Once an individual becomes exposed to a pathogen, the body's immune system responds faster against a second exposure to the same pathogen. Why does this occur? Stimulated memory cells quickly engulf pathogens
T cells recognize the pathogen and B cells terminate the pathogen
The proper antibodies are constantly circulating in the blood
Specific white blood cells quickly produce the proper antibodies

cell regulation is conducted by special proteins.

step 1: nitrogen fixation- conversion of atmospheric no₂ into forms useful to plants by lightning, bacteria, and cyanobacteria.

step 2: nitrification-process of converting ammonia to nitrite or nitrate.

step 3: assimilation-plant roots absorb new form of nitrogen.

step 4: ammonification-conversion of organic nitrogen to ammonia by bacteria involved in decomposition.

step 5: denitrification-bacteria turns the decomposed matter back into n₂o, which is released into atmosphere; repeat cycle.