If you have ever watched steam rise from a storm drain and disappear from view as it rises into the air owing to its own warmth, you have seen the diffusion of gas molecules in action.

When you spray air freshener into a room, and the scent in the area where you sprayed becomes gradually weaker, this is the result of different gas molecules invariably finding their way toward places in the local atmosphere that fewer of their "peers" have already reached.

Diffusion is one process by which molecules move through space. Sometimes this space is air, sometimes it is liquid, and at still other times it is localized to the area of a biological cell membrane. Were it not for various forms of diffusion, in fact, the cells of your body would be unable to do their jobs and would quickly suffocate and starve.

Diffusion is defined as the passive movement of a solute (such as a carbon dioxide molecule) across a permeable membrane. The word "passive" does a lot of work in this sentence; it means that no energy needs to be put into the system to cajole the solute across the membrane to the other side.

What is a permeable membrane? This is a name for a barrier (usually biological) that allows molecules to pass through under certain conditions. With diffusion, the energy is supplied by the concentration gradient. This is because a substance tends to move in any direction it can until the substance is evenly distributed throughout whatever space confines it and its molecular cohorts.

The rate of diffusion of a substance is influenced by a host of factors. Note that diffusion naturally continues until equilibrium is reached, and the substance is evenly distributed throughout its medium. Also, be aware that in a mixture of substances, each one has its own concentration gradient that is unaffected by other gradients in its midst (though the presence of these different molecules does affect their individual movements through sheer crowding).

Strength of the concentration gradient: As you might expect, the bigger the difference in concentration across a plasma membrane, the more rapidly the solute will diffuse across it. As equilibrium is approached, the diffusion rate slows.

Mass of the molecules: Lighter molecules, such as CH₄(methane), move more rapidly on average than do more massive ones, such as long segments of nucleic acids (e.g., DNA).

Area and thickness of the membrane: As the area of the membrane increases, so does the diffusion rate. But increasing thickness slows diffusion. Think of the effect on highway vehicular flow of adding more tollways to a highway without increasing the traffic (increased "area"); then consider the effect of needlessly making each narrow toll lane a half-mile long (increased "thickness").

Temperature: Molecules, like practically everything else you can think of, tend to diffuse faster as temperature increases, as this increases the random collision between molecules and increases the rate of diffusion.

Solute polarity: Non-polar or lipid-soluble materials more easily pass through plasma membranes than do polar materials, i.e., materials that have asymmetrical charge distributions across molecules with no net electrical charge.

Density of the solvent: As the density of the fluid in which diffusion occurs increases, diffusion slows. This is one reason dehydration causes problems; a thicker cell cytoplasm (cell interior) makes it harder for vital molecules to chug passively toward their enzymatic and other destinations.

Graham's law: When a gas is dissolved in a liquid, the relative rate of diffusion of a given gas is directly proportional to its solubility in that liquid, but inversely proportional to the square root of its molar mass. In the blood plasma human body, carbon dioxide is slightly heavier than oxygen gas, but its solubility is 22 times greater, giving it 19 times the diffusion rate of oxygen in this setting.

Distance of solute path: Again as you might guess, shorter paths of travel imply faster rates of molecular diffusion.