Page 1 - 7-Lesson notes-Nitrogen fixation and nitrogen cycle

SAI International School

Subject- Biology, Class-VIII
Topic- Microorganisms: Friend and Foe
Subtopic- Nitrogen fixation and Nitrogen cycle
Lesson Notes

What is Nitrogen Fixation?

• Nitrogen gas (N₂) makes up 78% of air, but plants cannot use it directly.
• Nitrogen fixation is the process of converting nitrogen gas into usable forms (like
nitrates).
• Done by:
❖ Nitrogen-fixing bacteria (e.g., Rhizobium in legume root nodules)
❖ Blue-green algae (cyanobacteria) in waterlogged soil
❖ Lightning, which converts nitrogen into compounds that mix with rainwater
❖ Industrial processes (used in making fertilizers)

What is the Nitrogen Cycle?

• The nitrogen cycle is the natural circulation of nitrogen between the atmosphere,
soil, plants, and animals.

Steps of the Nitrogen Cycle:

• Nitrogen Fixation – Nitrogen in air is converted into usable forms by microbes or
lightning.
• Absorption by Plants – Plants absorb nitrates from soil to make proteins.
• Consumption by Animals – Animals eat plants and use nitrogen to build body
proteins.
• Decomposition – Dead plants and animals decompose, and nitrogen returns to the
soil.
• Denitrification – Some bacteria convert nitrates back into nitrogen gas, releasing it
into the atmosphere.

Why is the Nitrogen Cycle Important?

• Maintains the balance of nitrogen in the atmosphere.
• Ensures a continuous supply of nitrogen to living organisms.