3 - Diffusion and directed transport

Summary

The movement of material within the cell and across membranes always requires a driving force. For diffusive processes, the driving force is the electrochemical gradient. The electrical component of this force requires a separation of charge: the negative and positive charges must be kept from one another until a conductive channel opens and the charged species can flow down their electrical gradient. Intact membranes, whether the cell membrane or those of organelles, are needed to provide the voltage buildup that will allow current to flow when conduction becomes possible. Within the cytoplasm or in the extracellular fluid, the charges are not kept separate, and without a voltage there will be no electrical gradient. In these cases, diffusion is entirely driven by concentration gradients. The generation of these gradients is an active process: functions linked to ATP hydrolysis are ultimately responsible for all diffusion gradients. Once the gradients are generated, however, they will produce the movement of material without further ATP input. Across the membrane, of course, there can be both concentration and electrical gradients for charged moieties. In some cases, like Na⁺ across the resting cell membrane, these gradients will be in the same direction, inward in this case, with a higher Na⁺ concentration on the outside and a negative charge on the inside attracting the positive sodium. In others, like K⁺ across the resting cell membrane, the electrical and concentration gradients are in opposite directions, with the higher K⁺ concentration on the inside driving K⁺ out countered by the negative charge on the inside drawing K⁺ in. If diffusion down a gradient does not require ATP directly, other intracellular transport processes do require ATP. The movement of vesicles, organelles, and other cargo by kinesin, dynein and non-polymerized forms of myosin requires the direct hydrolysis of ATP to power each step. Transport within the cell therefore comes in two forms: diffusion allows material to move in all directions, while ATP-driven processes move material in particular directions. In this chapter, both of these mechanisms, and the principles behind them, will be discussed.