Water is constantly moving into and out of living cells. Root hair cells take up water from the soil, plant cells become firm when they gain water, and red blood cells can swell or shrink when placed in different solutions. These situations may look very different, but they are all linked by the same process: osmosis.
Rather than memorising a separate rule for every example, begin with three questions: Where is the water potential higher? Where is it lower? What partially permeable membrane separates the two regions? Once those are identified, the direction of the net movement of water becomes much easier to predict.
Try these five questions before reading the chapter. They test water potential, partially permeable membranes and the effects of osmosis on living cells.
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- Water potential and cell reasoning
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What osmosis really means
Osmosis is sometimes shortened to “water moves from a dilute solution to a concentrated solution”. That shortcut may predict the direction correctly in simple situations, but it leaves out two ideas that matter in Biology: water potential and the partially permeable membrane.
A more precise way to think about osmosis is to follow the water itself. When two regions with different water potentials are separated by a membrane through which water can pass, there can be a net movement of water molecules from the region of higher water potential towards the region of lower water potential.
Exam definition: Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.
Why the word “net” matters
Individual water molecules are in constant random motion. Water molecules can therefore cross a suitable membrane in both directions.
If the water potentials on the two sides differ, however, the movements do not balance. More water crosses in one direction than in the other, so there is an overall, or net, movement of water.
Saying that water moves from higher to lower water potential describes the net movement. It does not mean that every individual water molecule travels in the same direction.
Understanding water potential
Water potential, represented by the symbol Ψ, describes the tendency of water to move from one region to another.
When ordinary solutions are compared under the same conditions, dissolving more solute lowers the water potential of the solution. This gives a useful relationship:
More dilute solution: generally has the higher water potential.
More concentrated solution: generally has the lower water potential.
This explains why water often moves from a dilute solution towards a more concentrated solution when the two are separated by a partially permeable membrane. In an exam, however, the more precise wording is: higher water potential → lower water potential.
Why the partially permeable membrane matters
A partially permeable membrane allows some substances to pass through more easily than others. In a simple osmosis model, water molecules can cross the membrane while the dissolved particles being considered cannot cross freely.
The membrane is therefore essential to the definition of osmosis. Without it, the movement would simply be described in terms of diffusion and mixing rather than osmosis across a membrane.
In cells: the cell membrane is partially permeable. The plant cell wall is not the partially permeable membrane responsible for osmosis. Water and many dissolved substances can pass relatively freely through the cell wall.
Osmosis is a passive process
A cell does not have to use ATP to make water move down a water potential gradient. Osmosis is therefore a passive process.
This does not mean that water molecules have no energy or are not moving. They are in constant random motion. The point is that the cell does not need to supply metabolic energy to drive the net movement of water.
Osmosis in animal cells
Animal cells are surrounded by a cell membrane but do not have a rigid cell wall. This means that large gains or losses of water can substantially change their volume.
Red blood cells are a useful example.
| Surroundings | Net movement of water | Effect on red blood cell |
|---|---|---|
| Higher water potential than the cytoplasm | Into the cell | The cell swells. If sufficient water enters, the cell membrane may rupture and the cell undergoes lysis. |
| Approximately the same water potential as the cytoplasm | No net movement | Water continues moving in both directions, but the movements balance and the cell maintains approximately its normal volume. |
| Lower water potential than the cytoplasm | Out of the cell | The cell loses water and shrinks. Shrinkage of red blood cells is called crenation. |
Why can an animal cell burst?
Suppose a red blood cell is surrounded by a solution with a much higher water potential than its cytoplasm. Water enters the cell by osmosis through its cell membrane.
As water enters, the cell volume increases. Because an animal cell has no rigid cell wall to resist continued expansion, excessive water entry may eventually rupture the membrane.
Higher external water potential: net water movement into cell → cell swells → cell may undergo lysis.
Why does a red blood cell shrink?
If the surrounding solution has a lower water potential than the cytoplasm, water instead leaves the cell by osmosis.
The cell loses volume and becomes shrunken. In red blood cells, this process is called crenation.
