Question

bacterial nitrogen fixation
nitrogen fixation is the process by which molecular nitrogen ((n_{2})) in the atmosphere is converted to ammonia ((nh_{3})) or related nitrogenous compounds. this occurs naturally in the soil by certain bacteria, such as rhizobium, which are associated with the root nodules of leguminous plants. nitrogen - fixing bacteria can also be free - living in the soil.

nitrogen - fixing symbioses in higher plants
root nodules are a root symbiosis between a nitrogen - fixing bacterium (usually rhizobium) and a leguminous plant. atmospheric nitrogen is fixed by the bacteria and made available to the plant, which in turn provides the bacteria with carbohydrates and other nutrients. the formation of root nodules is a highly specific interaction. the bacteria produce nod factors that trigger the formation of nodules on the plant roots. once inside the root cells, the bacteria differentiate into bacteroids, which are the nitrogen - fixing form of the bacteria.

the bacteria in these symbioses live in the nodules where they fix atmospheric nitrogen and convert it into a form that can be used by the plant. in return, they have access to a rich supply of carbohydrate. the fixation of atmospheric nitrogen to ammonia occurs within the nodule. the process is energy - consuming and is inhibited by oxygen, so the nodule provides a low - oxygen environment in which fixation can occur.

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

Explanation:

1. (a) Nitrogen - fixing bacteria in legume roots convert atmospheric nitrogen to ammonia. Lightning can also fix nitrogen by causing high - energy reactions that convert nitrogen gas to nitrogen oxides which then form nitrates in the soil. (b) Denitrification is the process by which bacteria convert nitrates back to nitrogen gas and release it into the atmosphere.
2. The atmosphere is the primary reservoir for nitrogen as it is about 78% nitrogen gas.
3. Nitrates ($NO_3^-$) are the form of nitrogen most readily available to most plants as they can be easily absorbed by plant roots and used in various metabolic processes.
4. Proteins are essential organic compounds that plants need nitrogen for as nitrogen is a key component of amino acids which are the building blocks of proteins.
5. Animals acquire the nitrogen they need by consuming plants or other animals. Plants have already incorporated nitrogen into their organic compounds, and when animals eat these plants or other animals that have eaten plants, they obtain the nitrogen.
6. Farmers may plow a crop of legumes into the ground rather than harvest it because legumes have symbiotic relationships with nitrogen - fixing bacteria. When plowed under, these legumes release the nitrogen they have accumulated into the soil, enriching it for future crops.
7. Nitrogen - fixing symbioses play a crucial role in the nitrogen cycle. For example, in the symbiosis between legumes and rhizobia bacteria, the bacteria convert atmospheric nitrogen into ammonia which is then used by the legume. When the legume dies or is plowed under, the nitrogen is released into the soil, making it available for other plants. Another example is the symbiosis between certain cyanobacteria and plants like Azolla, where the cyanobacteria fix nitrogen and contribute to the nitrogen supply of the plant and the surrounding ecosystem.

Answer:

1. (a) Nitrogen - fixing bacteria in legume roots, lightning. (b) Denitrification.
2. The atmosphere.
3. Nitrates ($NO_3^-$).
4. Proteins.
5. By consuming plants or other animals.
6. To enrich the soil with nitrogen.
7. Nitrogen - fixing symbioses convert atmospheric nitrogen into a usable form for plants. Examples: Legumes - rhizobia symbiosis, Azolla - cyanobacteria symbiosis.