limited, the
graph tails off
Enzyme concentration
Substrate concentration:
For a given amount of enzyme, the rate of an enzyme-controlled reaction increases, with an increase is substrate concentration up to a point. At low substrate concentrations, the active sites of an enzymes are not all used simply because there are not enough substrate molecules to occupy the enzymes. As there is more substrate concentration, the more enzymes’ active site will be used; increasing the substrate concentration wont necessarily increase the rate of reaction, as the amount of enzyme is the limiting factor. The graph below shows that the rate of reaction will be continuous up to a point where all the active sites of an enzyme are occupied.
Rate of reaction
Maximum rate of reaction
All active sites on
Enzyme are occupied
Rate of reaction
Temperature:
If the temperature is increased, the rate of reaction will be affected in two ways:
As the temperature increases, the kinetic energy of the substrate and enzyme molecules increases and so they move faster. The faster the molecules move, the more they will collide with one another resulting in a faster rate of reaction.
As the temperature is increased, the more the atoms in the enzyme molecules vibrate. This alters the precise shape of the enzyme by breaking the hydrogen bonds and other forces, which hold the shape of the molecule together. The three-dimensional shape is then changed so much that the active site of the enzyme will no longer fit the substrate molecule. At this point the enzyme is classed as “Denatured” and loses its catalytic properties
The optimum temperature of an enzyme varies considerably. Many arctic and alpine enzymes function efficiently at a temperature of about 10^0C, whereas those in algae inhabiting in hot springs continue to function at temperatures around 80^0C. For many more common enzymes the optimum temperature lies at about 40^0C and denatures at about 60^0C.
Rate of reaction
Rate at which reaction increases due to increased kinetic
energy of substrate and enzyme molecules
Temperature
Rate of reaction Rate of reaction
Temperature 40^0C
Rate at which reaction decreases due Temperature
To denaturisation of the enzyme Actual rate of reaction as a
result of the combined effect
of the other 2 influences
pH:
The precise three-dimensional shape of enzymes, which, is vital for the enzymes to function is partly due to hydrogen bonding. These bonds may be broken by the presence of hydrogen ions (H^+) present. pH is a measure of hydrogen ion concentration. It is measured on a scale of 1-14, with pH 7 being neutral. A pH below 7 is an acid and a pH above seven is an alkaline (base). The change of one point of pH represents a change in hydrogen ion concentration. Compared to water, which has a pH of 7, a solution which has a pH of 6 will be ten times more acidic, a solution of pH 5 will be a hundred times more acidic, a solution with a pH of 4 will be a thousand times more acidic and so on.
By breaking the hydrogen bonds, which give enzymes their shape, any change in pH can effectively denature enzymes. Each enzyme works efficiently at a particular pH, and any change from this optimum may result in denaturisation of the enzyme. The graph shows different pH optima of 4 different enzymes.
Rate of reaction
Pepsin Sucrase Salivary amylase Arginase
2 4.5 7 10
pH
Inhibition:
The rate of enzyme-controlled reactions may be decreased by the presence of inhibitors. There are 2 types: reversible inhibitors and non-reversible inhibitors.
Reversible inhibitors
The effect of this type of inhibitor is temporary and doesn’t cause any permanent damage to the enzyme. If removed again the activity of the enzyme is restored to normal again. There are 2 types: competitive and non-competitive inhibitors.
Competitive inhibition competes with active sites of enzyme molecules. The inhibitor may have a structure, which permits it to combine with the enzyme. While it remains bound to the active site, it prevents substrate molecules occupying them and so reduces the rate of reaction. The same quantity of the product is formed, because the substrate molecule continues to use any enzyme molecules which are unaffected by the inhibitor. However, it does take longer to make the products. Less inhibition if f the concentration of substrates is increased. This is because, as the substrate and inhibitor are in direct competition, the greater the proportion of substrate molecules, the greater their chance of finding the sites leaving fewer to be occupied by the inhibitor.
Substrate molecule occupying
the active site of the enzyme Inhibitor molecule occupying the active
site of the enzyme
Enzyme Enzyme
Substrate molecule unable to
Occupy the active site of the
enzyme
Non-competitive inhibitors do not attach themselves to the active site of the enzyme, but elsewhere on the molecule. Nevertheless, they alter the shape of the molecule in such a way that the active site can no longer accommodate the substrate molecule. As substrate and inhibitor molecules attach to different parts of the enzyme, they are not competing for the same sites. An increase in substrate concentration will not therefore reduce the effect of the inhibitor.
Substrate molecule occupying the active site of the enzyme
Enzyme
Substrate molecule still fits into the active site nut not in a
Way that allows the reaction to proceed
Enzyme shape is
changed due to the
presence of the
inhibitor molecule
Inhibitor molecule attached to
the enzyme molecule
Non-reversible inhibitors
Non-reversible inhibitors leave the enzyme permanently damaged and so unable to carry out its catalytic function. Heavy metal ions such as mercury and silver, cause disulphide bonds to break. These bonds help to maintain the shape of the enzyme molecule. Once broken, the enzyme molecules’ structure becomes irreversibly altered with the permanent loss of its catalytic properties.
Constants & Variables:
In this experiment, the only variable that will be changed is the surface area (which is the amount of enzyme). I will change this by cutting up different sizes of potato. I have mentioned the size of the potato cubes and the surface area of the potato in he hypothesis. The variables that affect the activity of the enzyme, Catalase, are considered and controlled so that they will not disrupt the success of the experiment.
Temperature: in this experiment I have decided to keep the temperature constant at room temperature which is about 23^0C. Any change in temperature will disrupt my experiment, so I have to be careful when handling the test tubes, so instead of using my hands I will use tongs to handle the test tubes.
pH: in this experiment I have decided to use the pH of 7 constant to maintain the pH suited to the enzyme by being equal to the natural environment of the enzyme. I will keep the pH constant by using a buffer.
Substrate concentration: To control substrate concentration I will use a pipette so that I can measure exactly the amount that I want (10cm^3)
Inhibition: In this experiment no inhibitors will be added so this will not affect the rate of reaction.
Method:
First you should start by collecting all the equipment listed. Then you should set up the diagram as shown on the diagram. After making sure everything is set up correctly, use the cork bore to cut the different sized potato cubes. Then you should put 10cm^3 of hydrogen peroxide and 2cm^3 buffer solution into a test tube. Thereafter put the first potato cube in the test tube. As soon as you drop the potato in, you must close the bung properly in order to make sure that no oxygen escapes. You must also make sure to start the stopwatch. Then, measure the amount of oxygen in the measuring cylinder and the more oxygen there is the faster the rate of Catalase activity and the slower the rate of Catalase activity, the lower the amount of oxygen being produced. The experiment should be repeated at least 3 times for the accuracy of results and so that it is a fair test.
Health & Safety:
Firs of all, because there is going to use of hydrogen peroxide (which is corrosive), goggles, gloves and a lab coat must be worn. I will use a safety tile so that the potato or bench aren’t going to be in any danger of germs. I will also have to consider other people because I will be working alongside my fellow classmates, so I will have to be aware of other people’s movements. The main term if safety is to be sensible and to act quickly if any spillage of hydrogen peroxide was to occur.
Bibliography:
Understanding Biology by Susan Toole and Glenne Toole
Diagram:
