The Negative Impact of Solutes on Water Potential [...]

Chorus Title: What is solute potential?

Response Title: It’s the Negative Impact of Solutes on Water Potential

I know, fascinating right? But there are a couple of things to note here.

Other explanations of solute potential often mention that “osmosis wants to equalize the saltiness” on two sides of a membrane. But when we say “want” in terms of physical processes, we are talking about physical mechanisms, not desires.

So let’s start with an observed phenomenon:

Here’s a beaker with a semi-permeable membrane. In this first picture, there are two equally full sides of the beaker, but one has saltier water on one side than the other. (We use salt here, but it could be other solutes as well).

Now what happens? Seriously, think about it for a minute. You leave this glass overnight, go to sleep, and come back in the morning: what do you see?

If you’re like most people, you probably think you see a beaker that looks just like the one above, but with the saltiness “averaged out”.

But if you thought that, you’d be wrong. Because here is what you’d find on your counter:

You’ll notice that the saltiness on each side of the membrane is equal, but this has been achieved by moving water, not salt molecules.

The reason for this is simple. For this semi-permeable membrane, water molecules can cross it, but the solute particles, which are larger and polar, cannot. As water molecules pass freely between the two sides, some of them bind to the solute, and are unable to pass back through the membrane. See this animation to see how that happens:

First, pure water (no solutes) has a solute potential (Ψπ) of zero. Solutes reduce water’s potential, limiting the ability of the solution to flow through a membrane.

Solute concentration relates to solute potential according to the given by the Van’t Hoff Equation:

Ψπ = − miRT

where m is the concentration in molarity of the solute, i is the Van’t Hoff factor, the ionization constant of the solute (1 for glucose, 2 for NaCl, etc.) R is the ideal gas constant, and T is the temperature.

The more solute molecules present in the , the more negative the solute potential is.

Solute potential has important implication for many living organisms. If a living cell with a lower solute concentration is surrounded by a concentrated solution, the cell will tend to lose water to the more negative water potential of the surrounding environment.

You can demonstrate this process in your kitchen using common eggs dropped in a hypertonic solution such as corn syrup:

Credits

Pieces of this explanation come from Wikipedia.