Passive transport mechanisms use no external energy source to bring about diffusion of a substance across a membrane. The most common passive transport systems are diffusion and facilitated transport.

Diffusion - Diffusion happens, and there is very little cells can do about it. Table 10.6 lists permeability coefficients for selected ions and molecules through membranes. Because the driving force for diffusion is a concentration gradient, active transport pumps, such as the sodium-potassium pump, create gradients of these two ions that are continually (though slowly) degraded by diffusion. Note in Table 10.6 that sodium and potassium ions do not have facilitated transport systems, so their permeability constants are very low.

Facilitated transport (or facilitated diffusion) - Includes pore-facilitated transport and carrier-facilitated transport systems. One notable feature of facilitated transport systems is that even though the driving force is also the process of diffusion and the end result is the same as diffusion, facilitated transport systems speed up diffusion by a factor of up to 10,000,000-fold.

Pore-facilitated transport - Band 3 protein of the erythrocyte is an example of a pore-facilitated transport system (Figure 10.20a). It contains a highly specific channel to transport bicarbonate ions out of cells as it transports chloride ions in. Note that the net charge difference in the transport is zero, so there is no electrical polarization of the membrane. Another kind of pore is gramicidin A, which is a simple 15-residue polypeptide that allows potassium and sodium ions to pass through it (Figure 10.22). Still another pore-facilitated system is that of the glucose transport protein of erythrocytes which strongly favors transport of D-glucose over other sugars.

Carrier-facilitated transport - Valinomycin, an antibiotic, is an example of a carrier-facilitated transport system. It contains a hydrophobic exterior for interacting with the hydrophobic portion of the membrane's lipid bilayer and an interior designed specifically to accommodate a potassium ion. It transports by the mechanism depicted in Figure 10.20b.

The different types of diffusion (facilitated versus simple diffusion) can be distinguished because facilitated systems have only a fixed number of sites through which transport occurs, but simple diffusion occurs across the entire surface of the cell. Thus, facilitated transport has a rate which is limited by the number of sites. Researchers attempting to increase the rate by increasing the concentration of transported molecule find that facilitated systems reach a maximum rate than cannot be substantially increased with increasing concentration of molecule. Facilitated diffusion systems are therefore saturable. Simple diffusion systems do not behave in this way. They slowly increase transport simply as the concentration of transported molecule increases.

An ionophore is a system that transports ions. If the result of the transfer is a change in charge, the process is called electrogenic; if there is no charge difference, it is called electroneutral.