Osmosis

This lesson covers osmosis as required by the Edexcel GCSE Combined Science specification (1SC0). You need to define osmosis, explain the concept of water potential, describe the effect of osmosis on plant and animal cells, and understand the practical investigation using potato cylinders.

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What Is Osmosis?

Osmosis is the movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) through a partially permeable membrane.

Key points:

• Osmosis is a special case of diffusion — it specifically involves water molecules.
• A partially permeable membrane has tiny pores that allow small molecules (water) to pass through but block larger molecules (e.g. sugar, starch).
• Osmosis is a passive process — it does not require energy from the cell.
• Water moves down the water potential gradient.

Exam Tip: The Edexcel specification uses the term "water potential" rather than "concentration". Water moves from high water potential to low water potential. A dilute solution has a higher water potential than a concentrated solution.

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Understanding Water Potential

Water potential is a measure of the tendency of water molecules to move from one area to another.

Solution	Water Potential	Description
Pure water	Highest water potential	Contains only water molecules
Dilute solution	High water potential	Contains a small amount of dissolved solute
Concentrated solution	Low water potential	Contains a large amount of dissolved solute

Water always moves from high water potential to low water potential through a partially permeable membrane.

graph LR
    A["Dilute solution (high water potential)"] -->|Water moves by osmosis| B["Concentrated solution (low water potential)"]
    subgraph Partially Permeable Membrane
        C[Small pores allow water through]
    end

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Effect of Osmosis on Plant Cells

In a Dilute Solution (e.g. Pure Water)

• Water moves into the cell by osmosis (from high water potential outside to low water potential inside).
• The vacuole swells and pushes the cytoplasm against the cell wall.
• The cell becomes turgid (firm and swollen).
• The cell wall prevents the cell from bursting.
• Turgid cells give the plant its structural support — the plant stands upright.

In a Concentrated Solution

• Water moves out of the cell by osmosis (from high water potential inside to low water potential outside).
• The vacuole shrinks and the cytoplasm pulls away from the cell wall.
• This is called plasmolysis — the cell is plasmolysed.
• The cell becomes flaccid (soft and limp).
• If many cells are plasmolysed, the plant wilts.

In an Isotonic Solution

• The concentration of the solution outside equals the concentration inside the cell.
• There is no net movement of water (water moves equally in both directions).
• The cell stays the same size — it is flaccid (neither turgid nor plasmolysed).

Exam Tip: Plant cells do not burst in pure water because of the cell wall. The cell wall is strong enough to withstand the pressure of the expanding vacuole.

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Effect of Osmosis on Animal Cells

Animal cells have no cell wall, so the effects of osmosis are different:

Solution	Effect on Animal Cell
Dilute (hypotonic)	Water enters by osmosis → cell swells → may burst (lyse)
Isotonic	No net movement of water → cell stays normal
Concentrated (hypertonic)	Water leaves by osmosis → cell shrinks (crenation)

Exam Tip: A key difference — plant cells become turgid in dilute solutions (cell wall prevents bursting); animal cells lyse (burst) because they have no cell wall.

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Practical Investigation: Osmosis with Potato Cylinders

This is a required practical that you must be able to describe in detail.

Aim

To investigate the effect of different sucrose concentrations on the mass of potato cylinders.

Method

1. Use a cork borer to cut several potato cylinders of the same length (e.g. 3 cm) and same diameter.
2. Blot each cylinder with a paper towel and weigh on a balance. Record the initial mass.
3. Prepare a range of sucrose solutions at different concentrations (e.g. 0.0 M, 0.2 M, 0.4 M, 0.6 M, 0.8 M, 1.0 M).
4. Place one potato cylinder into each solution. Leave for a set time (e.g. 24 hours) in a sealed container.
5. Remove each cylinder, blot dry and reweigh. Record the final mass.
6. Calculate the percentage change in mass for each cylinder.
7. Repeat with at least 3 cylinders per concentration and calculate a mean.

Calculating Percentage Change in Mass

$\text{Percentage change} = \frac{\text{Final mass} - \text{Initial mass}}{\text{Initial mass}} \times 100$Percentage change=Initial massFinal mass−Initial mass​×100

Expected Results

Sucrose Concentration	Direction of Water Movement	Mass Change	Cell State
Low (e.g. 0.0 M)	Water enters cells by osmosis	Mass increases	Turgid
Isotonic point (~0.4 M)	No net movement	No change	Flaccid
High (e.g. 1.0 M)	Water leaves cells by osmosis	Mass decreases	Plasmolysed

Worked Example

A potato cylinder has an initial mass of 2.5 g. After 24 hours in a dilute sucrose solution, its final mass is 2.9 g.

Percentage change = (2.9 − 2.5) ÷ 2.5 × 100 = 0.4 ÷ 2.5 × 100 = +16.0%

The positive value indicates the potato gained mass — water entered the cells by osmosis.

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Variables

Variable	Detail
Independent	Concentration of sucrose solution
Dependent	Percentage change in mass of potato cylinder
Control variables	Length and diameter of potato cylinders, volume of solution, temperature, time left in solution, type of potato

Exam Tip: Always use percentage change rather than just mass change, because the starting masses may differ slightly. Percentage change allows a fair comparison.

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Summary

• Osmosis is the movement of water from a region of higher water potential to a region of lower water potential through a partially permeable membrane. It is passive.
• Water moves from dilute (high water potential) to concentrated (low water potential) solutions.
• In dilute solutions, plant cells become turgid; animal cells may lyse (burst).
• In concentrated solutions, plant cells become plasmolysed; animal cells shrink (crenation).
• The potato cylinder practical measures percentage change in mass at different sucrose concentrations.
• Use the formula: percentage change = (final − initial) ÷ initial × 100.

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Deeper Look: Water Potential and the Cell

Osmosis is arguably the most important form of passive transport in biology. Plants stand upright because their cells are turgid; your red blood cells maintain their shape because they are in isotonic plasma; kidney dialysis relies on controlled osmosis across a dialysis membrane. Understanding osmosis means understanding how water moves in every part of every living organism.

What Is a Partially Permeable Membrane?