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

[Q]: Overpopulation takes place when the number of organisms in an area exceeds what?
[A]: carrying capacity

[Q]: The four major classes of biological macromolecules include carbohydrates, lipids, proteins, and what else?
[A]: nucleic acids

[Q]: What is the plasma membrane formed by?
[A]:
a phospholipid bilayer