Describe the nitrogen cycle with reference to: • decomposition of plant and animal protein to ammonium ions • nitrification • nitrogen fixation by lightning and bacteria • absorption of nitrate ions by plants • production of amino acids and proteins • feeding and digestion of proteins • deamination • denitrification

The nitrogen cycle is a crucial biogeochemical cycle that describes the movement of nitrogen in various forms through the environment, including its transformation and utilization by living organisms. Here’s a detailed explanation of the key processes involved in the nitrogen cycle, focusing on decomposition, nitrification, nitrogen fixation, absorption by plants, production of amino acids and proteins, feeding and digestion, deamination, and denitrification.

1. Decomposition of Plant and Animal Protein to Ammonium Ions

When plants and animals die or excrete waste products, their organic matter is broken down by decomposers such as bacteria and fungi. This process, known as ammonification, converts organic nitrogen (found in proteins and nucleic acids) into ammonium ions (NH₄⁺). The equation for this process can be simplified as follows:

Decomposers play a vital role in recycling nitrogen back into the soil, making it available for subsequent processes in the nitrogen cycle.

2. Nitrification

Nitrification is a two-step process carried out by specialized bacteria that convert ammonium ions into nitrates (NO₃⁻), which are more readily absorbed by plants.

• Step 1: Oxidation of Ammonium to Nitrite
  • Bacteria such as Nitrosomonas convert ammonium (NH₄⁺) into nitrite (NO₂⁻):
  NH4++1.5O2→NO2−+2H++2H2O
• Step 2: Oxidation of Nitrite to Nitrate
  • Another group of bacteria, such as Nitrobacter, further oxidizes nitrite to nitrate:
  NO2−+0.5O2→NO3−​

Nitrification is an aerobic process that requires oxygen and is essential for converting ammonia into forms usable by plants.

3. Nitrogen Fixation by Lightning and Bacteria

Nitrogen fixation is the process of converting atmospheric nitrogen gas (N₂) into ammonia or related compounds that can be utilized by living organisms.

• Biological Nitrogen Fixation: Certain bacteria, including those in the root nodules of legumes (e.g., Rhizobium), can convert atmospheric N₂ into ammonia (NH₃):

• Abiotic Nitrogen Fixation: Lightning also contributes to nitrogen fixation by providing the energy needed to convert N₂ into nitrates through high-temperature reactions during thunderstorms.

4. Absorption of Nitrate Ions by Plants

Plants absorb nitrates (NO₃⁻) from the soil through their root systems. This process is vital for plant growth as nitrates are a key source of nitrogen for synthesizing amino acids and proteins:

5. Production of Amino Acids and Proteins

Once absorbed, nitrates are converted into organic nitrogen compounds within plants. Through a series of biochemical processes, plants synthesize amino acids (the building blocks of proteins) from these nitrogen sources:

These proteins are essential for plant structure and function.

6. Feeding and Digestion of Proteins

When animals consume plants or other animals, they ingest these proteins. During digestion, enzymes break down dietary proteins into amino acids, which can then be utilized for growth and repair within the animal’s body.

7. Deamination

Deamination is the process by which excess amino acids are broken down in animals. This occurs primarily in the liver, where amino groups are removed from amino acids to produce ammonia (NH₃), which can then be converted to urea for excretion:

This process helps regulate nitrogen levels in the body.

8. Denitrification

Denitrification is the final step in the nitrogen cycle, where nitrates (NO₃⁻) are converted back into nitrogen gas (N₂) or nitrous oxide (N₂O) by denitrifying bacteria under anaerobic conditions. This process completes the nitrogen cycle by returning nitrogen to the atmosphere:

Denitrification is essential for maintaining soil health and preventing the accumulation of excess nitrates in ecosystems.