What you need to know
- 2.4Net movement of free water molecules through a partially permeableLets some molecules pass through but not others. membrane down a water-potential gradient.
By the end, you should be able to…
- Define osmosisThe net movement of water molecules from a higher water potential to a lower water potential across a partially permeable membrane. using water potentialThe tendency of water to move out of a solution. Pure water is 0; adding solutes makes it negative. Water moves from higher to lower water potential. and partial permeability.
- Predict water movement from numerical water potentials.
- Explain changes in plant and animal cells.
Build your understanding
Follow the water-potential gradient
osmosisThe net movement of water molecules from a higher water potential to a lower water potential across a partially permeable membrane. is the net movement of free water molecules through a partially permeableLets some molecules pass through but not others. membrane from higher water potentialThe tendency of water to move out of a solution. Pure water is 0; adding solutes makes it negative. Water moves from higher to lower water potential. to lower water potential. Such a membrane permits water movement while restricting at least some solutes. Osmosis is passive.
Water potential describes the tendency of water to move relative to a reference. Pure water at atmospheric pressure has a water potential of zero. Dissolved solutes lower water potential; pressure can raise it. A less negative value is higher: −200 kPa is higher than −600 kPa.
Cells change as water moves
An animal cell in a solution with higher water potential than its contents gains water and swells; sufficiently large uptake may cause lysis. In a solution of lower water potential it loses water and shrinks. These predictions assume the solutes causing the gradient do not rapidly cross the membrane.
Cells change as water moves (continued)
Plant cells have a cell wallA rigid layer outside the cell membrane. In plants it is made mainly of cellulose; it supports the cell and resists bursting. that resists expansion. As water enters, pressure builds and the cell becomes turgidDescribes a plant cell that is firm because water has entered and its contents push against the cell wall.. Increasing pressure raises the cell’s water potential until net entry stops. The wall does not prevent all water entry.
If a plant cell loses enough water, the protoplast shrinks and the cell surface membraneThe partially permeable membrane around a cell that controls which substances enter and leave. pulls away from the wall: plasmolysisWhen a plant cell loses so much water by osmosis that its membrane pulls away from the cell wall.. The wall itself does not shrink in the same way. A flaccid cell has little turgorThe pressure of a plant cell’s contents pushing outwards against the cell wall.; not every flaccid cell is already plasmolysed.
Use experimental changes carefully
In a tissue investigation, compare percentage mass change rather than raw change if starting masses differ: (final mass − initial mass)/initial mass × 100. Control tissue dimensions, exposure time and temperature, and use consistent blotting before weighing.
A solution concentration giving no net mass change estimates the tissue’s water potential under the test conditions. It does not show that water molecules stopped moving, nor that every cell is identical. Repeats and interpolation between appropriate values improve the estimate.
NOT: −200 kPa is higher than −600 kPa. Equal water potential need not mean equal solutes.
- Pure water at atmospheric pressure has water potential zero
- Solute lowers water potential; pressure can raise it
- Animal cell: lysis if too much water enters; it shrinks if water leaves
- Plant wall resists expansion, so pressure rises and the cell becomes turgid
Two traps this diagram blocks
- Do not treat a more negative number as a higher water potential.
- Do not say the wall stops all water entering. It allows pressure to build until the water potentials are equal.
From the diagram alone
Question
A cell has a water potential of −500 kPa. The surrounding solution is −150 kPa. Predict the initial net movement of water and explain it. [3]
Show a model answer
Water enters the cell. −150 kPa is higher than −500 kPa, so net water movement is from the solution into the cell through the partially permeable membrane. The prediction assumes the solutes causing the difference do not cross freely.
Make your reasoning visible
Compare negative values correctly
Water moves from −200 kPa towards −600 kPa, not the reverse. Name the partially permeableLets some molecules pass through but not others. membrane as well as the direction.
Identify the moving substance
osmosisThe net movement of water molecules from a higher water potential to a lower water potential across a partially permeable membrane. describes water movement. Do not write that sugar moves by osmosis, or rely on “dilute to concentrated” without identifying the solutions.
Key explanations
- No change in mass means no net water movement. Water molecules have not stopped moving, and solute concentrations need not be equal if pressure differs.
Avoid this misconception
The claim: At zero mass change no water crosses the membrane.
The correction: Water still crosses in both directions; there is no net gain or loss in the simplified interpretation.
Why a plant cell in water firms up, but a red blood cell bursts
Water moves into or out of cells by osmosis, from where the water potential is higher to where it is lower. A plant cell has a strong wall around it; an animal cell does not. That difference decides what happens when a cell takes in a lot of water.
What the model shows
A plant cell and a red blood cell sitting in the same salt solution. You choose how salty the solution is. Underneath is a practical with pieces of potato in sugar solutions, using made-up results.
What to look for
Which way the blue arrows point, how big each cell becomes, and the water-potential bars. In pure water, watch what the wall does for the plant cell. In the practical, find the sugar concentration where the potato neither gains nor loses mass.
What it tells us
Water moves towards the lower water potential. As a plant cell swells, its wall pushes back (pressure potential), so it ends up firm (turgid) instead of bursting. A red blood cell has no wall, so it bursts in a weak solution and shrinks in a strong one. The practical shows how to measure a tissue’s water potential.
Be careful what you claim
The numbers are simplified model values, not measurements from real cells. Real red cells differ from each other, so they burst over a range of about 0.45–0.5 % salt. The potato results are made up for practice.
Try it in the model: press Pure water and watch both cells. Then press Start again (new cells), set Salt (NaCl) outside to 0.6 %, then 0.4 %, and compare.
Same solution, two cells
A plant cell and a red blood cell in the same salt solution, with water-potential readings, followed by a potato-cylinder practical to find a tissue's water potential. Values are simplified model values.
