The driving force for passive transport is simply the process of diffusion. Though this may be masked a bit by movement of molecules through pores or via carrier molecules, the end result of passive transport is always an equal concentration of the transported molecule on both sides of the membrane.

Equation 10.4 defines the net rate of transport, J in terms of membrane thickness (l), the diffusion coefficient (D1), the partition coefficient (K), and the concentration difference (C2 - C1) of the compound across the membrane. This is simplified to J = -P(C2 - C1), where P is the permeability coefficient (Table 10.6).

The slow process of diffusion is insufficient to transport many needed molecules across cellular membranes, so cells have evolved a variety of mechanisms for speeding up diffusion. This process, called facilitated transport, includes pore-facilitated transport and carrier-facilitated transport. It is important to note, however, that though pores or carriers speed the diffusion process, the driving force for each process is still diffusion, with all of its built-in limitations.

Active transport, on the other hand, couples transport of compounds across the membrane to energetically favorable processes, such as hydrolysis of ATP. Because of the additional energy provided by the coupled process, active transport systems can "pump" molecules against a concentration gradient. Thus, with active transport provided by the sodium-potassium pump, cells can maintain a higher concentration of potassium ions inside of the cell than outside and a higher concentration of sodium ions outside than inside.

In the sodium-glucose cotransport system (Figure 10.27), energy for the transport is provided by the high sodium ion concentration outside the cell compared to inside. This might seem to be a passive transport process, because diffusion of sodium ions into the cell carries glucose with it. It is not, however, because the sodium ions are pumped back out as they enter the cell, so the sodium ion concentration never comes to equilibrium inside and outside the cell. The sodium-glucose cotransport system is thus an active transport system that derives its energy from another active transport system-the sodium ion gradient maintained by the sodium-potassium pump (Figure 10.26).