Osmosis (Including Required Practical)

This lesson covers osmosis — a special type of diffusion involving water — as required by the AQA GCSE Combined Science Trilogy specification (8464). You need to understand the definition, the effects of osmosis on plant and animal cells and be able to describe and analyse the Required Practical investigation.

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What Is Osmosis?

Osmosis is the movement of water molecules from a dilute solution (high water concentration) to a concentrated solution (low water concentration) through a partially permeable membrane.

More precisely:

$\text{Osmosis is the net movement of water molecules across a partially permeable membrane from a region of higher water potential to a region of lower water potential.}$Osmosis is the net movement of water molecules across a partially permeable membrane from a region of higher water potential to a region of lower water potential.

Key points:

• Osmosis is a passive process — it does not require energy from respiration.
• It is a special case of diffusion, but specifically involves water molecules.
• A partially permeable membrane (also called a selectively permeable membrane) has tiny pores that allow small molecules (like water) through, but block larger molecules (like sugar or protein).
• Water moves down its own concentration gradient — from where there is more water (dilute solution) to where there is less water (concentrated solution).

graph LR
    A["Dilute solution<br/>(high water<br/>concentration)"] -->|"Water molecules<br/>move by OSMOSIS"| B["Concentrated solution<br/>(low water<br/>concentration)"]
    C["Partially permeable<br/>membrane"] --- A
    C --- B

Exam Tip: Be precise with the definition. Osmosis involves: (1) water molecules, (2) movement from dilute to concentrated (or high to low water potential), and (3) across a partially permeable membrane. All three elements are needed for full marks.

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Water Potential

Water potential is a measure of the tendency of water molecules to move from one place to another. It is a more precise way to describe osmosis:

• Pure water has the highest water potential.
• Adding solute (e.g. sugar, salt) to water lowers the water potential.
• Water moves by osmosis from high water potential to low water potential.

Solution	Water Potential	Description
Pure water	Highest	No solute dissolved
Dilute solution	High	Small amount of solute
Concentrated solution	Low	Large amount of solute

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Effects of Osmosis on Plant Cells

Plant cells have a cell wall made of cellulose, which provides structural support. This affects how plant cells respond to osmosis.

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

1. Water moves into the cell by osmosis (higher water potential outside → lower water potential inside).
2. The vacuole swells and pushes the cytoplasm against the cell wall.
3. The cell becomes turgid (firm and swollen).
4. The cell wall prevents the cell from bursting.
5. Turgid cells provide support for the plant — this is why well-watered plants stand upright.

In a Concentrated Solution

1. Water moves out of the cell by osmosis (lower water potential outside → higher water potential inside).
2. The vacuole shrinks.
3. The cytoplasm pulls away from the cell wall — this is called plasmolysis.
4. The cell becomes flaccid (soft, limp).
5. If many cells become flaccid, the plant wilts.

graph LR
    A["Dilute solution<br/>(pure water)"] --> B["TURGID<br/>Cell swells<br/>Vacuole full<br/>Cell wall<br/>prevents bursting"]
    C["Isotonic solution<br/>(balanced)"] --> D["NORMAL<br/>No net movement<br/>of water"]
    E["Concentrated<br/>solution"] --> F["PLASMOLYSED<br/>Cell shrinks<br/>Vacuole shrinks<br/>Cytoplasm pulls<br/>from wall"]

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Effects of Osmosis on Animal Cells

Animal cells do not have a cell wall, so they respond differently to osmosis:

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

1. Water moves into the cell by osmosis.
2. The cell swells and may eventually burst (lysis) — because there is no cell wall to prevent it.

In a Concentrated Solution

1. Water moves out of the cell by osmosis.
2. The cell shrinks and becomes crenated (wrinkled).

In an Isotonic Solution

1. The concentration of water is the same inside and outside the cell.
2. There is no net movement of water.
3. The cell remains normal size.

Environment	Plant Cell	Animal Cell
Dilute solution	Turgid (firm) — cell wall prevents bursting	Swells and may burst (lysis)
Isotonic solution	Normal	Normal
Concentrated solution	Plasmolysed (flaccid) — cytoplasm pulls from wall	Crenated (shrivelled)

Exam Tip: A key difference: plant cells do not burst in water because of their cell wall; animal cells can burst because they lack a cell wall. This is a common comparison question.

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Required Practical — Investigating Osmosis

Aim

To investigate the effect of different concentrations of sugar (sucrose) solution on the mass of potato cylinders.

Equipment

• Potato, cork borer, ruler, scalpel, balance (to 2 decimal places), boiling tubes, sucrose solutions of different concentrations (e.g. 0.0 M, 0.2 M, 0.4 M, 0.6 M, 0.8 M, 1.0 M), distilled water, paper towels, timer.

Method

1. Use a cork borer to cut potato cylinders of equal diameter.
2. Cut them all to the same length (e.g. 3 cm) using a ruler and scalpel.
3. Blot each cylinder dry with a paper towel and measure its mass on a balance. Record the initial mass.
4. Place one cylinder into each boiling tube containing a different concentration of sucrose solution.
5. Leave the cylinders in the solutions for a set time (e.g. 30 minutes to 24 hours).
6. Remove each cylinder, blot dry gently and re-weigh to find the final mass.
7. Calculate the percentage change in mass for each cylinder.

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

Exam Tip: Always use percentage change rather than absolute change (in grams), because the starting masses may not be exactly the same. Percentage change allows a fair comparison.

Worked Example

Question: A potato cylinder has an initial mass of 2.40 g. After being placed in distilled water for 30 minutes, its final mass is 2.76 g. Calculate the percentage change in mass.

Solution:

$\text{Percentage change} = \frac{2.76 - 2.40}{2.40} \times 100 = \frac{0.36}{2.40} \times 100 = 15.0\%$Percentage change=2.402.76−2.40​×100=2.400.36​×100=15.0%

The mass increased by 15.0% — water moved into the potato cells by osmosis because the distilled water had a higher water potential than the cell contents.

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Expected Results

Sucrose Concentration	What Happens	Mass Change
0.0 M (distilled water)	Water moves into potato cells by osmosis (dilute → concentrated). Cells become turgid.	Increases (positive %)
Low concentration (e.g. 0.2 M)	Water still moves in — solution is more dilute than cell sap.	Increases (smaller positive %)
Isotonic point (approximately equal to cell sap concentration)	No net movement of water.	No change (0%)
High concentration (e.g. 0.8 M, 1.0 M)	Water moves out of potato cells by osmosis (concentrated outside). Cells become flaccid.	Decreases (negative %)

Graphing the Results

When the percentage change in mass is plotted against sucrose concentration: