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. Raise the total with pressure, lower it with solute; the total decides.
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.
2 · Watch
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Where it sits
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.