JAMB Biology 2023 Paper

Concept:Nitrogen fixation is the process that converts unusable atmospheric nitrogen into a form plants can absorb.Explanation:Plants cannot directly use atmospheric nitrogen gas ($\mathrm{N}_2$).Nitrogen-fixing bacteria convert $\mathrm{N}_2$ into ammonia ($\mathrm{NH}_3$) or ammonium ions ($\mathrm{NH}_4^+$).These bacteria may live freely in the soil or inside root nodules of plants like legumes.The fixed nitrogen can then be taken up by plant roots to build proteins and other vital compounds.This process is different from denitrification, which releases nitrogen back into the air.Nitrification involves converting ammonia into nitrates, while ammonification releases ammonia from dead organic matter.Therefore, the nutrient cycle step that makes atmospheric nitrogen available to plants is nitrogen fixation.Answer:B. Nitrogen fixation