What is a diffusion through plasma membrane?

Diffusion is the movement of particles from a high concentration area to a low concentration area. Things that can go through a membrane are ions (charged), small polar molecules. Big molecules can't go through the membrane due to its large size and disrupting the membrane. Passive transport is the movement of substances acrpss the cell membrane w/o the use of energy. Active transport needs energy to move substances across a cell membrane.