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The World of Microbes - Nitrogen Fixation and the Nitrogen Cycle

Grade 8CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Nitrogen makes up about 78%78\% of the Earth's atmosphere, but it cannot be taken directly by plants and animals.

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Nitrogen Fixation: The process of converting atmospheric nitrogen (N2N_2) into nitrogenous compounds like nitrates (NO3−NO_3^-) that plants can absorb.

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Biological Fixation: Symbiotic bacteria like Rhizobium, found in the root nodules of leguminous plants (e.g., peas, beans), help in fixing N2N_2.

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Atmospheric Fixation: During lightning, high temperatures and pressure convert atmospheric N2N_2 and O2O_2 into oxides of nitrogen, which dissolve in rain to form dilute nitric acid (HNO3HNO_3) and reach the soil as nitrates (NO3−NO_3^-).

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Nitrogen Assimilation: The process by which plants absorb nitrogenous compounds from the soil and convert them into plant proteins.

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Ammonification: When plants and animals die, bacteria and fungi in the soil convert organic nitrogenous waste back into ammonia (NH3NH_3) or ammonium salts (NH4+NH_4^+).

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Nitrification: The conversion of ammonia (NH3NH_3) into nitrites (NO2−NO_2^-) and then into nitrates (NO3−NO_3^-) by nitrifying bacteria such as Nitrosomonas and Nitrobacter.

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Denitrification: The process where certain bacteria like Pseudomonas convert nitrates (NO3−NO_3^-) in the soil back into nitrogen gas (N2N_2), which escapes into the atmosphere.

📐Formulae

N2 (Atmospheric Nitrogen)N_2 \text{ (Atmospheric Nitrogen)}

NH3 (Ammonia)NH_3 \text{ (Ammonia)}

NO3− (Nitrate Ion)NO_3^- \text{ (Nitrate Ion)}

NO2− (Nitrite Ion)NO_2^- \text{ (Nitrite Ion)}

NH4+ (Ammonium Ion)NH_4^+ \text{ (Ammonium Ion)}

💡Examples

Problem 1:

Explain how the amount of nitrogen in the atmosphere remains constant despite continuous use by living organisms.

Solution:

The nitrogen cycle maintains the balance. While plants and bacteria 'fix' N2N_2 into NO3−NO_3^-, denitrifying bacteria like Pseudomonas convert soil nitrates back into N2N_2 gas.

Explanation:

This cycle of fixation, assimilation, and denitrification ensures the atmospheric concentration of N2N_2 stays at approximately 78%78\%.

Problem 2:

What is the role of Rhizobium in leguminous plants?

Solution:

Rhizobium bacteria live in root nodules and perform biological nitrogen fixation, converting N2N_2 into a form the plant can use to synthesize proteins.

Explanation:

This is a symbiotic relationship where the plant provides food and shelter, and the bacteria provide usable nitrogen.

Problem 3:

Identify the process: NH3→NO2−→NO3−NH_3 \rightarrow NO_2^- \rightarrow NO_3^-.

Solution:

This process is known as Nitrification.

Explanation:

Nitrifying bacteria convert ammonia produced from decaying matter into nitrites and then nitrates, which are then available for plant uptake.