Passive cell transport processes such as simple diffusion, facilitated diffusion and osmosis do not expend any energy. These processes do not require energy because they involve substances that automatically move from areas of high particle concentration to low particle concentration.
Simple diffusion involves the moving of small molecules such as carbon dioxide and oxygen from one highly concentrated side of a cell membrane to the other, less concentrated side. Diffusion that occurs with the help of channel proteins is called facilitated transport. In facilitated transport, channel proteins create holes that allow substances to pass through a membrane to less concentrated areas. Osmosis refers to diffusion that features water as the primary moving substance.