1The idea in one paragraph

Water dissolves many substances because its molecules are polar and cluster round each solute particle. Where there are more solute particles, there are fewer water molecules free to move, so across a membrane that stops the solute, water moves on balance towards the side with more solute: from a hypotonic solution to a hypertonic one. A cell in a solution more dilute than itself gains water; in a more concentrated one it loses water; in an isotonic one the flows balance. Without a wall, a cell that gains too much water bursts, which is why animals keep their body fluids isotonic and freshwater protists pump water out. With a wall, a plant cell swells until the wall pushes back, and that pressure holds the plant up. At HL, all of this is put into one number, water potential, that tells you which way water will move.

2Water as a solvent: solvation

Solvation is the surrounding of a solute particle by solvent molecules. When the solvent is water, the solute dissolves if its particles attract water molecules more strongly than they attract each other. Figure 1 shows the three cases you need.

A water molecule is polar: its oxygen carries a slight negative charge (δ−) and its two hydrogens slight positive charges (δ+).

• Positive ions, such as Na⁺, attract the δ− oxygen ends of water molecules. The water molecules crowd round the ion with their oxygens facing in.
• Negative ions, such as Cl⁻, attract the δ+ hydrogen ends. The water molecules face the other way round.
• Polar molecules, such as glucose with its –OH groups, form hydrogen bonds with water molecules.

In each case the solute particle ends up inside a shell of water molecules, separated from the other solute particles. That is what "dissolved" means. The shell also matters for the rest of this page: water molecules held round solute particles are less free to move, which is the root of osmosis.

3Which way water moves: hypotonic, hypertonic and isotonic

The guide asks you to describe water movement in terms of solute concentration, not water concentration, and with three comparison words.

• Hypotonic: having a lower solute concentration than the solution it is being compared with.
• Hypertonic: having a higher solute concentration.
• Isotonic: having the same solute concentration.

All three are comparisons, so always say what with: "hypotonic to the cytoplasm".

Water moves by osmosis from a hypotonic solution to a hypertonic one: from lower solute concentration to higher.

Figure 2 shows why the word "net" matters.

Water molecules cross the membrane in both directions all the time. In panel (a), more cross from the hypotonic side, where more of them are free, than cross back from the hypertonic side, so there is a net movement towards the hypertonic side. In panel (b) the solutions are isotonic. Water still crosses, but equally in both directions, so the volume on each side stays the same. That is dynamic equilibrium: constant movement, no net change. "No water moves in an isotonic solution" is wrong.

4Water moving into and out of cells

The cytoplasm and vacuole of a cell are solutions, and the plasma membrane is partially permeable. So a cell behaves exactly like one side of Figure 2.

• Surroundings hypotonic to the cell (for example, pure water): net movement of water into the cell.
• Surroundings hypertonic to the cell (for example, strong salt solution): net movement of water out of the cell.
• Surroundings isotonic: no net movement; the cell's volume stays constant.

5Measuring it: plant tissue in a range of solutions

The standard practical measures how pieces of plant tissue change when bathed in solutions of different concentrations, and uses the results to find the concentration that is isotonic with the tissue.

Method in outline. Cut cylinders of one tissue (carrot, potato, beetroot) with a cork borer, trim them to the same length, and blot and weigh or measure each. Place replicate pieces in a range of sucrose or salt solutions, for example 0.0 to 0.5 mol dm⁻³, at the same temperature. After a fixed time, often 30 minutes to a few hours, blot them dry the same way and measure again. Pieces never start identical, so calculate the percentage change for each piece:

The data. The table gives invented results for carrot cylinders, all 50.0 mm long at the start, five repeats at each concentration.

Standard deviation and standard error. The standard deviation (SD) measures how spread out the repeats are around their mean. The standard error (SE) estimates how precisely the mean itself is known; it is the SD divided by the square root of the number of repeats, so it shrinks as you take more repeats. You do not need to memorise the formulae (a calculator or spreadsheet function does it), but you should see once how the numbers arise. Here are the five repeats at 0.2 mol dm⁻³.