What you see
- A plant cell and a red blood cell in the same salt solution. Gold specks are salt, which stays outside. Blue arrows show the net movement of water.
- Water-potential bars for the solution and for each cell. For the plant cell, the solute part (ψs) and the wall-pressure part (ψp) are shown separately.
- Below, a potato-cylinder practical with a table, a graph of mean % change and a place to estimate the tissue water potential.
How to use it
- Choose a salt concentration with the slider or the buttons. The cells take a few seconds to settle.
- A burst red cell stays burst. Press Start again for a new pair of cells.
- Run the practical as often as you like: each run gives a new set of made-up results.
What it shows
- Water moves from a higher to a lower water potential.
- A plant cell becomes turgid and does not burst, because wall pressure raises its water potential. Without a wall, a red cell bursts in a dilute solution.
- In strong solutions plant cells plasmolyse and red cells crenate.
- How to find a tissue’s water potential from the concentration that gives no change in mass.
Limits of the model
- Water potentials are simplified model values in kPa. Salt is treated as fully dissociated, so 1 % NaCl ≈ −830 kPa.
- The red cell follows a simple volume rule (part of the cell does not take up water), and bursts at 1.5 times its normal volume. Real red cells vary, and burst over a range around 0.45–0.5 % NaCl.
- The potato results are fictional, made by the model with some random variation.
Guided tour · step 1 of 7
- In Solution, just below these steps, press Pure water.
- Watch both cells until the red blood cell bursts.
Pure water has the highest water potential (0 kPa), so water moves into both cells by osmosis. The red cell has no wall, so it swells until it bursts.
Solution
A burst red cell stays burst: changing the solution cannot repair it.
Readings
Tissue water potential (fictional results)
Potato cylinders (about 2 g each) are blotted, weighed, left in sucrose solutions for 24 hours, then blotted and weighed again. Three repeats at each concentration. % change = (final − initial) ÷ initial × 100.
From your graph
Students often get the sign wrong: solutions and cells have negative water potentials, and pure water is 0 kPa.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
A cell is at −500 kPa and the surrounding solution at −150 kPa. Predict net water movement.
Show suggested answer
Water enters the cell, moving from the higher water potential of −150 kPa outside to the lower potential of −500 kPa inside through its partially permeable membrane.
Practice question
Retrieval and explanation · suggested answer
A tissue sample changes from 2.00 g to 2.16 g. Calculate percentage mass change.
Show suggested answer
(2.16 − 2.00)/2.00 × 100 = +8%. The sample gained mass, consistent with net water entry if other mass changes are negligible.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A plant cell gains water but eventually stops gaining mass while remaining in a solution containing less solute than the cell. Explain why this is possible.
Show a hint
Water potential depends on more than solute concentration.
Explore the reasoning
Water entry increases pressure as the wall resists expansion. The pressure contribution can raise the cell’s water potential until it equals the external potential. Equal water potential does not require equal solute concentration.
Recall this lesson
Lesson recall questions
Question 1 of 5: Compare a partially permeable membrane with a fully permeable membrane in the context of osmosis.
Osmosis and water potential
Practice question
Retrieval and explanation · suggested answer
Compare a partially permeable membrane with a fully permeable membrane in the context of osmosis.
Lesson: 2A 3: Osmosis: A Special Case of Diffusion
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Water has specialised routes
2A · Osmosis: A Special Case of DiffusionThe discovery of aquaporins showed that proteins can provide selective water channels in membranes. This connects membrane structure to the speed of water movement. In the work recognised…Specification 2.4 · 2.5(ii)Read & use the scienceClose article
Why this matters
The discovery of aquaporins showed that proteins can provide selective water channels in membranes. This connects membrane structure to the speed of water movement. In the work recognised by the 2003 Nobel Prize, comparing cells with and without an introduced channel helped test whether that protein increased water permeability. Faster water movement was a functional result, not just an image of a membrane component. Aquaporins provide a route; they do not supply energy to push water against its water-potential gradient. This distinction between permeability and driving force is useful whenever research identifies a new transport protein and claims it changes cell behaviour.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.4 understand what is meant by osmosis in terms of the movement of free water molecules through a partially permeable membrane, down a water potential gradient
- 2.5(ii) understand the involvement of carrier and channel proteins in membrane transport
Use the science
Would adding more water channels necessarily reverse the direction of osmosis?
Explore a possible response
No. It can change permeability and therefore rate, but net direction still follows the water-potential gradient.
Explore the source
- Nobel Prize: discovery of water channelsNobel Prize
Making drinking water by reversing the usual flow
2A · Osmosis: A Special Case of DiffusionReverse-osmosis plants use pressure and a selectively permeable membrane to separate water from dissolved salts. Without the imposed pressure, water tends to move towards the more concentrated solution…Specification 2.4Read & use the scienceClose article
Why this matters
Reverse-osmosis plants use pressure and a selectively permeable membrane to separate water from dissolved salts. Without the imposed pressure, water tends to move towards the more concentrated solution under otherwise comparable conditions. The pressure contribution to water potential changes that situation, allowing a net flow towards the freshwater side. Engineers have to balance water recovery against energy use, membrane performance and the concentrated brine left behind. This is a useful local and global water-supply application of osmosis, but it is not a biological carrier pump using ATP. The membrane and the external energy supply play different roles.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.4 understand what is meant by osmosis in terms of the movement of free water molecules through a partially permeable membrane, down a water potential gradient
Use the science
If the pressure supply stopped, why would the same membrane not continue producing freshwater at the same rate and direction?
Explore a possible response
The pressure contribution that drives reverse flow would be lost; the remaining water-potential difference would favour ordinary osmotic movement towards the saltier side.
Explore the source
- DesalinationUS Geological Survey