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

Example Input: The nitrogen that enters living systems by nitrogen fixation is successively converted from organic nitrogen back into nitrogen gas by what?
Example Output: bacteria

Example Input: Which process helps absorptive protists absorb food molecules across their cell membranes?
Example Output: diffusion

Example Input: What is the small, dense region at the center of the atom that consists of positive protons and neutral neutrons?
Example Output:
nucleus