Intro to Anatomy 4: Cell Structure and Function

Transport of Substances Across Cellular Membranes: Diffusion

When you consider the movement of substances across cellular membranes, there are three things to keep in mind. First of all, molecules are always in motion, even if they’re simply vibrating in place. Second, in a fluid such as water (well over 90% of the molecules in a typical cell are water molecules), the molecules are free to move and are not constrained to vibrate in place. Finally, movement of molecules from areas of high concentration to areas of low concentration is spontaneous.

Let’s consider that last point for a moment. Remember that the molecules in a fluid are moving randomly – in other words, in all directions. If you concentrate the molecules, they will tend to spread out again until they fill the available space uniformly.

Maybe it’s easier to understand why this occurs if we consider only two dimensions for now, instead of three. Imagine I have a container that is divided into two halves by a partition. Now imagine that I put a 10% sugar solution into the right half of the container. (This means that 10% of the solution is sugar, and 90% is water. If you prefer, you could think of it as a 90% water solution.) Now imagine that I put a 50% sugar solution (i.e. a 50% water solution) into the left half of the container. What will happen if I remove the partition that separates the solutions?

Since the solution in the left half of the container contains five times the sugar as does the solution in the right half of the container, this means that for every sugar molecule that happens to be moving from the right half of the container to the left half, there will be five sugar molecules (on average) moving from left to right. So, sugar molecules will tend to migrate from the left half of the container, where they’re more concentrated, to the right half of the container, where they’re less concentrated. This will continue until the sugar concentrations in both halves of the container are the same.

When the concentration of sugar molecules in both sides of the container is equal, the solution is said to be in equilibrium. At that point, for every sugar molecule moving from left to right, there will be one moving from right to left, and so there will be no net movement of sugar molecules, even though the individual molecules will still be moving about quite energetically.

(Incidentally, the water molecules would be moving too. The water was initially more concentrated in the right side of the container, so there would be a net movement of water from right to left, until the concentrations were equal.)

This spontaneous movement of molecules in solution from high concentration to low concentration until equilibrium is reached is called diffusion.

Diffusion across a cellular membrane.
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