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
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5Water Potential and Osmosis
Osmosis is a special case of diffusion that applies only to water.
Water potential (Ψ)
- 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 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
- Solutes lower Ψ Dissolved solute particles attract water molecules, so a more concentrated solution has a more negative water potential.
Osmosis
- 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
- Partially permeable membrane The phospholipid bilayer lets small uncharged molecules such as water pass but restricts larger or charged solutes.
Equilibrium
- Equilibrium Net movement stops when the water potential is equal on both sides, although molecules still cross in both directions.
Section 2 of 5Cells in Different Solutions
Cells in Different Solutions
Animal cell in dilute solution
- 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
- 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
- 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
- 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 |
Section 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.
Section 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.
Section 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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