Question

Nearly 80% of nitrogen found in human tissues originate from ________ nitrogen fixation.

• A. physical
• B. biological
• C. industrial
• D. symbiotic

Hint:

Food chain link: Microbes fix nitrogen → plants absorb → animals & humans obtain.

Answer 1

The Correct Option is B

Concept:
Nitrogen fixation is the process of converting atmospheric nitrogen ($N_2$) into usable forms like ammonia.

Step 1: Types of nitrogen fixation.
• Physical fixation → Lightning
• Industrial fixation → Haber process
• Biological fixation → By microorganisms (e.g., Rhizobium, cyanobacteria)

Step 2: Source of nitrogen in humans.
• Humans obtain nitrogen through food (plants and animals)
• Plants depend mainly on biologically fixed nitrogen Thus, most nitrogen in human tissues originates indirectly from biological nitrogen fixation.

Step 3: Analyze options.
• (A) Physical → Minor contribution $\times$
• (B) Biological → Major contribution \checkmark
• (C) Industrial → Limited role $\times$
• (D) Symbiotic → Only a type of biological fixation $\times$

Step 4: Conclusion. \[ \text{Major nitrogen source in humans = Biological nitrogen fixation} \]