Given a scientific question, generate a correct answer to it.

[EX Q]: The nitrogen that enters living systems by nitrogen fixation is successively converted from organic nitrogen back into nitrogen gas by what?
[EX A]: bacteria

[EX Q]: What is the name of the organelle that helps make and transport proteins and lipids?
[EX A]: endoplasmic reticulum

[EX Q]: What hydrocarbons contain only single bonds between carbon atoms?
[EX A]:
saturated hydrocarbons