Lower external water potential: net water movement out of cell → cell loses volume → cell shrinks.
Why are suitable saline solutions used with blood cells?
A solution placed in contact with blood cells should not cause a large net movement of water into or out of them. An isotonic saline solution is formulated so that the cells maintain approximately their normal volume rather than swelling or shrinking because of osmosis.
Osmosis in plant cells
Plant cells respond differently because they have both a partially permeable cell membrane and a strong cell wall. A typical mature plant cell also contains a large central vacuole filled with cell sap.
The cell wall does not stop water from entering. Instead, its strength becomes important when the contents of the cell expand.
When a plant cell gains water
If the surroundings have a higher water potential than the cell contents, there is a net movement of water into the cell by osmosis.
The vacuole becomes larger and the cell contents press more firmly against the cell wall. The cell wall resists further expansion, so pressure develops inside the cell. The cell becomes turgid.
Turgidity: water enters → vacuole enlarges → cell contents press against cell wall → cell becomes turgid.
Turgidity is important in supporting non-woody parts of plants. When many cells are turgid, leaves and young stems remain firm.
Why does a plant cell not normally burst?
As water enters, the expanding cell contents press increasingly strongly against the cell wall. The strong wall resists further expansion.
Eventually the tendency for further net water entry can be balanced by the pressure that has developed inside the cell. Water molecules still move across the membrane, but there is then no net movement of water.
When a plant cell loses water
If the surrounding solution has a lower water potential than the cell contents, water moves out of the cell by osmosis.
The vacuole becomes smaller and the cell loses turgor. The cell becomes flaccid.
If still more water is lost, the living contents of the cell shrink sufficiently for the cell membrane to pull away from the cell wall. The cell is then described as plasmolysed.
Do not treat “flaccid” and “plasmolysed” as identical. A plant cell can lose turgor and become flaccid before enough water has been lost for the membrane to pull visibly away from the cell wall.
Turgid, flaccid and plasmolysed are not interchangeable
These terms describe different conditions of a plant cell. Keeping them separate prevents one of the most common mistakes in osmosis questions.
| State | What has happened? | Key feature |
|---|---|---|
| Turgid | The cell has gained water and developed turgor pressure. | The cell contents press firmly against the cell wall. |
| Flaccid | The cell has little or no turgor and is no longer firm. | The cell contents no longer press strongly against the cell wall. |
| Plasmolysed | The cell has lost sufficient water for the living contents to shrink markedly. | The cell membrane has pulled away from the cell wall in places. |
How osmosis helps support plants
Many young stems and leaves do not depend entirely on rigid woody tissue for support. Water entering their cells generates turgor pressure, helping the tissues remain firm.
If a plant loses water faster than it replaces it, its cells lose turgor and become flaccid. The tissues become less firm and the plant begins to wilt.
Well-watered plant: many cells are turgid → tissues are firm.
Water-deficient plant: cells lose water and turgor → tissues become less firm → plant may wilt.
Investigating osmosis using plant tissue
Osmosis can be investigated using pieces of plant tissue such as potato cylinders. The basic idea is to place similar pieces of tissue into solutions of different concentrations and measure whether the tissue gains or loses water.
A suitable experimental method
- Cut several potato cylinders so that they have similar dimensions.
- Measure and record the initial mass of each potato cylinder.
- Place the cylinders into equal volumes of solutions with different known concentrations, such as different sucrose concentrations.
- Keep variables such as temperature, duration and the dimensions of the potato pieces as constant as possible.
- After the same period of time, remove each cylinder and gently blot away excess solution from its surface in a consistent manner.
- Measure the final mass.
- Calculate and compare the changes in mass.
Percentage change in mass:
percentage change = (final mass − initial mass) ÷ initial mass × 100%
Interpreting the results
| Observation | What it suggests |
|---|---|
| Potato gains mass | There has been a net movement of water into the potato tissue. The surrounding solution had a higher water potential than the tissue. |
| Potato loses mass | There has been a net movement of water out of the potato tissue. The surrounding solution had a lower water potential than the tissue. |
| Little or no change in mass | There is approximately no net movement of water. The water potential of the surrounding solution is approximately equal to that of the tissue under the experimental conditions. |
Why calculate percentage change?
