Question

which process allows small molecules to pass directly through the cell membrane without using energy?
a diffusion
b facilitated diffusion
c active transport

Explanation:

Diffusion is the passive movement of small molecules from an area of higher concentration to an area of lower concentration directly through the cell - membrane without energy input. Facilitated diffusion requires a transport protein. Active transport uses energy to move molecules against their concentration gradient.

Answer:

A. diffusion