How Does Nitrogen Turn Into Protein? The Complete Guide

December 1, 2025 •

4 min read

Approximately 78% of Earth's atmosphere consists of nitrogen gas, yet most living organisms cannot directly use it. This inert nitrogen must first be converted into usable forms through a series of biochemical reactions, known as the nitrogen cycle, before it can be incorporated into amino acids and ultimately, protein.

Quick Summary

The conversion of inert nitrogen into protein is a multi-step biological process that relies heavily on microbes to convert atmospheric nitrogen into usable forms like nitrates and ammonium. Plants absorb these compounds to synthesize amino acids, which are the building blocks of protein, and animals obtain them by consuming plants or other animals.

Key Points

Nitrogen Fixation: Specialized bacteria convert inert atmospheric nitrogen ($N_2$) into usable ammonia ($NH_3$) or nitrates ($NO_3^-$), a process called nitrogen fixation.
Plant Assimilation: Plants absorb fixed nitrogen from the soil through their roots and use it to synthesize amino acids, the basic components of all proteins.
Animal Consumption: Animals obtain nitrogen by eating plants or other animals, and their digestive systems break down these proteins into amino acids for their own use.
Protein Synthesis: Inside the cells of organisms, ribosomes assemble amino acids into complex protein molecules, guided by the genetic code in DNA and mRNA.
The Nitrogen Cycle: The conversion is a part of the continuous, Earth-wide nitrogen cycle, which ensures that nitrogen is constantly recycled and made available to living things.
Essential Amino Acids: Animals cannot synthesize all necessary amino acids themselves and must acquire 'essential' amino acids through their diet.
Microbial Dependence: The entire process is fundamentally dependent on the enzymatic reactions carried out by various microorganisms, including nitrogen-fixing, nitrifying, and decomposing bacteria.

The journey of nitrogen into protein is a complex but elegant biological pathway, fundamental to all life on Earth. Since atmospheric nitrogen ($N_2$) is unusable by most organisms, its conversion is a multi-stage process involving a critical cast of microorganisms, plants, and animals. The entire process is known as the nitrogen cycle.

The Journey Begins: Nitrogen Fixation

The first, and arguably most important, step is nitrogen fixation, which transforms inert atmospheric nitrogen into a reactive, usable form.

Biological Nitrogen Fixation

The Key Players: Specialized bacteria and archaea, known as diazotrophs, are the primary agents for biological nitrogen fixation. Prominent examples include symbiotic bacteria like Rhizobium, which live in the root nodules of legumes (e.g., peas, beans), and free-living bacteria like Azotobacter in the soil.
The Nitrogenase Enzyme: This conversion is catalyzed by the nitrogenase enzyme complex, which breaks the powerful triple bond in the $N_2$ molecule to produce ammonia ($NH_3$). This process is extremely energy-intensive, requiring 16 ATP molecules for every $N_2$ molecule fixed.

Other Forms of Nitrogen Fixation

Atmospheric Fixation: Lightning strikes provide enough energy to break the nitrogen bond, converting atmospheric nitrogen into nitrogen oxides ($NO_x$) that dissolve in rain and enter the soil as nitrates.
Industrial Fixation: The Haber-Bosch process artificially fixes nitrogen by combining nitrogen and hydrogen under high pressure and temperature to create ammonia for use in fertilizers.

Building Blocks: Assimilation and Amino Acid Synthesis

Once nitrogen has been fixed into ammonia, it is then made available for assimilation, where organisms use it to build complex molecules.

Plant Assimilation

Plants absorb usable nitrogen—primarily as ammonium ($NH_4^+$) and nitrates ($NO_3^-$)—from the soil via their roots. These inorganic nitrogen compounds are then incorporated into organic molecules, such as amino acids, which are the fundamental units of protein. A key metabolic pathway for this is the glutamine synthetase–glutamate synthase (GS-GOGAT) cycle, which incorporates ammonium into glutamine and glutamate. From these, other amino acids are synthesized through transamination reactions.

Animal Assimilation

Animals cannot fix nitrogen or synthesize all the amino acids they require. They acquire nitrogen-containing compounds by consuming plants or other animals. During digestion, proteins are broken down into individual amino acids, which are then used by the animal to build its own proteins for growth, repair, and other metabolic functions. The nitrogen from excess amino acids is excreted, typically as urea.

