Diffusion, in biochemistry, refers to one of many processes by which molecules can move into and out of cells through the plasma membrane, or cross membranes within the cell, such as the nuclear membrane or the membrane that encloses mitochondria.
Think of diffusion as a "drifting" movement. While it refers to a random and unguided process, and one that does not require an input of energy, it does follow one rule: Particles move from areas of higher concentration to areas of lower concentration, even while individual molecules are free to move in all directions.
Understanding Chemical Gradients
What does it mean for something to move from a region of high concentration to one of low concentration? First, it is necessary to know what "concentration" means in this context. Most of the time, concentration refers to the number of molecules per unit volume (e.g., milliliters, or ml).
Think of what happens when you take a drink of orange juice from the bottle or carton. Chances are that you perceive the drink as sweet, because the high concentration of sugar in the juice exceeds that of the fluids in your system.
However, if you mix the juice with plain water so that the resulting solution contains 10 parts water for every 1 part juice, wait a few minutes, and take another sip, you will perceive the fluid as dilute, because it is now at lower concentration – less concentrated, at any rate, than your body fluids.
Because the molecules of sugar in the juice tend to mix with the water molecules until the concentration of sugar is equal throughout the solution, it is said that diffusion occurs in the direction of equilibrium.
Importantly, equilibrium does not mean a cessation of molecule movement, but rather that the movement of the molecules has reached a point of true randomness because all concentration gradients have been eliminated.
The Process of Diffusion
While some substances can simply diffuse across cell membranes when the concentration gradient favors this, others are too large to make it between the phospholipid molecules in the membrane, or they carry a net electric charge that opposes their movement.
The plasma membrane is thus a semipermeable membrane: Small, uncharged molecules such as water (H2O) and carbon dioxide (CO2) can simply meander on through, whereas others require help or are unable to cross the membrane outright.
Simple diffusion is exactly what it sounds like – the movement of molecules across a membrane down a concentration gradient as if the membrane were, in effect, not there. In facilitated diffusion, however, substances such as ions (charged particles) move down a concentration gradient, but they also must cross the membrane through specialized transport channels made of protein.
Diffusion tends to proceed until equilibrium concentration is reached. At this point, molecules tend to leave the region only by active transport mechanisms powered by ATP, or adenosine triphosphate – the "energy currency" of cells.
Pros and Cons of Diffusion
On the plus side, the diffusion process is "free" compared to other forms of transport in that it does not require energy. This is a major asset given that efficiency is enormously desirable in biological systems and energy, just as in the "macro" world, is at a premium.
The down side of diffusion is that is it obviously insufficient to move substances up a concentration gradient, and it is not difficult to envision a scenario in which molecules are needed inside a cell despite an already higher concentration of these substances on the inside than on the outside. More often, such substances must be moved across an electrochemical gradient.
This is a different physical form of resistance, but it's one that only an investment of ATP can overcome. This is done using membrane "pumps" that continually fight the tide of the electrochemical gradient that opposes their work.