D2.3 Water potential

A solvent is a liquid that dissolves other substances, the solutes, to make a solution. Water dissolves more kinds of substance than any other common liquid, which is why the cytoplasm, blood plasma, xylem sap and phloem sap are all aqueous solutions. Dissolving in water is called solvation.

Water can do this because its molecules are polar: the oxygen atom carries a slight negative charge (δ−) and the two hydrogen atoms slight positive charges (δ+). See A1.1 Water for how this polarity arises.

• Positive ions such as Na⁺ and K⁺ attract the δ− oxygen ends of water molecules.
• Negative ions such as Cl⁻ attract the δ+ hydrogen ends.
• Polar molecules such as glucose and amino acids form hydrogen bonds with water through groups such as –OH and –NH₂.

In each case the particle becomes surrounded by a shell of water molecules and is carried away from the others, so it dissolves. Substances that are attracted to water in this way are hydrophilic.

Non-polar substances such as fats, oils and steroids have no charges or partial charges for water to attract. Water molecules form hydrogen bonds with each other instead, so non-polar molecules are pushed together and do not dissolve: they are hydrophobic. This is why lipids are transported in blood inside lipoproteins and why the hydrophobic core of a membrane is a barrier to ions.

Comparing solutions

Three terms compare the solute concentration of one solution with another:

The terms only make sense as a comparison. A 0.3 mol dm⁻³ sucrose solution is hypertonic to pure water but hypotonic to a 0.6 mol dm⁻³ solution.

Why water moves

Osmosis is the net movement of water across a partially permeable membrane. As in B2.1 Membranes and membrane transport, explain it with three ideas:

1. Water molecules move randomly and cross the membrane in both directions all the time.
2. The membrane is impermeable to the solute, so solute particles cannot move to even out their concentrations.
3. The solute concentrations differ on the two sides. In any period of time, more water molecules cross from the side with the lower solute concentration (hypotonic) into the side with the higher solute concentration (hypertonic) than cross the other way. (At AHL this is described using water potential: the more concentrated solution has the lower water potential.)

So the net movement of water is from the less concentrated (hypotonic) solution to the more concentrated (hypertonic) solution. It is passive: no energy from ATP is needed.

Predicting movement into and out of cells

The cytoplasm of a cell is a solution, so the same rules apply across its plasma membrane:

• Hypotonic surroundings (lower solute concentration than the cytoplasm): net movement of water into the cell.
• Hypertonic surroundings (higher solute concentration): net movement of water out of the cell.
• Isotonic surroundings: water molecules still move in and out, but at equal rates, so there is no net movement. This is a dynamic equilibrium, not an absence of movement.

Animal cells and many unicellular organisms have a plasma membrane but no cell wall, so nothing resists a change in volume. Osmosis can therefore damage them.

Red blood cells

• In a hypotonic solution, such as pure water, water enters by osmosis. The cell swells, and because the plasma membrane is not strong enough to resist the rising pressure, it bursts. For red blood cells this is called haemolysis: the haemoglobin is released, leaving an empty membrane “ghost”.
• In a hypertonic solution, water leaves. The volume of cytoplasm falls but the area of plasma membrane does not, so the cell shrinks and its surface becomes wrinkled and spiky: it is crenated.
• In an isotonic solution the cell keeps its normal biconcave shape.

Freshwater unicellular organisms

Protists such as Paramecium and Amoeba that live in ponds, lakes and rivers have cytoplasm that is hypertonic to the water around them, so water enters continuously by osmosis. They remove it with contractile vacuoles. Water drains into the vacuole, often along radiating canals; the full vacuole then contracts and expels the water through a pore in the plasma membrane. This uses energy from ATP, and it must be repeated every few seconds to keep pace with the water entering.

Multicellular animals

The cells of an animal’s body are bathed in tissue fluid (and red blood cells in plasma). If this fluid became hypotonic or hypertonic, cells would swell or shrink and be damaged. Animals therefore keep their extracellular fluids isotonic with their cells. In humans the kidneys do this by regulating the water and solute content of the blood (osmoregulation, D3.3 Homeostasis).

Plant cells, fungi and many bacteria have a cell wall outside the plasma membrane. In plants it is made largely of cellulose fibres, which are very strong. The wall is freely permeable to water and solutes; the partially permeable barrier is still the plasma membrane. In plant cells the large vacuole holds most of the cell’s water and solutes.

