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REPEATER 2024 — BIOLOGY STD 12 Science — Question

Maharashtra BoardEnglish MediumSTD 12 ScienceBIOLOGYREPEATER 20243 Marks
Question
Explain the role of diazotrophs in nitrification.
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Answer

$(1)$ Diazotrophs are microorganisms that have the ability to fix atmospheric nitrogen gas into a usable form, such as ammonia.
$(2)$ Diazotrophs are either symbiotic or free living. The cyanobacteria fix significant amount of nitrogen in specialized cells called heterocysts.
$(3)$ The best known nitrogen fixing symbiotic bacterium is Rhizobium.
$(4)$ Diazotrophs, while essential for nitrogen fixation, do not directly participate in nitrification.
$(5)$ Diazotrophs convert atmospheric nitrogen $(N_2)$ into ammonia $(NH_3).$
$(6)$ This process, called nitrogen fixation, requires $16 ATP$ molecules per nitrogen molecule.
$(7)$ It is an endothermic reaction.
$N _2+8 H ^{+}+8 e^{-}+16 A T P \longrightarrow 2 NH _3+ H _2+16 ADP +16 Pi$
$(8)$ Then nitrifying bacteria which are responsible for nitrification, converts ammonia into nitrate, the form of nitrogen which can be used by plants and animals.
$(9)$ Nitrification involves two steps:
$(i)$ Nitrosation: Ammonia is converted into nitrite $(NO^{2-})$ by bacteria like Nitrosomonas
and Nitrosococcus.
$(ii)$ Nitrate oxidation: Nitrite is converted into nitrate by bacteria like Nitrobacter.

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