Overview
This lecture explains the nitrogen and phosphorus cycles, focusing on their importance to living things, their biological availability, and the impact of human intervention through fertilizers.
Importance of Nitrogen and Phosphorus
- Living things require nitrogen for amino acids, proteins, DNA, and RNA.
- Phosphorus is necessary for DNA, RNA, ATP, and cell membranes.
- Nitrogen is abundant in the atmosphere, phosphorus in rocks, but both are often not in usable forms for organisms.
The Nitrogen Cycle
- Atmospheric nitrogen (N₂) makes up 78% of the air but has a strong triple bond that most organisms cannot break.
- Nitrogen-fixing bacteria use the enzyme nitrogenase to convert N₂ into ammonia (NH₃), which becomes ammonium (NH₄⁺) in water, usable by plants.
- Some bacteria form symbiotic relationships with plant root nodules, especially in legumes.
- Decomposers (fungi, protists, bacteria) convert organic nitrogen from dead matter and waste into ammonia.
- Nitrifying bacteria turn ammonia into nitrites (NO₂⁻) and then nitrates (NO₃⁻), which plants can assimilate more easily.
- Denitrifying bacteria convert nitrates back into nitrogen gas using nitrate reductase, completing the cycle.
- Lightning and synthetic processes (fertilizer production) can also fix atmospheric nitrogen.
The Phosphorus Cycle
- Phosphorus is mainly found in rocks as inorganic phosphates and does not cycle through the atmosphere.
- Weathering of rocks releases phosphates into soil and water, where plants assimilate them.
- Animals obtain phosphorus by eating plants or other animals.
- Decomposers return phosphates to the soil or water from waste or dead organisms.
- Phosphates are quickly reused by plants but can enter aquatic systems and cycle among aquatic organisms.
- In deep waters, phosphorus can become sediment and eventually form new rocks, restarting the cycle.
Human Impact and Fertilizers
- Humans manufacture synthetic fertilizers rich in nitrogen and phosphorus to increase food production.
- Excessive fertilizer use causes ecosystem imbalances and environmental harm, which will be discussed next lecture.
Key Terms & Definitions
- Nitrogen Fixation — Process by which bacteria convert nitrogen gas (N₂) to ammonia (NH₃).
- Nitrogenase — Enzyme that breaks the triple bond in atmospheric nitrogen.
- Nitrification — Conversion of ammonia to nitrites and nitrates by bacteria.
- Denitrification — Conversion of nitrates back to nitrogen gas by bacteria.
- Phosphorus Cycle — Movement of phosphorus from rocks, through living things, and back to rocks.
- Lithotrophs — Bacteria that obtain energy from inorganic compounds like rock phosphates.
Action Items / Next Steps
- Watch the recommended video on Fritz Haber and the history of synthetic fertilizers.
- Prepare for next week’s lecture on human impacts on the biosphere.
- Review the phosphorus and nitrogen cycles for better understanding.