Each different type of enzyme will usually act on only one substrate to catalyse one biological reaction. Enzymes are specific because different enzymes have differently shaped active sites. The shape of the active site of an enzyme is complementary to the shape of its specific substrate. This means they are the correct shapes to fit together.
Temperature has an effect on enzyme activity.
The graph shows that enzymes have a lower activity at low temperatures and high temperatures. The temperature at which enzyme activity is highest is called the optimum temperature.