AS 1 osmosis question — recurring in most AS 1 series · 5 marks
Plant cells were placed in a concentrated sucrose solution. Explain, in terms of water potential, what happens to the cells.
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AS · AS 1 Molecules and Cells
Water potential: the right way to explain osmosis
The short answer
Water potential (Ψ) is the tendency of water to move out of a system, measured in kilopascals. Pure water is 0 kPa and adding solute makes it negative. Water always moves by osmosis from a less negative (higher) water potential to a more negative (lower) one, through a partially permeable membrane.
The equation and its two parts
Ψ = Ψs + Ψp. Solute potential (Ψs) is always negative and becomes more negative as solute concentration rises. Pressure potential (Ψp) is usually positive in a turgid plant cell, where the cell wall pushes back on the protoplast.
- Turgid: Ψp is high, the cell is full and the wall prevents bursting.
- Incipient plasmolysis: the membrane just begins to pull away, so Ψp = 0 and Ψ = Ψs.
- Plasmolysed: the protoplast has shrunk away from the wall in a strongly hypertonic solution.
Animal cells behave differently
Animal cells have no cell wall, so there is no pressure potential to resist water entry. In a hypotonic solution they swell and burst (lysis or haemolysis in red blood cells); in a hypertonic solution they shrink and crenate. This is exactly why osmoregulation matters physiologically.
Answering osmosis questions
Almost every mark comes from three clauses joined together: name the direction of the water potential gradient, say the water moves by osmosis through a partially permeable membrane, then state the consequence for the cell. Always compare two named water potentials rather than saying 'more water'.
Phrases that earn the marks
- water moves from a higher (less negative) to a lower (more negative) water potential
- by osmosis through a partially permeable membrane
- down a water potential gradient
- Ψ = Ψs + Ψp
- the cell becomes turgid / plasmolysed
Where marks get lost
- Writing 'water concentration' instead of water potential.
- Saying water moves from low to high water potential, or getting 'less negative' backwards.
- Leaving out 'partially permeable membrane'.
- Using 'hypotonic' and 'hypertonic' without saying which solution is which.
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These questions are written in the style of the CCEA GCE Biology (2016) papers and are not reproduced from any live paper. CCEA owns the copyright in its question papers and mark schemes — always download the official paper and mark scheme from CCEA and use these worked answers alongside them. BioCCEA is an independent study tool and is not endorsed by CCEA.
Exam questions on water potential a level biology, marked
Two AS-standard questions in CCEA paper style, each with the full mark scheme, a full-mark answer written the way you should write it, and the point where most candidates drop marks.
AS 1 osmosis question — recurring in most AS 1 series · 5 marks
Plant cells were placed in a concentrated sucrose solution. Explain, in terms of water potential, what happens to the cells.
Command word: Explain — how to answer itMark scheme — 5 creditworthy points
- The sucrose solution has a lower (more negative) water potential than the cell contents
- Water moves out of the cell by osmosis, down a water potential gradient
- Movement is through the partially permeable cell surface membrane
- The protoplast shrinks and pulls away from the cell wall — plasmolysis
- The cell becomes flaccid and loses turgor pressure
Full-mark answer
The concentrated sucrose solution has a lower, more negative water potential than the contents of the cell. Water therefore moves out of the cell by osmosis, down a water potential gradient, through the partially permeable cell surface membrane. As water leaves, the cell loses turgor pressure and becomes flaccid; if enough water is lost the protoplast shrinks and pulls away from the cell wall, and the cell is plasmolysed.
Examiner insight: The direction must be stated as 'higher (less negative) to lower (more negative) water potential'. Writing 'from high to low concentration' is not credited at A level.
AS 1 / A2 1 definition question · 4 marks
Explain the difference between water potential, solute potential and pressure potential in a plant cell, and state the water potential of pure water.
Command word: Explain — how to answer itMark scheme — 4 creditworthy points
- Water potential (Ψ) is the tendency of water to move from one place to another / free energy of water
- Solute potential (Ψs) is the reduction in water potential caused by dissolved solutes and is always negative
- Pressure potential (Ψp) is the pressure exerted by the cell wall on the protoplast and is usually positive
- Ψ = Ψs + Ψp; the water potential of pure water at standard temperature and pressure is 0 kPa
Full-mark answer
Water potential (Ψ) is the tendency of water molecules to move from one place to another. Solute potential (Ψs) is the reduction in water potential caused by dissolved solutes, so it is always negative; the more concentrated the solution, the more negative it is. Pressure potential (Ψp) is the pressure exerted by the cell wall on the cell contents as the cell becomes turgid, and it is usually positive. The two combine as Ψ = Ψs + Ψp. Pure water at standard temperature and pressure has a water potential of 0 kPa, which is the maximum possible value.
Examiner insight: State the sign of each component — 'always negative' and 'usually positive' are creditworthy. Quote the unit kPa with the zero value.
These questions are written in the style of the CCEA GCE Biology (2016) papers and are not reproduced from any live paper. CCEA owns the copyright in its question papers and mark schemes — always download the official paper and mark scheme from CCEA and use these worked answers alongside them. BioCCEA is an independent study tool and is not endorsed by CCEA.
Download the official CCEA paper and mark schemeCommon questions
- Why is water potential always negative?
- Pure water is defined as 0 kPa, the maximum. Dissolving anything lowers the tendency of water to leave, so any solution has a negative value.
- What is incipient plasmolysis?
- The point at which the plasma membrane just begins to pull away from the cell wall. Pressure potential is zero, so water potential equals solute potential.
- Which way does water move between two cells?
- From the cell with the less negative water potential to the one with the more negative water potential, until the two are equal.
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