Over the past 200 years the level of carbon dioxide in the atmosphere has
a. decreased slightly
b. increased until recently, then decreased
c. increased steadily
d. stayed aboutbthe same

the answer is d p: cell, q: organ, r: organ system, s: tissue

the nitrogen cycle provides nitrogen to the ecosystem from the atmosphere, ground and oceans. nitrogen is an important component of complex molecules such as amino acids and nucleotides, which lead to the creation of proteins and dna, the building blocks of all life. plants absorb nitrogen in the form of ammonium, nitrate ion and, on rare occasions, as amino acids. animals receive nitrogen necessary for biological processes from feeding on living or dead organic matter.