Experiments have shown that many factors can affect the action of enzymes in living cells. These factors affect only the rate of catalyzed reaction. The products formed by the reaction do not change.
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Small amounts of enzymes can cause the reaction of large quantities of substrate. The time needed for an enzyme substrate to form and a reaction to occur if very short. A single enzyme can catalyze thousands of substrate reactions each second. Thus only small amounts of any enzyme need to be present in a cell in any given time.

Enzymes enable cell reactions to take place at normal temperatures. Many chemical reactions that take place at ordinary temperatures can be speeded up by raising temperatures. Although high temperatures can kill living cells. Enzymes speed up reactions in a cell without requiring high temperatures.

Enzymes work best at certain temperatures. Enzyme actions depends on the random motion of molecules because this motion brings the substrates into contact with the enzymes. The motion increases as the temperature rises. At higher temperatures the enzymes become more effective, because complexes are forming at a faster rate. At higher temperatures,however, the enzyme itself starts to break down. This process is called denaturation. When the shape of the enzyme molecule changes , its active site no longer fits the substrate molecule. Therefore, it looses its effectiveness. There is a certain temperature in which the enzyme works best at.
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Each enzyme works best at a certain pH. The effectiveness of the enzyme depends on the pH of the surrounding medium. For example, the pH of the contents of the human stomach is slightly acidic. The enzyme pepsin starts the dige stion of protiens in the stomach. Pepsin is most effective at this pH level. The in the intestines in the human digestive system is slightly basic on the pH scale. At this pH the enzyme, trypsin, works best.

The rate of enzyme controlled reaction depends on the concentrations of the enzyme and substrate. The rate of an enzyme controlled reaction depends on how often enzyme and substrate molecules bump into each other . When there is little enzyme but a great deal of substrate, the concentration of enzyme limits the rate of the reaction. Adding more enzyme, therefore, increases the number of substrate molecules that can be reacting at any moment. The reaction rate increases until a maximum rate is reached. After this point, adding more enzyme will not increase the rate
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