Imagine one potato cylinder begins with a mass of 2.0 g while another begins at 6.0 g. A change of 0.2 g represents a much larger relative change for the smaller piece.
Using percentage change in mass allows samples with slightly different starting masses to be compared more fairly.
Why must the potato be blotted before weighing?
Drops of solution remaining on the outside of a potato cylinder would add to its measured mass even though that liquid had not entered the tissue.
Blotting removes excess surface liquid so that the final measurement better represents the mass of the potato tissue itself. The same blotting method should be used for every sample.
Osmosis compared with diffusion and active transport
Diffusion, osmosis and active transport all involve the movement of substances, but they describe different processes.
| Process | What moves? | Direction | Membrane required? | Energy supplied by cell? |
|---|---|---|---|---|
| Diffusion | Particles | Net movement down a concentration gradient | Not necessarily | No |
| Osmosis | Water molecules | Net movement from higher to lower water potential | Yes — a partially permeable membrane | No |
| Active transport | Specific substances such as ions or glucose | Can move against a concentration gradient | Occurs across cell membranes | Yes |
One useful way to distinguish these processes is to ask what substance is actually moving. If the movement is specifically the net movement of water through a partially permeable membrane because of a difference in water potential, the process is osmosis.
Common Mistakes
| Common mistake | Why it is wrong | Better approach |
|---|---|---|
| Saying that sugar or salt “moves by osmosis” | Osmosis specifically describes the movement of water molecules. | Identify water as the substance undergoing osmosis. |
| Leaving the partially permeable membrane out of the definition | The membrane is an essential part of the definition of osmosis. | Include through a partially permeable membrane. |
| Saying water moves from “high concentration to low concentration” | This is ambiguous because it does not state whether you mean water or dissolved solute. | Use higher water potential → lower water potential. |
| Saying that all water molecules move in one direction | Individual molecules continue moving randomly in both directions. | State that there is a net movement of water. |
| Saying the plant cell wall is partially permeable | The cell wall is relatively freely permeable. The cell membrane is partially permeable. | Distinguish the supporting role of the cell wall from the selective permeability of the cell membrane. |
| Saying a plant cell bursts when water enters | The strong cell wall resists excessive expansion. | State that the cell normally becomes turgid. |
| Calling every water-losing plant cell plasmolysed | A plant cell can lose turgor and become flaccid before the membrane pulls away from the cell wall. | Use plasmolysed only when sufficient water loss has caused the living contents to shrink away from the wall. |
| Saying water stops moving when equilibrium is reached | Random movement continues. | State that there is no net movement of water. |
| Saying osmosis requires energy from respiration | Osmosis is a passive process. | State that the cell does not need to supply metabolic energy. |
| Explaining potato mass changes without mentioning water potential | Saying only that the potato “absorbed water” does not explain why water moved. | Compare the water potentials and then state the direction of net water movement. |
What you should be able to do
Define osmosis precisely
State that it is the net movement of water molecules from higher water potential to lower water potential through a partially permeable membrane.
Use water potential correctly
Predict the direction of net water movement by identifying the regions of higher and lower water potential.
Explain net movement
Recognise that water molecules move in both directions even though the overall movement may favour one direction.
Explain the role of the membrane
Recognise that the cell membrane is partially permeable while the plant cell wall is relatively freely permeable.
Explain osmosis in animal cells
Predict when an animal cell will swell, undergo lysis or shrink because of water movement.
Distinguish plant-cell states
Explain the differences between turgid, flaccid and plasmolysed plant cells.
Explain turgor
Link water entry, vacuole expansion and pressure against the cell wall to the development of turgidity.
Explain plasmolysis
Describe how sufficient water loss causes the living contents to shrink and the cell membrane to pull away from the cell wall.
