Answer the following question given this paragraph: 

Small molecules can pass through the plasma membrane through a process called diffusion. Diffusion is the movement of molecules from an area where there is a higher concentration (larger amount) of the substance to an area where there is a lower concentration (lower amount) of the substance ( Figure below ). The amount of a substance in relation to the total volume is the concentration . During diffusion, molecules are said to flow down their concentration gradient , flowing from an area of high concentration to an area of low concentration. Molecules flowing down a concentration gradient is a natural process and does not require energy.


Q: What is the process of small molecules passing through the plasma membrane called?


A:
diffusion