The Final Step: Protein Synthesis

Protein synthesis is the final stage where nitrogen-containing amino acids are assembled into functional proteins. This complex process occurs inside cells and involves a series of steps:

Transcription: A gene's DNA sequence is copied into a messenger RNA (mRNA) molecule in the cell's nucleus.
Translation: The mRNA moves to a ribosome, where the genetic code is translated. Transfer RNA (tRNA) molecules bring the correct amino acids, based on the mRNA sequence.
Polypeptide Chain Formation: The ribosome links the amino acids together in a specific order, forming a polypeptide chain. This chain then folds into a unique three-dimensional structure to become a functional protein.

Comparison of Nitrogen Assimilation by Plants and Animals


Feature	Plant Nitrogen Assimilation	Animal Nitrogen Assimilation
Starting Material	Inorganic nitrogen compounds ($NH_4^+$ and $NO_3^-$) from soil.	Organic nitrogen compounds (proteins and amino acids) from diet.
Process Dependence	Depends on nitrogen-fixing bacteria and metabolic pathways like GS-GOGAT to create amino acids.	Depends on digestive enzymes to break down dietary protein into amino acids.
Essential Amino Acids	Can synthesize all 20 standard amino acids needed for protein production.	Requires certain amino acids (essential amino acids) from their diet, as they cannot synthesize them.
Energy Cost	High energy cost for nitrogen fixation and reduction of nitrates to ammonium.	Energy is primarily required for digestion and the synthesis of new proteins.
Protein Synthesis Location	Occurs within the plant's cells, particularly in organelles like chloroplasts.	Occurs in the cells of the animal's body, using ribosomes to assemble amino acids.

Conclusion

From the inert nitrogen gas that makes up most of our atmosphere, a cascade of microbial, plant, and animal-based biochemical reactions builds the complex proteins essential for all life. Nitrogen fixation by bacteria provides the initial entry point, transforming atmospheric nitrogen into a usable form. Plants assimilate this nitrogen to produce amino acids, which are then transferred up the food chain. This intricate recycling process, the nitrogen cycle, ensures that this vital element is continuously converted and reused, supporting the growth and function of every living organism. Without the diligent work of nitrogen-fixing bacteria, life would be unable to access and utilize this abundant but otherwise inaccessible element.

The Role of Microorganisms in the Nitrogen Cycle

The conversion of nitrogen to protein would be impossible without the intricate work of various microorganisms throughout the nitrogen cycle. The steps below illustrate their indispensable contributions:

Nitrogen-fixing bacteria (e.g., Rhizobia) convert atmospheric nitrogen into ammonia.
Nitrifying bacteria (e.g., Nitrosomonas, Nitrobacter) oxidize ammonia into nitrates, which plants can absorb.
Denitrifying bacteria (e.g., Pseudomonas) reduce nitrates back into atmospheric nitrogen, completing the cycle.
Decomposers (e.g., bacteria, fungi) break down dead organic matter and waste, releasing ammonium back into the soil in a process called ammonification.
Plants incorporate the soil's nitrates and ammonium into amino acids and proteins through their roots.
Animals consume plants, digesting their proteins into amino acids for their own use.

Frequently Asked Questions

Nitrogen fixation is the process of converting inert atmospheric nitrogen ($N_2$) into biologically available nitrogen compounds, such as ammonia. It is crucial because most organisms, including plants, cannot use atmospheric nitrogen directly, and this process makes it accessible for creating essential molecules like proteins.

Plants can synthesize all 20 standard amino acids from inorganic nitrogen compounds they absorb from the soil. Animals, however, must obtain certain 'essential' amino acids by consuming plants or other animals, as they lack the metabolic pathways to produce them themselves.

Bacteria are central to this process. Nitrogen-fixing bacteria convert atmospheric nitrogen into ammonia, nitrifying bacteria convert ammonia to nitrates, and decomposing bacteria break down organic matter to release ammonium. These usable nitrogen compounds are then taken up by plants and enter the food web.

After plants use nitrogen to synthesize amino acids, these amino acids are used as building blocks for the plant's own proteins. When an animal eats the plant, it digests the plant's proteins into amino acids to form its own proteins.

No, humans cannot use atmospheric nitrogen directly. We must consume nitrogen in the form of pre-made proteins and amino acids found in our food, which ultimately derive their nitrogen from the larger nitrogen cycle involving bacteria and plants.

The GS-GOGAT (glutamine synthetase–glutamate synthase) cycle is a key metabolic pathway used by plants and microorganisms to incorporate ammonium into organic compounds. It is a critical step in turning inorganic nitrogen into amino acids.

Nitrogen moves through the ecosystem via the nitrogen cycle. After organisms die, decomposers release nitrogen back into the soil as ammonium. This nitrogen can then be re-assimilated by plants, converted back to atmospheric gas by denitrifying bacteria, or otherwise recycled.