Written for this site from the class slides, with textbook page references for each statement. The chapter is on pages 681–690 of the course textbook. Image credits are on slide 14.
In solvation, water molecules surround a solute. Water is polar, so positive ions attract the partially negative oxygen, and negative ions attract the partially positive hydrogen. Polar molecules such as glucose form hydrogen bonds with water.
| Term | Solute concentration compared with the other solution |
|---|---|
| Hypotonic | Lower |
| Hypertonic | Higher |
| Isotonic | The same |
Net movement of water is from the hypotonic solution to the hypertonic solution.
Water enters a cell in a hypotonic solution, and leaves a cell in a hypertonic solution.
Watch out: in an isotonic solution, water still crosses in both directions, at equal rates. This is a dynamic equilibrium with no net movement.
Tissue in a hypotonic solution gains mass and length, and tissue in a hypertonic solution loses them. Calculate percentage change, because starting sizes differ. On a graph of percentage change against concentration, the point at zero change gives the isotonic concentration. Standard deviation and standard error show how far you can trust the means.
In a hypotonic medium, these cells swell and may burst. In a hypertonic medium, they shrink, with crenation in red blood cells. Freshwater unicellular organisms use contractile vacuoles to expel water that enters. Multicellular animals keep tissue fluid isotonic with their cells.
In a hypotonic medium, water enters and pressure builds against the wall, giving turgor pressure. The wall stops the cell from bursting. In a hypertonic medium, water leaves and the membrane pulls away from the wall, which is plasmolysis.
Intravenous fluids, such as 0.9% sodium chloride, and solutions for bathing organs before transplant are isotonic. They prevent net water movement, so cells do not swell, burst or shrink.
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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 seven statements, from quick recall to data questions and a longer answer. It has 36 marks and takes about 35 minutes, or you can do one section at a time.
Your teacher has the mark scheme.
Questions built on realistic experimental data, in the style of Paper 1B and Paper 2. Work them on paper and show your working. 9 marks in total. Your teacher has the mark scheme.
Cylinders of potato tuber (Solanum tuberosum), each about 2.5 g, were blotted, weighed and placed in sucrose solutions at 20 °C. After 24 hours they were blotted and weighed again. Three cylinders were used for each concentration. The graph shows the mean percentage change in mass. Error bars show ± 1 standard deviation.
You will need: Potato cylinders, sucrose solutions from 0 to 1.0 M, balance, paper towel, boiling tubes
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.
Measuring mass change in tissue lets us infer water movement across membranes, even though we never see the molecules move.
Mass changes are the data, and osmosis is the explanation. Consider how far an inference goes beyond what was measured.
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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