Interpret an osmosis investigation
Use changes in mass or percentage mass to infer the direction of net water movement.
Distinguish transport processes
Separate osmosis from diffusion and active transport by considering what moves, the gradient involved, the membrane and the need for cellular energy.
These ten questions test whether you can use water potential to explain unfamiliar situations involving membranes, animal cells, plant cells and practical investigations.
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Frequently asked questions
Osmosis: questions students often ask
Once the definition is secure, most confusion comes from water potential, the meaning of net movement and the different responses of plant and animal cells.
Why is osmosis only about water?
Osmosis is defined specifically as the net movement of water molecules through a partially permeable membrane. If glucose, oxygen, ions or other particles are moving, another transport process such as diffusion or active transport may be involved.
Can I say that water moves from a dilute solution to a concentrated solution?
This can work as a shortcut for simple solutions compared under the same conditions, because the more dilute solution normally has the higher water potential. However, the more precise exam wording is that water moves from higher water potential to lower water potential.
Do water molecules move in only one direction during osmosis?
No. Water molecules are moving randomly and may cross a suitable membrane in both directions. Osmosis describes the net movement of water from higher water potential to lower water potential.
Does water stop moving when the water potentials become equal?
No. Water molecules continue moving randomly. When the water potentials are equal, movement in the two directions balances overall, so there is no net movement of water.
Why must a partially permeable membrane be present?
Osmosis depends on water being able to cross a membrane while some dissolved particles cannot cross freely. The selective nature of the membrane allows a water potential difference to produce net water movement.
Is the plant cell wall partially permeable?
The plant cell wall is relatively freely permeable to water and many small dissolved substances. The cell membrane is the partially permeable boundary involved when water enters or leaves the cell.
What is the difference between turgid, flaccid and plasmolysed?
A turgid cell has gained water and its contents press firmly against the cell wall. A flaccid cell has little or no turgor and is less firm. A plasmolysed cell has lost enough water for the living contents to shrink and the cell membrane to pull away from the wall.
Why does a plant cell not burst when water enters?
As water enters, the expanding cell contents press against the strong cell wall. The wall resists further expansion, allowing pressure to develop inside the cell. The cell therefore becomes turgid instead of continuing to expand freely.
Why can a red blood cell burst in distilled water?
Distilled water has a higher water potential than the contents of the red blood cell, so there is a net movement of water into the cell. Because a red blood cell has no cell wall, excessive water entry can cause its membrane to rupture.
What does crenation mean?
Crenation is the shrinkage of a red blood cell when it loses water to surroundings of lower water potential.
Why is isotonic saline suitable for blood cells?
An isotonic solution avoids a large net movement of water into or out of the blood cells. This helps the cells maintain approximately their normal volume.
Why does a potato gain mass in a dilute solution?
If the surrounding solution has a higher water potential than the potato tissue, there is a net movement of water into the cells through their partially permeable membranes. The tissue therefore gains water and its mass increases.
Why does a potato lose mass in a concentrated solution?
If the surrounding solution has a lower water potential than the potato tissue, there is a net movement of water out of the cells. The tissue therefore loses water and its mass decreases.
What does zero percentage change in potato mass tell me?
It suggests that there was approximately no net movement of water during the experiment. The water potential of the surrounding solution was therefore approximately equal to that of the potato tissue under those experimental conditions.
Does osmosis require energy from respiration?
No. Osmosis is passive. The cell does not need to use ATP to move water down a water potential gradient.
What is the safest way to explain osmosis in an exam?
Identify where the water potential is higher and where it is lower. Then state that there is a net movement of water molecules through a partially permeable membrane from higher water potential to lower water potential. Finally, link that movement to the observed effect on the cell or tissue.
Osmosis becomes much easier once every situation is reduced to the same underlying principle. Identify the two water potentials, locate the partially permeable membrane and follow the net movement of water from higher water potential to lower water potential. From there, the swelling of an animal cell, turgidity, flaccidity, plasmolysis and changes in the mass of potato tissue are all consequences of the same process.