OCR · A-Level · Biology
Course version: H420 · Version from Sept 2025
(e) (i) the movement of water across membranes by osmosis and the effects…
Full specification wording
Biological membranes: (e) (i) the movement of water across membranes by osmosis and the effects that solutions of different water potential can have on plant and animal cells
Osmosis is the movement of water across a partially permeable membrane from a region of higher water potential to a region of lower water potential.
Checked against the OCR H420 specification (opens in a new tab) on .
- Point 7 of 8
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Osmosis is the movement of water across a partially permeable membrane from a region of higher water potential to a region of lower water potential. Water potential is measured in kilopascals (kPa); pure water has a water potential of 0 kPa, and solutions have negative values. If a plant cell sits in a solution with higher water potential than its contents, water enters, the vacuole swells and the cell becomes turgid, which supports the plant. In a solution with lower water potential, water leaves, the vacuole shrinks and the membrane pulls away from the cell wall, giving plasmolysis. Animal cells lack a cell wall: in a higher water potential solution they swell and may burst (lysis); in a lower water potential solution they shrink and become crenated.
What earns the marks
- Osmosis is the net movement of water across a partially permeable membrane from higher to lower water potential.
- Water potential is measured in kPa; pure water is 0 kPa and solutions have negative water potential.
- In a solution of higher water potential than the cell contents, water enters a plant cell, the vacuole swells and the cell becomes turgid.
- In a solution of lower water potential, water leaves a plant cell, the vacuole shrinks and the membrane pulls away from the cell wall (plasmolysis).
- Animal cells have no cell wall, so in a higher water potential solution they may swell and burst (lysis).
- In a lower water potential solution, animal cells lose water, shrink and become crenated.
Common mistakes and how to put them right
Saying water moves from low to high water potential.
Put it right: Water moves from higher to lower water potential.
Treating a concentrated solution as having high water potential.
Put it right: A more concentrated solution has a lower (more negative) water potential.
Assuming animal cells become turgid like plant cells.
Put it right: Without a cell wall, animal cells swell and may lyse rather than becoming turgid.
Exam tips
- Always compare water potentials explicitly, stating which side is higher and which is lower.
- Use the correct terms: turgid and plasmolysed for plant cells, lysed and crenated for animal cells.
- Remember that water potential values for solutions are negative, so a 'more negative' value means a lower water potential.
Try the lesson
The start of the Biological membranes: (e) (i) the movement of water across membranes by osmosis and the effects that solutions of different water potential can have on plant and animal cells lesson, made from checked teaching notes for this specification point.
Have a go first
Which of the following best describes osmosis?
Listen
Osmosis is the movement of water across a partially permeable membrane from a region of higher water potential to a region of lower water potential. Water potential is measured in kilopascals (kPa); pure water has a water potential of 0 kPa, and solutions have negative values. If a plant cell sits in a solution with higher water potential than its contents, water enters, the vacuole swells and the cell becomes turgid, which supports the plant. In a solution with lower water potential, water leaves, the vacuole shrinks and the membrane pulls away from the cell wall, giving plasmolysis. Animal cells lack a cell wall: in a higher water potential solution they swell and may burst (lysis); in a lower water potential solution they shrink and become crenated. A strong answer earns marks for these points. Osmosis is the net movement of water across a partially permeable membrane from higher to lower water potential. Water potential is measured in kPa; pure water is 0 kPa and solutions have negative water potential. In a solution of higher water potential than the cell contents, water enters a plant cell, the vacuole swells and the cell becomes turgid. In a solution of lower water potential, water leaves a plant cell, the vacuole shrinks and the membrane pulls away from the cell wall (plasmolysis). A common slip: Saying water moves from low to high water potential. Instead, water moves from higher to lower water potential. Exam tip: Always compare water potentials explicitly, stating which side is higher and which is lower.
Key words
Say what each word means before you turn the card.
Osmosis
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Water potential
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Lysis and crenation in animal cells
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See every definition
- Osmosis
- The movement of water across a partially permeable membrane from a region of higher water potential to a region of lower water potential.
- Water potential
- Measured in kilopascals (kPa); pure water has a water potential of 0 kPa, and solutions have negative values.
- Lysis and crenation in animal cells
- Lacking a cell wall, animal cells in a higher water potential solution swell and may burst (lysis), and in a lower water potential solution they shrink and become crenated.
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