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

What is the last step in the nitrogen cycle?

1. Animal waste returns nitrogen directly into the air

2. Nitrifying bacteria convert nitrates back into nitrogen gas

3. Plants absorb nitrogen from the air

4. Decomposers create ammonia from dead animals

The correct answer is: Nitrifying bacteria convert nitrates back into nitrogen gas

The last step in the nitrogen cycle involves nitrifying bacteria converting nitrates back into nitrogen gas, a process known as denitrification. This step is crucial as it completes the cycle by returning nitrogen to the atmosphere, balancing the nitrogen levels in the ecosystem. During denitrification, specific types of bacteria break down nitrates and nitrites into nitrogen gas, which can then be released into the atmosphere. This process is essential for closing the nitrogen cycle, ensuring that the nitrogen used by plants and animals is ultimately recycled back into the environment. In contrast, the other choices represent steps that occur earlier in the nitrogen cycle. For example, while animal waste does return nitrogen to the soil, it typically does so in the form of ammonium or other compounds rather than directly into the air. Plants absorb nitrogen in different forms, such as nitrates, but this occurs before it is converted into nitrogen gas. Decomposers breaking down dead organisms to create ammonia is a vital part of recycling nitrogen but occurs earlier in the cycle before denitrification takes place. Thus, the last step correctively identifies the process that finalizes nitrogen's return to the atmospheric pool.