In a hypotonic solution: turgor

Water enters by osmosis, and the vacuole and cytoplasm expand. The protoplast (everything inside the wall) presses outwards on the wall, and the wall presses back. This builds up turgor pressure inside the cell, which becomes turgid. The wall stretches only slightly, so the cell does not burst. As the pressure rises it opposes further entry of water, and eventually water enters and leaves at equal rates.

Turgor pressure matters to plants:

• It keeps non-woody stems and leaves rigid and upright; without it the plant wilts.
• It drives cell expansion during growth and powers movements such as the opening of stomata.

In a hypertonic solution: plasmolysis

Water leaves the cell. First turgor pressure is lost and the cell becomes flaccid (soft, with no pressure on the wall). If water continues to leave, the protoplast shrinks, and the plasma membrane pulls away from the cell wall. This is plasmolysis. Because the wall is freely permeable, the space between the wall and the membrane fills with the external solution. The point at which the membrane is just starting to leave the wall is called incipient plasmolysis.

Brief plasmolysis can be reversed if the cell is returned to water, but prolonged or severe plasmolysis damages the membrane and usually kills the cell.

The water relations of a plant tissue can be investigated by bathing samples of it in solutions of different concentration. Mass and length change because of water entering or leaving the cells.

Method

1. Make a range of solutions, for example sucrose or sodium chloride from 0.0 mol dm⁻³ upwards (a serial dilution of a stock solution works well).
2. Cut cylinders or cubes of tissue (potato, sweet potato, carrot, beetroot) of the same size and shape, from the same source, with a cork borer and scalpel.
3. Blot each sample dry, then measure its mass on an electronic balance and its length with a ruler marked in millimetres.
4. Put several samples (repeats) into each solution at the same temperature, for the same time (typically from one hour up to a day).
5. Remove, blot gently in the same way, and measure again.
6. Calculate % change = (final − initial) ÷ initial × 100. Percentage change, not absolute change, is used because the samples’ starting values differ slightly.

Results and the isotonic concentration

In hypotonic solutions the tissue gains mass and becomes firm and turgid; in hypertonic solutions it loses mass and becomes soft and flaccid as its cells plasmolyse. Plot mean % change (y-axis) against concentration (x-axis), draw a line of best fit, and read off where it crosses 0 % change: the x-intercept. At that concentration there would be no net movement of water, so it is the estimate of the solution isotonic with the tissue.

Reliability: standard deviation and standard error

Repeats at each concentration let you judge how reliable the means are (the formulae do not need to be learned):

• Standard deviation (SD) shows how spread out the repeat results are around their mean.
• Standard error of the mean (SE) = SD ÷ √n, where n is the number of repeats. It shows how reliably the sample mean estimates the true mean, and gets smaller as n increases.

Error bars of ± 1 SE drawn on each mean make reliability visible. Short bars mean the mean is well estimated; if the bars of two means overlap widely, the difference between them may not be significant. Comparing the SDs of mass and length data shows which measurement is more reliable: length changes are smaller and a ruler reads only to 1 mm.

Human cells have no cell wall, so if they are exposed to a fluid that is not isotonic they swell and may burst, or shrink. Any fluid that comes into contact with living tissue during medical treatment must therefore be isotonic (or very close to it) with the cells.

Intravenous fluids

Patients who are dehydrated, have lost blood or cannot drink are given fluids directly into a vein by intravenous (IV) drip. The fluid mixes with blood plasma, so it is made isotonic with the plasma: for example normal saline, a sterile solution of about 0.9 % sodium chloride.

• If pure water were infused, the plasma would become hypotonic, and red blood cells would take in water, swell and burst (haemolysis).
• A strongly hypertonic fluid would draw water out of red blood cells, which would shrink and crenate.

Organs for transplantation

A kidney, liver or heart removed from a donor must be kept alive until it is transplanted. It is flushed through and bathed in a cold, sterile preservation solution that is isotonic with its cells, then packed in a bag surrounded by ice (often an isotonic slush) for transport. The isotonic solution stops the cells swelling or shrinking by osmosis, and the low temperature slows their metabolism so they use less oxygen and ATP.