Question

why is nitrogen fixation necessary?

a. photosynthesis requires excess nitrogen.

b. respiration uses nitrogen that is attached to oxygen.

c. plant life cannot use nitrogen atoms unless they are fixed into pairs.

d. most plants use single nitrogen atoms, not n₂ molecules.

Explanation:

Nitrogen fixation is the process of converting atmospheric nitrogen (\(N_2\)) into forms that plants can use. Plants cannot directly use \(N_2\) molecules. They need nitrogen in a more reactive form (such as ammonia \(NH_3\) or nitrate \(NO_3^-\)). Option A is incorrect because photosynthesis mainly requires carbon dioxide (\(CO_2\)), water (\(H_2O\)), and sunlight. Option B is wrong as respiration primarily involves the use of oxygen (\(O_2\)) and the release of carbon dioxide (\(CO_2\)). Option C is incorrect because it's not about fixing into pairs. Nitrogen fixation breaks the strong triple - bond in \(N_2\) to make nitrogen available for plants.

Answer:

D. Most plants use single nitrogen atoms, not \(N_2\) molecules.