Some pituitary hormones cause the ovaries to secrete hormones. these ovarian hormones then influence the production of the pituitary hormones. this type of control mechanism is known as
1) a menstrual cycle
2) ovulation
3) gametogenesis
4) negative feedback

the wires in the toaster turning red and the toast crisping and turning brown

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.

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.