Advanced Placement (AP) Biology Practice Exam

What role do enzymes play in chemical reactions?

1. They increase the amount of energy needed

2. They speed up reactions by lowering activation energy

3. They are consumed in the reaction

4. They act independently of substrates

The correct answer is: They speed up reactions by lowering activation energy

Enzymes are biological catalysts that play a crucial role in facilitating chemical reactions within organisms. The primary function of enzymes is to speed up reactions by lowering the activation energy, which is the minimum amount of energy required for a reaction to occur. This is important because many biochemical reactions would proceed too slowly at the temperatures compatible with life if enzymes were not present. By lowering the activation energy, enzymes make it easier for reactants to collide with enough energy to form products, thereby increasing the reaction rate. They achieve this through mechanisms such as stabilizing the transition state of the reaction, providing an optimal environment for the reaction to occur, or bringing substrates together in a favorable orientation. In contrast, the other choices suggest incorrect roles for enzymes. Enzymes do not increase the amount of energy needed for reactions; rather, they decrease the energy barrier. Additionally, enzymes are not consumed in the reaction; they can be reused multiple times after facilitating a reaction. Lastly, enzymes rely on substrates to function; they do not act independently. Instead, they specifically bind to substrates to form an enzyme-substrate complex, which is crucial for their catalytic activity.