Written for this site from the class slides, with textbook page references for each statement. The chapter is on pages 690–692 of the course textbook. Image credits are on slide 12.
Water potential measures the stored energy held by a given volume of water, in kPa relative to pure water, which is set at 0 kPa. Solutes form hydrogen bonds with water molecules and lower their freedom to move, so solutions have negative water potentials.
Water moves from higher (less negative) to lower (more negative) water potential, until the two are equal.
Water potential = solute potential + pressure potential. Solute potential is zero or negative. Pressure potential is usually positive in cells with walls, as the protoplast pushes on the wall, but negative in xylem under tension.
In a hypotonic solution, water enters: solute potential becomes less negative and pressure potential rises until the cell is turgid. In a hypertonic solution, water leaves: solute potential becomes more negative and pressure potential falls to zero, and the cell may plasmolyse.
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Print this guide and complete it as you work through the slides and videos. It asks for your own explanations, so close the notes while you try each section.
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A printable worksheet covering all four statements, from quick recall to data questions and a longer answer. It has 20 marks and takes about 20 minutes, or you can do one section at a time.
Your teacher has the mark scheme.
You will need: Potato cylinders, sucrose solutions from 0 to 1.0 M, balance, table of solute potentials
Safety: Take care with cork borers and knives. Wipe up spills.
Carry this out with your teacher's supervision and follow your school's lab rules.
Water potential cannot be seen directly, but Scholander's pressure chamber of the 1960s made it possible to measure tension in plants.
Water potential is a concept defined by a calculation. Consider whether it is a real property or a useful construct.
Summarised in our own words. Use each code to find the exact wording in your textbook or the IB subject guide.
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