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Before we begin a discussion of transport, we must consider how molecules and ions move in a liquid. This process is called diffusion.


Molecules and ions dissolved in a liquid are called solutes and are constantly moving in a random fashion. If the solute is more highly concentrated in one area of the liquid, a concentration gradient is formed. Due to random movement, the solutes will move, or diffuse, down the concentration gradient: in other words, the molecules and ions will move from an area of high concentration to an area of low concentration. This will occur until the solute is evenly distributed throughout the liquid and equilibrium is achieved. The particles will continue to move, but the concentration will not change. This is called dynamic equilibrium.
The rate of diffusion is influenced by many factors, such as:
  • The “steepness” of the concentration gradient: Steeper gradients will cause faster diffusion, and, as the gradient decreases, the diffusion slows.
  • Temperature: The higher the temperature, the faster the diffusion.
  • Size of the solute: Smaller molecules and ions move faster.
  • Presence of an electrical gradient: Ions on one side of the membrane will help attract solutes with the opposite charge, thereby increasing the steepness of the concentration gradient of the solute.
  • Presence of a pressure gradientPressure can increase the gradient of a solute. 
Transport across membranes can occur by diffusion. Certain solutes that are small, nonpolar and “lippidy,” such as O2, can diffuses freely across the membrane. All other molecules cannot cross the cell membrane by diffusion, but can cross by the other methods discussed in this chapter. However, the principles of diffusion, especially the notion of concentration gradients, are crucial to most of these other methods of transport.

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