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

What do nitrifying bacteria do in the nitrogen cycle?

1. Convert nitrates into ammonia

2. Break down complex proteins into amino acids

3. Convert ammonia into nitrates

4. Absorb nitrogen gas from the atmosphere

The correct answer is: Convert ammonia into nitrates

Nitrifying bacteria play a crucial role in the nitrogen cycle by converting ammonia into nitrates. This process is significant because ammonia, which can be toxic to plant life at high concentrations, is transformed into nitrates, which are a more usable form of nitrogen for plants. The conversion occurs in two main steps. First, ammonia is oxidized to nitrites by a group of nitrifying bacteria known as ammonia-oxidizers. Then, another group of bacteria, called nitrite-oxidizers, further oxidizes the nitrites into nitrates. This transformation is essential for maintaining the fertility of soils, providing plants with the necessary nutrients to grow. This process not only supports plant health but also contributes to the overall balance of nitrogen in ecosystems, highlighting the significance of nitrifying bacteria in both agriculture and natural environments.