MATH: Water Potential

Why does water move from areas where solutes are less concentrated to areas where they are more concentrated?

This is actually a complicated question. To answer it, let’s take a step back and refresh our memory on why diffusion happens. In diffusion, molecules move from a region of higher concentration to one of lower concentration—not because they’re aware of their surroundings, but simply as a result of probabilities. When a substance is in gas or liquid form, its molecules will be in constant, random motion, bouncing or sliding around one another. If there are lots of molecules of a substance in compartment A and no molecules of that substance in compartment B, it’s very unlikely that a molecule will randomly move from B to A. On the other hand, it’s extremely likely that a molecule will move from A to B. You can picture all of those molecules bouncing around in compartment A and some of them making the leap over to compartment B. So, the net movement of molecules will be from A to B, and this will be the case until the concentrations become equal.

In the case of osmosis, you can once again think of molecules (this time, water molecules) in two compartments separated by a membrane. If neither compartment contains any solute, the water molecules will be equally likely to move in either direction between the compartments. But if we add solute to one compartment, it will affect the likelihood of water molecules moving out of that compartment and into the other—specifically, it will reduce this likelihood. The most generally accepted reason for this is that there is less water bound to the solute on the side with less solute, so more water is free to move across.

Water Potential

Water potential (ψ) is a mathematical description of the tendency for water to move from one area to another. Overall water potential (ψ) is a combination of the contributions of the solute potential (ψ_s) and the pressure potential (ψ_p) of the system. Here is the equation set given to you on the AP Biology equation sheet:

Water Potential ( ψ )

ψ = ψ p + ψ s

ψ p = pressure potential

ψ s = solute potential

First, watch the Water Potential video below about how to use these equations to predict water movement.

Click through the Water Potential Presentation for calculations and some good examples of using water potential to predict which way the water moves. Remember, water moves from a higher value to a lower value (always to the more negative water potential).

Now, work through these practice problems below.

Complete the learning object on Water Potential Practice below.

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