The effect of Temperature on the Rate of Enzyme Catalysed Reactions
In an exam it is most likely that if you get a question about enzymes and temperature you will be interpreting data. Don't be put off by a question that's about an enzyme you've never heard of, the principal is always the same. Also, different enzymes have different optimum temperatures, so don't be thrown if you are given data that doesn't match what you have learned in class. You should keep in mind that enzymes are proteins and make connections between their behaviour and the chemical structure of them.
Explaining the graph above
First you must understand what it means to explain what a graph shows. You should be able to pick out key features of the graph and use your kwoledge of the subject so say why the graph is like that. I would break the above graph down into 3 stages:
1. Kinetic Energy
At low temperatures it can clearly be seen that as temperature increases the rate of enzyme activity (take this from the axis) also increases. (here you have stated the relationship). This is because as temperature increases the molecules of both the enzyme and the substrate have more kinetic energy. This makes them more likely to collide successfully with enough energy to form an enzyme substrate complex. (now you have given the reason)
2. Optimum
At ____degrees C the rate of enzyme reaches it's optimum. (Shown with the blue dashed line on the graph above. There is not a great deal more you can say here. You should use the word optimum though and you should state the temperature according to the data you have been given- remember, enzymes have different optimum temperatures so don't estimate or make assumptions).
3. Denatured
Above the optimum temperature the rate of enzyme activity is reduced (again, state what the graph shows). This is because above this temperature the enzyme may change shape. Hydrogen bonds in particular are easily broken at high temperature and the tertiary structure is altered. This means that the active site no longer fits the substrate and the enzyme can no longer facilitate the reaction. The enzyme is said to have been denatured. (now you have given the reason)