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MCAT Question A Day - 7/21/13 - Answer!

Certain bacteria in the soil and in the root nodules of plants called legumes are unique in that they can convert nitrogen gas in the air into ammonia. Without nitrogen fixation, the first step in the nitrogen cycle, other living organisms would be unable to synthesize

A.  proteins.
B.  nucleic acids.
C.  ATP.
D.  All of the above

The correct answer is (D). By converting nitrogen gas from the air into a form that can eventually be utilized by plants (ammonia must later be converted to nitrates by nitrifying bacteria), nitrogen-fixing bacteria provide a means for all living things to synthesize nitrogen-containing molecules. These include proteins (which contain amino acids), nucleic acids (which contain the nitrogen bases adenine, guanine, cytosine, thymine, or uracil), and ATP (which contains adenine).