Diffusion is the tendency of molecules to spread into an available space. This tendency is a result of the intrinsic thermal energy (heat) found in all molecules at temperatures above absolute zero. Without other outside forces at work, substances will move/diffuse from a more concentrated environment to a less concentrated environment. No work is performed for this to happen, as diffusion is a spontaneous process.

Passive transport is the diffusion of substances across a membrane. As we stated above, this is a spontaneous process and cellular energy is not expended. Molecules will move from where the substance is more concentrated to where it is less concentrated.

As illustrated in the image above: "This cartoon illustrates passive diffusion. The dashed line is intended to indicate a membrane that is permeable to the molecules or ions illustrated as red dots. Initially all of the red dots are within the membrane. As time passes, there is net diffusion of the red dots out of the membrane, following their concentration gradient. When the concentration of red dots is the same inside and outside of the membrane the net diffusion ceases. However, the red dots still diffuse into and out of the membrane, but the rates of the inward and outward diffusion are the same resulting in a net diffusion of O."- Steven Berg

Although the process is spontaneous, the rate of diffusion for different substances is affected by membrane permeability. Since membranes are selectively permeable (only some substances can pass), different molecules will have different rates of diffusion. For instance, water diffuses freely across membranes, an obvious benefit for cells since water is crucial to many cellular processes. Some molecules however must be helped across the cell membrane through a process called facilitated diffusion.