catalyst. The source of catalase is in the potato cells.


Preliminary work For this, I wanted to see how many potato discs would be a good number to do the actual experiment. I used the same experiment I am going to use to find the effect of temperature on enzymes but instead of varying the temperature, as I am going to do, I varied the amount of potato discs in the test tube. Overall, I took 6 readings of a different number of discs. At first, I used ; then I went onto 4, and so on up to 1. (, 4, 6, 8, 10, 1) I found out that the ideal number of discs was 8 so I am going to use 8 in my experiment of varying temperature.


Results of prelim work


No. of Potatoes


(amount) Time to reach 5cm


(seconds)


4 41.4


6 .


8 0.4


10 0.1


1 18.4


Apparatus I have decided to use the following equipment in order to carry out my experiment


? -Water Baths


? -Ice Baths


? -Test Tube/Boiling tube


? -10 cm Measuring cylinder


? - cm of Hydrogen Peroxide


? -1 cm of circular potato chips


? -Manometer


? -Borer


? -Stopwatch


Fig 1.1 is a hand drawn diagram of the equipment I will be using listed above


Methods At first, I will have to get the potato so I will use a Borer to cut a cylinder of potato out of the whole one and from there I will cut up the potato cylinder into segments of 1 cm using a razor. I will then put them into a test tube containing 10 cm of pH.7 buffer solution and place it in their designated water/ice baths along with the 10 cm of Hydrogen Peroxide. The water baths range from 0°C to 60°C, with intervals of 10°C. Time will not affect my experiment, as I will leave all the potato chips in the water baths for an equal length of time, my only variable being the temperature, with a range increasing by 10°C each time. I will use this range because I think it will improve the accuracy of my results. Once the Hydrogen Peroxide and buffer solution are at the temperature I want them to be, I will place the Manometer over the test tube as soon as I have put the buffer solution in with the potato discs. I will then start the stopwatch as soon as I have tightened the tap on the manometer, measuring the time it takes for the red dye to move 5cm. This distance is taken from the level point of the dye and will be measured using a ruler and the 5cm mark is drawn with a black chinagraph pencil. I have chosen 5cm because the Manometer tube is not much longer than 5cm. I will repeat this three times to find a mean for this temperature. The procedure will be repeated for the following temperatures 0°C, 0°C, 0°C, 40°C, 50°C and 60°C. I have evolved on this plan as a result of preliminary work on the topic in which a number of procedures and variables were demonstrated.


Key Variables


? Heat


? pH


? Time for the red dye to reach the 5 cm mark


? Concentration of enzyme of substrate


? Potato


? The surface area of the potato


I am going to vary a (the temperature), from above, and control all of the other variables.


I shall only be altering the temperature, as that is my main variable, and so I will therefore keep the others the same in order to make it a fair test, and a comprehensive study without many variables. I will keep my variables the same by making sure that I have implied the same strategy for each test taken. The amount of Hydrogen Peroxide is carefully measured out using the measuring cylinder; enabling me to make sure it is kept constant throughout the experiment.


Safety Precautions I will have to be careful when using the Hydrogen Peroxide, as it is a corrosive chemical, so I will attempt to overcome this problem by wearing goggles. Hydrogen Peroxide is a bleaching agent, so I will be wearing a lab coat so it doesn�t bleach my clothes. I will also use a peg to retrieve the test tubes from the boiling water baths. I will be using sharp razors during the experiment so I will have to be cautious about that.


Hypothesis My prediction is supported by Kinetic Theory in that if I apply twice as much heat there will be twice as much particle vibration therefore the reaction will happen twice as quickly. It is also backed by Collision Theory in that if I apply twice as much heat there will be twice as many collisions and therefore the rate of reaction will double. This will only be so until the enzyme denatures after its optimum temperature 40 - 45°C.


Predictions I predict that the enzyme will become denatured, and therefore will work at a slower rate after 40 - 45°C. I think the reason for this prediction is because every enzyme has a temperature range of optimum activity. Outside that temperature range the enzyme is rendered inactive. This occurs because as the temperature changes enough energy is supplied to break some of the molecular bonds. When these forces are disturbed and changed the active site becomes altered in its ability to accommodate the substrate molecules it was intended to catalase. Most enzymes in a human body shut down beyond certain temperatures. This can happen if body temperature gets too low (hypothermia), or too high (hypothermia).


