Water Potential and Osmosis Quantified
Water potential puts a number on the tendency of water to move, made from solute potential and pressure potential. Cells are then predicted to gain or lose water from the numbers alone.
What a learner can do afterwards
- Adds solute potential and pressure potential to get the water potential of a cell.
- Predicts the direction of net water movement between two solutions from their water potentials.
- Explains why pure water sits at zero and every solution sits below it.
1 · Read
Water potential is a single number for how much water wants to move. It is built by adding pieces: solute potential, which dissolved stuff drags down, and pressure potential, which squeezing raises and pulling lowers, plus smaller effects from gravity and clinging to surfaces. Pure water with nothing dissolved and no pressure sits at exactly zero, the reference everything is measured against. Water always moves from higher potential to lower potential.
Take a plant with pressure potential 0.21 and solute potential minus 3.50. Add the two numbers to get its total water potential: minus 3.29. Right after rain the wet soil sits near zero, which is higher, so water flows from soil into plant. After months of drought the dry soil drops below the plant, the flow reverses into the soil, and the leaf pores called stomata close to cut evaporation loss.
A U-shaped tube shows the same adding logic. Push the left side with pressure and its water potential rises, so water is pushed right and the right level climbs. Now add solute to the left instead: the solute potential there falls and drags the total back down. Add just enough to cancel the pressure boost and both sides match, so movement stops.
Use the number, not molecule pictures, to predict osmosis. When two solutions sit across a membrane only water can cross, water moves toward the side with more dissolved solute. What matters is the count of dissolved particles, not their size: more particles means lower water potential on that side. So compare the two totals and point the arrow from higher to lower every time.
Add solute and pressure potentials, then move water from higher to lower.
More to read
A Water Potential Word Problem
A plant has a pressure potential of Yp = 0.21 MPa and a solute potential of Ys = -3.50 MPa. Add those two numbers together and you get the plant's total water potential.
Right after it rains, the soil is full of water, so the soil's water potential is close to zero, about as high as it gets. Water moves from high water potential to low water potential, so water flows from the wet soil into the plant.
Now fast forward three months with no rain. The soil dries out, which drags its water potential way down, lower than it was right after the rain. Once the soil's water potential drops below the plant's, the flow can reverse. Water starts leaving the plant and moving back into the dry soil instead of the other way around.
That's also why a plant's stomata, the tiny pores on its leaves, tend to close up during a drought. Closing them cuts down on water loss through evaporation, which helps the plant hang on to whatever water potential it has left.
What Water Potential Is Made Of
Water potential is a single number that tells you how much water wants to move. It's built from a few pieces added together: solute potential (how much dissolved stuff lowers the number) and pressure potential (how much squeezing or pulling raises or lowers it), plus smaller effects from gravity and how tightly water clings to surfaces.
Pure water, with nothing dissolved in it and no pressure on it, gets a water potential of exactly zero. That's the reference point everything else gets measured against.
Add solutes to water and the water potential drops below zero, because dissolved particles make water molecules want to stay put more. Push on water with pressure and you raise its water potential back up; pull on it, like the tension inside a plant's stem, and you lower it further.
Water always moves from a spot with higher water potential to a spot with lower water potential, whether that's soil to root, root to stem, or stem to leaf. Plants use this one rule to move water all the way from their roots to the top of their leaves without any pump at all.
Balancing Pressure and Solute in a Tube
Picture a U-shaped tube with a membrane in the middle that only water can cross. If you raise the pressure potential (Yp) on the left side, the water potential on the left goes up, so water gets pushed over to the right, and the right side's water level rises.
Could you bring the two water levels back to equal by adding solute instead? Yes. Adding solute to the left side lowers the solute potential there, which drags the total water potential on the left back down. Add just enough solute to cancel out the boost from the pressure, and the water potential on both sides matches again, so the water stops moving.
Why More Solute Means More Pull
When two solutions sit on either side of a membrane that only water can pass through, water moves toward whichever side has more dissolved solute in it. That movement is osmosis: water crossing a semipermeable membrane to try to even out the concentration on both sides.
What matters is the number of dissolved particles, not how big those particles are. More particles means a lower water potential on that side, so water gets pulled toward it.
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Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.