Simple diffusion (simple diffusion) refers to the transmembrane diffusion of substances from the side of the membrane with a high concentration through the gaps between lipid molecules to the side with a low concentration. This is a physical phenomenon that does not involve biological transport mechanisms, requires no metabolic energy, and is considered passive transport, also known as simple diffusion. Substances transported by simple diffusion are all lipid-soluble (non-polar) substances or a few uncharged polar small molecules, such as O2, CO2, N2, steroid hormones, ethanol, urea, glycerol, water, etc.
