Biology STAAR (State of Texas Assessments of Academic Readiness) Practice Test

In the nitrogen cycle, which organisms convert nitrogen into a usable form for plants?

1. Herbivores

2. Bacteria

3. Carnivores

4. Decomposers

The correct answer is: Bacteria

In the nitrogen cycle, bacteria play a crucial role in converting atmospheric nitrogen into forms that plants can absorb and use. Specifically, nitrogen-fixing bacteria, such as Rhizobium, reside in the root nodules of certain plants, especially legumes. They convert nitrogen gas (N₂) from the atmosphere into ammonia (NH₃) through a process known as nitrogen fixation. This ammonia can then be transformed into other forms, such as nitrates and nitrites, by different types of bacteria (nitrifying bacteria), which further enrich the soil and make nitrogen available to plants. The process is essential because nitrogen is a critical component of amino acids, proteins, and nucleic acids, which are fundamental for plant growth and development. Thus, bacteria are integral to ensuring that nitrogen, an abundant but inaccessible element in the atmosphere, becomes usable for living organisms in the ecosystem.