From my background knowledge it is evident that as temperature increases, the rate of reaction also increases. However, the stability of the protein also decreases due to thermal degradation. Holding the enzyme at a high enough temperature for a long period of time may cook the enzyme. Reaction rate is the speed at which the reaction proceeds toward equilibrium. For an enzyme-catalysed reaction, the rate is usually expressed in the amount of product produced per minute. The energy barrier between reactions and products governs reaction rate. In general, energy must be added to the reactants to overcome the energy barrier. This added energy is termed activation energy, and is recovered as the reactants pass over the barrier and descend to the energy level of the products. Enzymes can accelerate the rate of a reaction. Catalysts accelerate the rates of reactions by lowering the activation energy barrier between reactants and products. All chemical reactions speed up as the temperature is raised. As the temperature increases, more of the reacting molecules have enough kinetic energy to undergo the reaction. Since enzymes are catalysts for chemical reactions, enzyme reactions also tend to go faster with increasing temperature. However, if the temperature of an enzyme catalysed reaction is raised still further, an optimum is reached above this point the kinetic energy of the enzyme and water molecules is so great that the structure of the enzyme molecules starts to be disrupted. The positive effect of speeding up the reaction is now more than offset by the negative effect of denaturing more and more enzyme molecules. Many proteins are denatured by temperatures around 40 - 50°C, but some are still active at 70 - 80°C, and a few withstand being boiled. So, my first prediction is that the enzyme will become denatured at around 40°C, and secondly, that as the temperature increases the reaction rate will increase by 50%, due to the molecules colliding together at a higher speed (kinetic theory) due to their extra energy obtained by the increase in temperature. My prediction is supported by Kinetic Theory in that if I apply twice as much heat there will be twice as much particle vibration therefore the reaction will happen twice as quickly. It is also backed by Collision Theory in that if I apply twice as much heat there will be twice as many collisions and therefore the rate of reaction will double. This will only be so until the enzyme denatures after its optimum temperature 45°C.


The graph I have predicted (fig 1.) is as follows


OBTAINING


Table of Results


Temperature


(oC) Time taken to get to 5 cm on the Manometer


(Seconds)


Data no.1 Data no. Data no. Average data


0 75 77 80 77


4 (Room) 45 47 46 46


0 0 1


40 4 6 5 5


50 7 7 76 7


60 100 10 104 10


Below is a graph to show Rate of Reaction against temperature (fig 1.)


ANALYSIS


Analysis of results Fig 1. is a free hand graph of the table above.


From the graph, I am able to back up my theory. I can see when the enzyme is most active and when it starts to denature. From the graph, I have found out that, as the temperature increases, so does the catalase activity, as it does not take as long to move the same distance, up to a certain point (60°C) where the activity ceases altogether. I found that the optimum for a catalase is at 40°C. This is where the greatest number of collisions takes place between the enzyme and the substrate and therefore the highest rate of reaction is.


The rate was higher at the higher temperatures (up to 40°C) because as the temperature is raised, so is the energy level of the enzymes and substrate molecules. This means that they have more kinetic energy so they collide more often and therefore more reactions take place between them. This, in turn, means that the rate increases as more oxygen (O) is produced. The enzyme denatured at about 60°C because the weak bonds, which hold the molecule into the specific shape for one substrate, are broken. The increase in molecular collisions and vibrations at higher temperatures is great enough to permanently change the shape of the active site. The enzyme is said to be denatured because it can no longer form an enzyme-substrate complex as its active site has been unalterably changed.


My prediction was correct in that there was very little activity in the ice bath because the speed at which the enzymes and substrate molecules were moving was very slowly so there were not many collisions between them. The optimum temperature was nearly the same as I predicted at 40°C instead of 45°C. The temperature at which the enzyme denatured and the activity stopped, was therefore nearly the same temperature at 60°C instead of 70°c as was predicted.


I included the anomalies (room temperature and 50°C) in my graph because I did not think they would affect the graphs line of best fit. These results may be anomalous because I started the stopwatch too early or too late. This was a very big problem in the experiment and is all explained in my evaluation.


EVALUATION


Evaluation Although I conducted the experiment as accurately as I could there were many sources of error in the method that I used. Firstly, some help from a friend was needed to start the experiment and this lead to a small delay in starting the stopwatch. I needed to put the Hydrogen Peroxide into the test tube containing pH 7 Buffer solution and 8 potato discs, put the bung of the Manometer onto the test tube and start the stopwatch all at the same time. I also think it would have been better if I had used the same potato from the whole experiment but was unable to due to the time restrictions. I had to conduct the experiment over a number of days and could not therefore use the same potato. This is a source of error because the concentration of catalase in the potatoes may have been different which may have produced an inconsistent rate of reaction. This might be why the values I obtained for the 4°C and 50°C do not quite fit the pattern of the graph that I expected. To remove this problem, I could repeat the experiment not only with three readings at each temperature, but also with three different potatoes, which would provide an even more accurate reading, as I could calculate an average. Another problem was that I had to leave the buffer solution and potato discs to acclimatise for longer than I expected would be necessary.


I think that any other anomalous results where mostly due to a longer acclimatisation or the fact that I did not allow the Hydrogen Peroxide to acclimatise in a different tube, which I would definitely, do if I repeated the experiment. There could also have been a slight variation in the length of the potato discs because it was very difficult to get them all the exact same size even though I was using a ruler and razor. Next time I would use longer discs, maybe even the same weight so that it would be easier to judge how long they are. If I repeated the experiment I would also take more readings for example at every 5°C because if I did this I would be able to plot a more accurate graph and it would be easier and more accurate to tell when the enzyme got to the optimum and denaturing temperatures.


The evidence that I obtained is sufficient enough to support the conclusions I have come to about the values for the optimum and denaturing temperatures because I conducted my experiment as accurately as I could whit the method I used and did quite a large range and number of repetitions to the results reliable.





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