Ch 2 · Osmosis · O Level Biology 5090
Topic 2 of 3 in Movement into and out of Cells — O Level Biology 5090 — 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, if you want to deliver packages (nutrients) or pick up trash (waste), you don't just throw them across the street; you put them in a vehicle that can move through the streets. In your body, water is that vehicle. It dissolves molecules so they can move freely through fluids like blood or cytoplasm.
📖 Definitions
🔍 Worked example — Easy
State the role of water as a solvent in the process of digestion.
🔍 Worked example — Standard
A plant cell is placed in a solution with a lower water potential than the cell sap. Describe the effect on the cell's appearance using appropriate terminology.
🔍 Worked example — Exam-style
Calculate the percentage change in the mass of a plant tissue piece used in an osmosis experiment. Initial mass = 0.50 g, final mass = 0.42 g.
⚠️ Common errors
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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
Think of osmosis as a crowd of people trying to balance out a room. Imagine a room divided by a screen that only allows tiny marbles to pass through, but blocks large basketballs. If one side of the room is packed with marbles and the other side has very few, the marbles will naturally spread out through the screen until both sides are equally 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. Initial mass = 5.20 g, final mass = 4.85 g.
🔍 Worked example — Exam-style
Explain why a red blood cell placed in distilled water bursts (hemolysis).
⚠️ Common errors
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🎯 Exam question
Key ideas — Plant tissues in different solutions
Animal cells. Animal cells have no cell wall. In pure water they take in water by osmosis, swell and burst. In a concentrated solution they lose water, shrink and become crenated. This is why the water potential of blood plasma must be kept constant by the kidneys.
🧑🏫 Teacher's explanation
Think of osmosis as a crowd of people (water molecules) trying to balance out the density of a room. If one room is packed with people and the next is nearly empty, people will naturally move toward the crowded room to spread out. In biology, 'crowded' means a low concentration of solutes and a high concentration of water molecules.
📖 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 and the force exerted on the cell wall.
🔍 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
Compare the response of a red blood cell and a plant parenchyma cell when placed in pure water.
⚠️ Common errors
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🎯 Exam question
(a) Explain why the mass of the potato decreased. [2] (b) Describe what would happen to the appearance of the potato if it were placed in pure water. [2] [4 marks]
Key ideas — Investigating osmosis
🧑🏫 Teacher's explanation
Imagine a crowded room where everyone is trying to move to a quieter area. In biology, water molecules behave similarly; they naturally move from where they are 'crowded' (high water potential) to where they are 'sparse' (low water potential). This movement isn't just random; it's a directed flow through a selective barrier.
📖 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, its mass increases to 5.4 g. Calculate the percentage change in mass.
🔍 Worked example — Standard
A student uses dialysis tubing filled with 10 cm^3 of 0.4 M sucrose solution. The tubing is placed in distilled water. Predict the change in volume of the solution inside the tubing and explain why.
🔍 Worked example — Exam-style
In an experiment, potato cylinders were placed in various sucrose concentrations. The results showed: 0.0 M (+10\% mass), 0.2 M (+4\% mass), 0.4 M (0\% mass), 0.6 M (-6\% mass). Determine the concentration of the cell sap in the potato.
⚠️ Common errors
🧠 Quick check 1
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🎯 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 water potential of the cell sap is -200 kPa. State the direction of the net movement of water and the resulting state of the cell. [2]
🔍 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. [2]
🔍 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
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