Question

bacterial nitrogen fixation
nitrogen fixation is a process by which molecular nitrogen is converted to ammonia, nitrites, or nitrates, which can be absorbed by plants and converted to protein and nucleic acids. biological nitrogen fixation is carried out by various species of bacteria (including cyanobacteria) free - living in the soil or water, or in symbioses with plants, protists, or lichen - forming fungi.
nitrogen fixing symbioses in higher plants

• root nodules are a root symbiosis between a higher plant and a bacterium. the bacteria fix atmospheric nitrogen and are extremely important to the nutrition of many plants, including the economically important legume family. root nodules are extensions of the root tissue caused by entry of a bacterium. in legumes, this bacterium is rhizobium. other bacterial genera are involved in the root nodule symbioses in non - legumes.
• the bacteria in these symbioses live in the nodule where they fix atmospheric nitrogen and provide the plant with most, or all, of its nitrogen requirements. in return, they have access to a rich supply of carbohydrate. the fixation of atmospheric nitrogen to ammonia occurs within the nodule, using the enzyme nitrogenase. nitrogenase is inhibited by oxygen and the nodule provides a low o₂ environment in which fixation can occur.

1. describe three processes that fix atmospheric nitrogen:

(a)
(b)
(c)

1. what process releases nitrogen gas into the atmosphere?
2. what is the primary reservoir for nitrogen?
3. what form of nitrogen is most readily available to most plants?
4. name one essential organic compound that plants need nitrogen for:
5. how do animals acquire the nitrogen they need?
6. why might farmers plow a crop of legumes into the ground rather than harvest it?
7. describe the ecological role of nitrogen - fixing symbioses in the nitrogen cycle. use examples to illustrate your answer.

Explanation:

1. (a) Bacterial nitrogen - fixing in soil or water by free - living bacteria like cyanobacteria. (b) Nitrogen - fixing symbiosis in root nodules of legumes by Rhizobium. (c) Nitrogen - fixing symbiosis in non - legumes by other bacterial genera.
2. Denitrification is the process that releases nitrogen gas into the atmosphere. It is carried out by denitrifying bacteria that convert nitrates to nitrogen gas.
3. The atmosphere is the primary reservoir for nitrogen, containing about 78% nitrogen gas.
4. Nitrates ($NO_3^-$) are the form of nitrogen most readily available to most plants. They can be easily absorbed by plant roots.
5. Proteins are essential organic compounds that plants need nitrogen for. Nitrogen is a key component of amino acids, which are the building blocks of proteins.
6. Animals acquire the nitrogen they need by consuming plants or other animals. Plants take up nitrogen and incorporate it into organic compounds, which are then passed up the food chain.
7. Farmers might plow a crop of legumes into the ground rather than harvest it because legumes have root nodules with nitrogen - fixing bacteria. Plowing them in enriches the soil with nitrogen, which benefits future crops.
8. Nitrogen - fixing symbioses play a crucial role in the nitrogen cycle. For example, in legume - Rhizobium symbiosis, the bacteria convert atmospheric nitrogen into ammonia, which is then used by the plant. When the plant dies and decomposes, the nitrogen is released back into the soil, making it available for other plants. This enriches the soil and supports plant growth, which in turn affects the entire ecosystem.

Answer:

1. (a) Bacterial nitrogen - fixing by free - living bacteria

(b) Nitrogen - fixing symbiosis in legumes by Rhizobium
(c) Nitrogen - fixing symbiosis in non - legumes by other bacteria

1. Denitrification
2. The atmosphere
3. Nitrates ($NO_3^-$)
4. Proteins
5. By consuming plants or other animals
6. To enrich the soil with nitrogen from the root nodules of legumes
7. They convert atmospheric nitrogen into a form usable by plants. For example, Rhizobium in legume root nodules fix nitrogen, and when the plant decomposes, nitrogen is released into the soil for other plants.