Reading the graph. Figure 3 plots the means, with error bars of ±1 SE.

Below about 0.24 mol dm⁻³ the cylinders got longer: the solution was hypotonic to the cells, water entered by osmosis, and the cells swelled. Above it they got shorter: the solution was hypertonic, water left. Where the line crosses zero there was no net change, so that solution is isotonic with the tissue. Read it off the graph, or interpolate between the readings either side of zero:

The error bars are short and no two of them overlap, so the differences between concentrations are larger than the scatter between repeats: the trend is reliable. If the bars of two neighbouring points overlapped widely, you could not be confident that those two means really differ.

Length or mass? The same experiment can record mass. SE lets you compare how reliable the two measurements are. In pure water, the same five carrot pieces (invented data) gave a mean length change of +7.8% with SE 0.14, and a mean mass change of +16.0% with SE 0.70.

Relative to the size of the change, the mass readings scatter more, most likely because blotting leaves a variable film of liquid on each piece. In this data set length was the more reliable measure, though mass changes are larger and easier to detect.

6Cells without a cell wall

An animal cell, or a protist, has only a plasma membrane round it. It has nothing to resist swelling. Figure 4 (top row) shows a red blood cell in three solutions.

• Hypotonic: water enters, the cell swells and, because the membrane cannot stretch far, it bursts. For red blood cells this is called haemolysis.
• Isotonic: no net movement; the cell keeps its normal biconcave shape.
• Hypertonic: water leaves, the cell shrinks and its surface becomes wrinkled and spiky, called crenation.

Both extremes damage or kill cells, so organisms without walls must control their water balance.

Freshwater unicellular organisms. Pond water is strongly hypotonic to the cytoplasm of a protist such as Paramecium, so water enters by osmosis all the time. The cell would burst unless it removed the water. It does so with contractile vacuoles (Figure 5), which collect excess water from the cytoplasm and expel it through the membrane. This uses energy from ATP, and the vacuoles fill and empty continually.

Multicellular organisms. Your cells are bathed in tissue fluid, which comes from blood plasma. If tissue fluid became hypotonic or hypertonic to the cells, they would swell or shrink, with harmful effects everywhere at once. So the solute concentration of blood, and therefore of tissue fluid, is kept isotonic with the cells, mainly by the kidneys (D3.3).

7Cells with a cell wall

Plant cells behave differently because of the wall, a strong layer of cellulose outside the membrane that is fully permeable to water and solutes. Figure 4 (bottom row) shows the three cases.

Hypotonic: turgor. Water enters, and the vacuole and cytoplasm swell until the plasma membrane presses against the wall. The wall is strong and barely stretches, so it pushes back. The pressure that builds up inside is turgor pressure. When it is high enough, water stops entering, even though the cell's contents are still more concentrated than the water outside. The cell is turgid, and it does not burst. Turgid cells pressing against each other are what keep a non-woody plant upright; a plant short of water loses turgor and wilts.

Isotonic: flaccid. No net movement of water; the membrane touches the wall but does not press on it. The cell is flaccid.

Hypertonic: plasmolysis. Water leaves, the vacuole and cytoplasm shrink, and the plasma membrane pulls away from the wall. This is plasmolysis. Because the wall is fully permeable, the space between wall and membrane fills with the external solution.

Without a wall, a cell in a hypotonic solution bursts. With a wall, it becomes turgid instead.

8Isotonic solutions in medicine

Anything that touches living tissue for long must be isotonic with it, for the reasons in section 6.

Intravenous fluids. Fluid given into a vein, for example to replace lost blood volume or to treat dehydration, mixes directly with blood plasma. It must be isotonic with the plasma, or red blood cells and other cells would swell and burst (if it were hypotonic) or shrink (if it were hypertonic). The commonest is normal saline, 0.9% sodium chloride, which is isotonic with blood plasma.

Organs for transplantation. A donor kidney or liver may be out of the body for hours before it is transplanted. It is bathed in a cold, isotonic solution. Isotonic, so its cells neither take up water and swell nor lose it and shrink; cold, to slow its metabolism and reduce the damage caused by lack of oxygen.