What is osmosis?

What is osmosis? The definition to learn

Different courses word it slightly differently. All of these get the marks at their level:

Three parts earn the marks, whichever version you use: it is water that moves, it moves across a partially permeable membrane (called semipermeable or selectively permeable in the US), and it moves from dilute to concentrated.

How osmosis works

Picture a membrane with tiny holes, big enough for water molecules but too small for sugar molecules. On the left is pure water. On the right is sugar solution.

   Dilute (pure water)  |  Concentrated (sugar solution)
                        |
   ~  ~  ~  ~  ~        |   ~  S  ~  S  ~
   ~  ~  ~  ~  ~   -->  |   S  ~  S  ~  S
   ~  ~  ~  ~  ~        |   ~  S  ~  S  ~
                        |
   ~ = water molecule   partially permeable   S = sugar molecule
   net flow of water: left to right            (sugar can't cross)

Water molecules move about at random and cross the membrane in both directions all the time. On the right, some of the space next to the membrane is taken up by sugar molecules, so fewer water molecules hit the holes from that side. More water crosses from left to right than from right to left. The overall result, the net movement, is water flowing into the sugar solution.

Water keeps moving until the concentrations are equal, or until pressure pushes back hard enough to stop it. Even then, molecules still cross both ways, just at the same rate. You can run this yourself in our Cell Simulator.

Osmosis vs diffusion

Osmosis is a special case of diffusion. Diffusion is the net movement of any particles from where they are more concentrated to where they are less concentrated, like oxygen moving into a cell. Osmosis is only about water, and only across a partially permeable membrane.

Is osmosis active or passive?

Osmosis is passive. The water moves because of the random motion of its molecules, down its own concentration gradient, so the cell spends no energy (no ATP from respiration) on it. Many cell membranes contain channel proteins called aquaporins that let water through faster, but they are open channels, not pumps, so osmosis through them is still passive.

Try it: this question gives you examples of things crossing membranes and asks you to sort them by type of transport.

Osmosis in animal cells

What happens depends on the solution outside the cell compared with the cytoplasm inside. US courses use three words for this, and they are worth knowing at any level.

Animal cells have no cell wall to stop them swelling, so a red blood cell put in pure water takes in water until the membrane bursts. That is why a saline drip given into a vein is isotonic with blood, and why your kidneys work hard to keep the concentration of your blood steady.

Osmosis in plant cells

Plant cells have a strong cell wall outside the membrane, which changes the story.

• In a dilute solution (hypotonic), water moves into the vacuole and the cell swells until it presses hard against the wall. The cell is turgid. The wall stops it bursting, and the pressure, called turgor pressure, is what keeps leaves and young stems firm.
• In an isotonic solution, there is no net movement. The cell is flaccid: soft, but not shrunken.
• In a concentrated solution (hypertonic), water leaves the vacuole and the cytoplasm shrinks. The membrane pulls away from the wall, which is called plasmolysis, and the plant wilts.

Roots take in water by osmosis. Root hair cells have a long, thin shape that gives a big surface area, and their cytoplasm is more concentrated than the soil water around them, so water flows in. Too much fertiliser, or salt spread on winter roads, can make the soil water more concentrated than the roots, and then water flows the wrong way.

Osmosis examples in everyday life

• Wilting and recovery: a wilted plant stands up again a few hours after watering, as its cells take water back in and become turgid.
• Salting food: salt draws water out of bacteria by osmosis, which is one reason salted meat and fish keep longer.
• Limp salad: salad left in dressing goes floppy as salt and vinegar draw water out of the leaf cells.
• Sea water: drinking it makes you more dehydrated, because it is saltier than your body fluids and, as NOAA explains, your kidneys need more water than you drank to get rid of the salt.

The osmosis practical: potato cylinders

At GCSE, AQA's required practical 3 is to investigate how a range of salt or sugar solutions affects the mass of plant tissue. The usual method:

1. Cut equal cylinders of potato with a cork borer and trim them to the same length.
2. Blot each one dry and record its starting mass.
3. Put each cylinder in a different concentration of sugar solution, for example 0, 0.2, 0.4, 0.6 and 0.8 mol/dm³, for the same time.
4. Take them out, blot dry the same way, and record the final mass.
5. Work out the percentage change in mass for each one.

Worked example: percentage change in mass

A cylinder goes from 2.80 g to 3.12 g. The change is 3.12 − 2.80 = 0.32 g, and 0.32 ÷ 2.80 × 100 = +11.4%. It gained water, so the solution was more dilute than the potato cells. Another goes from 2.20 g to 1.85 g: a change of −0.35 g, and −0.35 ÷ 2.20 × 100 = −15.9%. It lost water, so that solution was more concentrated than the cells.

Using percentage change, not just the change in grams, lets you compare cylinders that started at different masses. Here is a full set of example results:

Here x is the sugar concentration in mol/dm³ and y is the percentage change in mass. Where the line crosses zero, at about 0.30 mol/dm³, the potato neither gains nor loses water, so that is roughly the concentration inside its cells. At A-level, the same intercept is used to find the water potential of the tissue. You can plot your own results with a best fit line in the Practical Data Lab.

Water potential (A-level)

A-level courses describe osmosis with water potential, written ψ (the Greek letter psi) and measured in kilopascals (kPa). It measures how freely water molecules can move. Pure water has the highest water potential, set at 0 kPa, and dissolving anything in it lowers the value below zero. Water always moves by osmosis from a higher (less negative) water potential to a lower (more negative) one. A cell at −700 kPa placed in a solution at −300 kPa gains water, because −300 is higher than −700.

Common mistakes

• Saying solutes move in osmosis. Only water moves. If sugar moves, that is diffusion.
• Writing "from high concentration to low concentration". Say whose concentration: from a dilute solution to a concentrated one.
• Leaving out the membrane. Without a partially permeable membrane it isn't osmosis.
• Saying plant cells burst. The cell wall stops them. They become turgid.
• Calling osmosis active. It needs no energy from the cell.
• Saying water stops moving at equilibrium. It still crosses both ways; there is just no net movement.

Read next

Biology · CellsOsmosis required practical: potato method and resultsThe osmosis required practical with potato cylinders: equipment, method, variables, results, percentage change in mass, the graph, errors and exam tips.