The rate of enzyme reactions will be affected by external factors such as temperature and pH value. In general, the rate increases by an increase in temperature until a certain temperature is reached. In the human body, the rate is fastest at 37?C which is the normal body temperature and is close to the optimum temperature. At temperatures above 40?C, the rate decreases rapidly because the shape of the enzyme molecule changes and so does the shape of the active site. The enzyme is denatured by the high temperature. Figure 1 is an experiment to demonstrate the effect of temperature on the action of amylase on starch. The optimum temperature is 37?C because the brown area is the largest. The enzyme is denatured at 50?C because there is no brown area. When the temperatures of the dishes at 4?C, 20?C and 30?C are increased to 37?C, the respective brown areas remain the same as that of 37?C. When the temperature of the dish at 50?C is lowered to 37?C, the brown area will still not appear. Effect of pH Enzymes are highly sensitive to the pH value of the medium. This means that a small change in pH will greatly affect the rate of enzyme reaction because the shape of the active site will be changed. The rate is fastest at the optimum pH. However, different en zymes have different optimum pH. Denaturation caused by a small change in pH is reversible after the pH is adjusted, but denaturation caused by a drastic change in pH is usually irreversible. For example, the effect of pH on the action of catalase on hydrogen peroxide can be demonstrated by an experiment as shown in Figure 2. It can be found that the optimum pH is about 9. ERIC LEE