What is the natural tendency of molecules to spread from an area of high concentration to an area of low concentration?

This concept is known as diffusion. Diffusion is a passive process, as it requires no energy for it to occur. In a cell, it is used as a method of passive transport since it requires no energy.

Diffusion

Explanation:

Diffusion is a process by which a substance or certain molecule moves from a region of high concentration to a region of low concentration due to the concentration gradient and continues till the concentration become uniform.

This is a passive mode of transportation.

For example - Nutrients move across the cell membrane through a lipid bilayer of a cell membrane by the process of diffusion