instruction:
Given a scientific question, generate a correct answer to it.
question:
What is the term for a substance that increases the rate of a chemical reaction but is not changed or used up in the reaction?
answer:
a catalyst

question:
What is the term for a loop in which a product feeds back to control its own production?
answer:
feedback mechanism

question:
Regulated by guard cells, stomata allow what to enter and exit the plant?
answer:
gases