Diffusion and Concentration Gradients

The direction of the passage of particles through the channel is also dependent on concentration gradients. A concentration gradient exists whenever a concentrated solution is in contact with a less concentrated solution. Because the solutions are in contact, particles may flow between the two solutions (or between two regions of the same solution) by the process known as diffusion. Diffusion is a term used to describe the mixing of two different substances that are placed in contact. The substances may be gases, liquids, or solids. Diffusion is the migrating by random motion of these different particles. Although particles move in every direction, there is a net flow from the more concentrated solution to the less concentrated solution ("down the concentration gradient"). As the number of particles in the more concentrated solution diminishes and the number of particles in the less concentrated increases, the difference in concentration between the two solutions decreases. Hence, the concentration gradient is said to get smaller (Movie 1). All else being equal, the concentrations of the solutions change more rapidly when the difference in their concentrations is greater. This diffusion process continues until the concentrations of the two solutions are equal. This state is known as dynamic equilibrium. When the two solutions are in dynamic equilibrium, particles continue to move between the two solutions, but there is no net flow in any one direction, i.e., the concentrations do not change.

equillibrium

Figure 6

The graph at the top of this figure plots the time course of the changes in concentration that occur after a solution (A) with a 1.0 M concentration of some particle is placed in contact (via a semipermeable membrane) with another solution with a 0.0 M concentration of the particle. The blue line represents the concentration of the particle in solution A, and the magenta line represents the concentration of the particle in solution B. Over time, the concentrations become equal and no longer change; at this point, the solutions are said to be in dynamic equilibrium.

The schematic at the bottom shows the two solutions approximately 2 seconds after the solutions are placed in contact with one another.

To view a QuickTime movie showing the movement of the particles by diffusion between these two solutions, please click on the pink button below. Click the blue button below to download QuickTime 6.5 to view the movie.

In biological systems such as the kidney, the two solutions are often separated by a membrane. Protein channels in the membrane allow particles to cross the membrane, flowing "down the concentration gradient" until equilibrium is reached. Sometimes these channels may be closed, so that particles will not travel across the membrane, even if there is a strong concentration gradient. (In effect, the two solutions are no longer in contact when the channels are closed.) In other cases, the proteins in the membranes act like "pumps," using energy to move particles "against the concentration gradient" (i.e., so the more concentrated solution becomes even more concentrated); examples are the light-driven proton pump that occurs in the photosynthetic thylakoid membrane discussed in the introduction to the Experiment, the proton pumps used in the synthesis of ATP, the body's energy currency (which you will encounter in the tutorial entitled "Energy for the Body: Oxidative Phosphorylation"), and the sodium pumps discussed below.