Osmosis is a process by which molecules of a solvent pass through a semipermeable membrane from a less concentrated solution into a more concentrated one, thus balancing the concentrations on each side of the membrane. Diffusion is the net movement of molecules or atoms from an area of high concentration, which is high chemical potential to a state of low concentration, which is low chemical potential.
This state is also referred to as the movement of a substance down a concentration gradient. Active transport is the progression of ions or molecules across a cell membrane into a region of a higher concentration, assisted by enzymes and requiring energy. Passive transport is the transport of biochemicals and other atomic or molecular substances across cell membranes without the need for energy input.