OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

What happens to enzymes when exposed to excessively high temperatures?

1. The reaction rate increases indefinitely

2. They maintain their activity without change

3. The active site denatures and stops functioning

4. They operate at peak efficiency

The correct answer is: The active site denatures and stops functioning

Enzymes are proteins that act as catalysts to speed up biochemical reactions. Each enzyme has an optimal temperature range in which it functions effectively. When enzymes are exposed to excessively high temperatures, the heat can disrupt the hydrogen bonds and other interactions that help maintain the enzyme's three-dimensional structure. As a result, the active site of the enzyme, which is the specific part where substrates bind, undergoes a structural change known as denaturation. This alteration means that the enzyme can no longer properly bind to its substrates, thereby halting its catalytic activity. Consequently, the reaction rates decrease significantly or stop altogether, as the enzyme is no longer functional. In contrast, maintaining activity without change would imply that the enzyme remains effective, which is not the case under high-temperature conditions. Similarly, operating at peak efficiency is only true within the optimal temperature range, not at excessively high temperatures. The notion that the reaction rate increases indefinitely is also inaccurate, as extreme conditions ultimately lead to enzyme loss of function. Thus, denaturation due to high temperatures explains why the active site of the enzyme stops functioning.