Ch 2 · Osmosis · IGCSE Biology 0610
Topic 2 of 3 in Movement into and out of Cells — IGCSE Biology 0610 — 48 lessons.
Introduction
Water makes up most of the mass of every cell, and its movement decides whether a plant stands upright or wilts and whether an animal cell survives a change in its surroundings. Water crosses membranes by a special case of diffusion called osmosis. This lesson covers water as a solvent, osmosis itself, its effects on plant and animal cells, and the standard practical investigations.
Key ideas — Water as a solvent
🧑🏫 Teacher's explanation
Think of water as the 'universal delivery driver' of the biological world. In a crowded city, goods cannot move effectively if they are stuck in solid blocks; they need to be dissolved in a liquid to flow through streets and reach their destination. In your body, if your nutrients were solid chunks, they couldn't move through your blood or cross into your cells.
📖 Definitions
🔍 Worked example — Easy
State the role of water in the process of digestion.
🔍 Worked example — Standard
Compare the role of water in transport within animals and plants.
🔍 Worked example — Exam-style
A student observes that a plant cell placed in a highly concentrated salt solution becomes plasmolysed. Explain why this occurs in terms of water movement.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Key ideas — Osmosis
Cell membranes are partially permeable: they let water molecules through but not larger dissolved molecules. Osmosis is the diffusion of water through a partially permeable membrane. Water moves from a dilute solution, which contains a lot of water, to a more concentrated solution, which contains less water. This is how water moves into and out of cells, and no energy from respiration is needed.
🧑🏫 Teacher's explanation
Imagine a crowded party in one room and a very empty hallway outside. If the doors are slightly ajar (the partially permeable membrane), people (water molecules) will naturally tend to move from the crowded room to the empty hallway to balance things out. This isn't because they are being pushed, but because there is more 'room' for them to move into where it is less crowded.
📖 Definitions
🔍 Worked example — Easy
A plant cell is placed in a solution with a lower water potential than the cell sap. Describe the effect on the cell.
🔍 Worked example — Standard
Calculate the percentage change in mass for a potato cylinder that starts at 2.50 g and ends at 2.15 g after being placed in a salt solution.
🔍 Worked example — Exam-style
Explain why a red blood cell bursts when placed in distilled water.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Key ideas — Plant tissues in different solutions
🧑🏫 Teacher's explanation
Think of osmosis like a crowd of people trying to balance out a room. If one room is packed with people (low water potential) and the next room is empty (high water potential), people will naturally move toward the crowded room to spread out. In biology, 'people' are water molecules and 'rooms' are the compartments inside and outside your cells.
📖 Definitions
🔍 Worked example — Easy
A plant cell is placed in a solution with a higher water potential than the cell sap. Describe the change in the cell.
🔍 Worked example — Standard
Calculate the percentage change in mass for a plant tissue sample that weighs 0.50 g initially and 0.42 g after being immersed in a concentrated sugar solution.
🔍 Worked example — Exam-style
An animal cell is placed in a solution of very low water potential. Explain why the cell may burst if placed in pure water instead.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Key ideas — Investigating osmosis
🧑🏫 Teacher's explanation
Imagine you have two rooms connected by a door that only allows small, specific people to pass through. If one room is packed with people (high solute concentration) and the other room is mostly empty space (high water concentration), people will naturally move toward the crowded room to balance things out. In biology, water acts like those people, moving to where the 'stuff' (solutes) is more concentrated.
📖 Definitions
🔍 Worked example — Easy
A potato cylinder has an initial mass of 5.0 g. After being placed in a 0.5 M sucrose solution for 30 minutes, its mass is 5.4 g. Calculate the percentage change in mass.
🔍 Worked example — Standard
A student places a Visking tubing bag containing 20 cm^3 of 0.8 M sucrose solution into a beaker of pure water. The volume in the capillary tube increases by 4 cm^3. Explain why the liquid level rises.
🔍 Worked example — Exam-style
In an experiment, the percentage change in mass of potato cylinders was recorded. The results were: 0.4 M sucrose arrow +5\%; 0.2\ ext M sucrose arrow +2\%; 0.0 M sucrose arrow 0\%; 0.2 M sucrose arrow -3\%; 0.4 M sucrose arrow -7\%. Determine the approximate concentration of the cell sap in the potato.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
Potato cylinders were placed in sucrose solutions for 30 minutes. Percentage changes in mass: 0.0 mol/dm³, +8.2%; 0.2 mol/dm³, +3.1%; 0.4 mol/dm³, −2.5%; 0.6 mol/dm³, −7.4%.
🔍 Worked example — Easy
A plant cell is placed in a solution with a water potential of -500 kPa. The cell sap has a water potential of -200 kPa. State the direction of the net movement of water and the resulting state of the cell.
🔍 Worked example — Standard
A 5.0 g potato cylinder is placed in a 0.4 mol/dm^3 sucrose solution. After 1 hour, the mass is 4.8 g. Calculate the percentage change in mass.
🔍 Worked example — Exam-style
Potato cylinders were placed in sucrose solutions for 30 minutes. Percentage changes in mass: 0.0 mol/dm^3, +8.2\%; 0.2 mol/dm^3, +3.1\%; 0.4 mol/dm^3, -2.5\%; 0.6 mol/dm^3, -7.4\%. (a) Explain why the cylinder in 0.6 mol/dm^3 sucrose lost mass. [3] (b) Estimate the concentration of the potato cell sap and explain your reasoning. [2] (c) Explain why the cylinders were blotted dry before weighing. [1]
⚠️ Common errors
Common mistakes
Practice
Answers