Enzymes and Reactions for AP Biology

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By — McGraw-Hill Professional
Updated on Oct 24, 2011

Practice problems for these concepts can be found at:  Chemistry Review Questions for AP Biology


Teacher (CT): "The topic of enzymes is full of essay material. Know it well."

Enzymes are proteins that act as organic catalysts and will be encountered often in your review for this exam. Catalysts speed up reactions by lowering the energy (activation energy) needed for the reaction to take place but are not used up in the reaction. The substances that enzymes act on are known as substrates.

Enzymes are selective; they interact only with particular substrates. It is the shape of the enzyme that provides the specificity. The part of the enzyme that interacts with the substrate is called the active site. The induced-fit model of enzyme-substrate interaction describes the active site of an enzyme as specific for a particular substrate that fits its shape. When the enzyme and substrate bind together, the enzyme is induced to alter its shape for a tighter active site–substrate attachment. This tight fit places the substrate in a favorable position to react, speeding up (accelerating) the rate of reaction. After an enzyme interacts with a substrate, converting it into a product, it is free to find and react with another substrate; thus, a small concentration of enzyme can have a major effect on a reaction.


Every enzyme functions best at an optimal temperature and pH. If the pH or temperature strays from those optimal values, the effectiveness of the enzyme will suffer. The effectiveness of an enzyme can be affected by four things:

  1. The temperature
  2. The pH
  3. The concentration of the substrate involved
  4. The concentration of the enzyme involved

You should be able to identify the basic components of an activation energy diagram if you encounter one on the AP exam. The important parts are identified in Figure 5.9.

The last enzyme topic to cover is the difference between competitive and noncompetitive inhibition. In competitive inhibition (Figure 5.10), an inhibitor molecule resembling the substrate binds to the active site and physically blocks the substrate from attaching. Competitive inhibition can sometimes be overcome by adding a high concentration of substrate to outcompete the inhibitor. In noncompetitive inhibition (Figure 5.11), an inhibitor molecule binds to a different part of the enzyme, causing a change in the shape of the active site so that it can no longer interact with the substrate.




pH: Acids and Bases

The pH scale is used to indicate how acidic or basic a solution is. It ranges from 0 to 14; 7 is neutral. Anything less than 7 is acidic; anything greater than 7 is basic. The pH scale is a logarithmic scale and as a result, a pH of 5 is 10 times more acidic than a pH of 6. Following the same logic, a pH of 4 is 100 times more acidic than a pH of 6. Remember that as the pH of a solution decreases, the concentration of hydrogen ions in the solution increases, and vice versa. For the most part, chemical reactions in humans function at a neutral pH. The exceptions to this rule are the chemical reactions involving the enzymes of the digestive system.


There are five types of reactions you should know for this exam:

  1. Hydrolysis reaction. A reaction that breaks down compounds by the addition of H2O.
  2. Dehydration synthesis reaction. A reaction in which two compounds are brought together with H2O released as a product.
  3. Endergonic reaction. A reaction that requires input of energy to occur.
  4. A + B + energy → C

  5. Exergonic reaction. A reaction that gives off energy as a product.
  6. A + B → energy + C

  7. Redox reaction. A reaction involving the transfer of electrons. Such reactions occur along the electron transport chain of the mitochondria during respiration.


 Practice problems for these concepts can be found at:  Chemistry Review Questions for AP Biology

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