Organisms and their Environment - Nutrient cycles (Carbon and Nitrogen)

The Carbon Cycle: The process by which carbon is exchanged between the atmosphere, biosphere, geosphere, and hydrosphere. Key processes include photosynthesis ($6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$), respiration ($C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$), decomposition, and combustion.

Carbon Sinks and Sources: Reservoirs that store carbon are sinks (e.g., oceans, forests as biomass, fossil fuels), while processes that release $CO_2$ are sources (e.g., volcanic activity, burning fossil fuels).

The Nitrogen Cycle: Nitrogen is essential for synthesizing proteins and nucleic acids (DNA/RNA). Since atmospheric $N_2$ is inert due to a triple covalent bond, it must be 'fixed' into reactive forms like $NH_3$ or $NO_3^-$.

Nitrogen Fixation: Conversion of atmospheric $N_2$ gas into ammonia ($NH_3$) or ammonium ions ($NH_4^+$) by nitrogen-fixing bacteria (e.g., Rhizobium in root nodules of legumes or Azotobacter in soil) or via lightning.

Nitrification: A two-step aerobic process where nitrifying bacteria convert ammonium ($NH_4^+$) into nitrites ($NO_2^-$) (by Nitrosomonas) and then into nitrates ($NO_3^-$) (by *Nitrobacter$).

Denitrification: The conversion of nitrates ($NO_3^-$) back into nitrogen gas ($N_2$) by denitrifying bacteria (e.g., Thiobacillus denitrificans) typically occurring in anaerobic conditions like waterlogged soils.

Ammonification/Decomposition: Saprobionts break down organic nitrogen (from dead matter and urea) into ammonia ($NH_3$), which then forms ammonium ions ($NH_4^+$) in the soil.