Simple diffusion is a process of diffusion that occurs without the aid of an integral membrane protein. This type of diffusion occurs without any energy, and it allows substances to pass through cell membranes.
Simple diffusion can only diffuse a small category of substances. For a compound to be able to pass through a cell membrane via simple diffusion, it must be able to penetrate the cell's phospholipid bilayer. Since the core of the phospholipid bilayer is hydrophobic, compounds must be hydrophobic themselves, and they must also be small. Common molecules that utilize simple diffusion include carbon dioxide and oxygen.