Final answer:
Plants depend on symbiotic nitrogen-fixing bacteria to convert atmospheric nitrogen, which they cannot use, into a form they can absorb and use to grow.
Step-by-step explanation:
Plants are unable to fix atmospheric nitrogen directly and thus depend on the nitrogen fixation process carried out by various types of bacteria. While plants can absorb carbon dioxide from the atmosphere to perform photosynthesis, they rely on nitrogen in the form of nitrates, which can be absorbed through their roots from the soil.
This is possible thanks to the action of symbiotic nitrogen-fixing bacteria, such as Rhizobium living in the root nodules of legumes, and free-living bacteria, like Azotobacter. These bacteria convert atmospheric nitrogen, which plants cannot use, into ammonia and subsequently into nitrates, a form that plants can uptake and utilize to synthesize essential organic compounds. Cyanobacteria also play a crucial role in nitrogen fixation in aquatic ecosystems.