what is simple diffusion

Simple diffusion is the process by which molecules, atoms, or ions diffuse through a semipermeable membrane down their concentration gradient without the assistance of transporter proteins. In essence, it is the movement of particles from an area of high concentration to an area of low concentration. Simple diffusion is an unaided form of diffusion that does not require the assistance of membrane proteins. It is a type of passive transport where molecules diffuse relatively easily and without the need for ATP expenditure. Simple diffusion is exemplified by the simple transport of molecules at the cellular level. For example, small nonpolar molecules like oxygen or carbon dioxide can diffuse down their concentration gradient by passing directly through the plasma membrane. Simple diffusion is different from facilitated diffusion, which requires the assistance of membrane proteins.