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

Ex Input:
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?

Ex Output:
a catalyst

Ex Input:
What is at the surface of a cell, arranged in similar bilayer?

Ex Output:
phospholipids

Ex Input:
Which theory describes the distribution of electrons in molecules in much the same way that the distribution of electrons in atoms is described using atomic orbitals?

Ex Output:
molecular orbital theory