2.1.4Unit 2: Cells, Development, Biodiversity and Conservation
Osmosis — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 5 parts Lesson 11 of 26 Unit quiz
Part 1 of 5Water Potential and Osmosis
Osmosis is a special case of diffusion that applies only to water.
Water potential (Ψ)
- A measure of the tendency of water molecules to move;
- it is measured in kilopascals (kPa) and is highest in pure water.
Pure water
- Pure water at standard pressure is defined as having a water potential of zero, and every solution has a value below this.
Solutes lower Ψ
- Dissolved solute particles attract water molecules, so a more concentrated solution has a more negative water potential.
Osmosis
- The net movement of water molecules across a partially permeable membrane from a region of higher water potential to one of lower water potential.
Partially permeable membrane
- The phospholipid bilayer lets small uncharged molecules such as water pass but restricts larger or charged solutes.
Equilibrium
- Net movement stops when the water potential is equal on both sides, although molecules still cross in both directions.
Part 2 of 5Cells in Different Solutions
Cells in Different Solutions
Animal cell in dilute solution
- Water enters by osmosis, and with no cell wall to resist, the cell swells and may burst, which is called lysis.
Animal cell in concentrated solution
- Water leaves by osmosis and the cell shrinks and develops a crinkled outline, which is called crenation.
Plant cell in dilute solution
- Water enters, the vacuole swells and pushes the membrane against the cell wall, making the cell firm or turgid.
Plant cell in concentrated solution
- Water leaves, the cytoplasm shrinks away from the wall, and the cell becomes flaccid then plasmolysed.
| Solution outside | Water movement | Plant cell | Animal cell |
|---|---|---|---|
| Higher Ψ than cell | Into the cell | Turgid | Lysis |
| Same Ψ as cell | No net movement | Flaccid | Normal |
| Lower Ψ than cell | Out of the cell | Plasmolysed | Crenated |
Part 3 of 5Investigating Osmosis in Tissue
1
Cut identical cylinders of potato with a cork borer and trim them to the same length, then blot them dry and measure the mass of each.
2
Place each cylinder in a different sucrose concentration, including distilled water, and keep volume, temperature and time the same.
3
After a fixed time, remove the cylinders, blot off surface liquid gently, and reweigh each one.
4
Calculate percentage change in mass for each concentration so that cylinders of different starting mass can be compared fairly.
5
Plot percentage change against concentration; the point where the line crosses zero shows the concentration with the same water potential as the tissue.
Part 4 of 5Worked Example
Question
- A potato cylinder has a starting mass of 2.40 g.
- After 60 minutes in a sucrose solution its mass is 2.64 g.
- Calculate the percentage change in mass and state what this shows about the solution.
- Change in mass = final mass − initial mass = 2.64 − 2.40 = 0.24 g.
- Percentage change = (change ÷ initial mass) × 100 = (0.24 ÷ 2.40) × 100.
- (0.24 ÷ 2.40) = 0.10, so the percentage change is +10.0%.
Answer
- +10.0%;
- the cylinder gained mass, so water entered by osmosis, meaning the solution had a higher water potential than the potato cells.
Part 5 of 5Exam Checklist & Tips
2.1.4 – Key Points for the Exam
- Water potential
- The tendency of water to move out of a solution;
- pure water is 0 kPa and solutions are negative.
- The tendency of water to move out of a solution;
- Osmosis Net movement of water down a water potential gradient through a partially permeable membrane.
- Turgid A plant cell that is full of water, with the membrane pressed firmly against the cell wall.
- Plasmolysis The membrane pulling away from the cell wall of a plant cell that has lost water.
- Lysis Bursting of an animal cell that has taken in too much water by osmosis.
Common Mistakes
- Saying water moves from high concentration to low concentration;
- it should be described as moving from higher to lower water potential.
- Writing that a concentrated solution has a high water potential, when more solute makes the value more negative.
- Forgetting that plant cells do not burst because the cell wall resists further expansion.
- Not blotting tissue before reweighing, so surface water gives a false increase in mass.
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