Transport in Cells: Osmosis and Active Transport

This lesson covers osmosis and active transport as required by AQA GCSE Biology specification 4.1.3. You need to understand the process of osmosis, how to investigate it experimentally, the effects of osmosis on plant and animal cells, and how active transport differs from diffusion and osmosis.

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What Is Osmosis?

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

Osmosis is a special case of diffusion — it specifically describes the movement of water molecules only, and it always involves a partially permeable membrane.

Key Definitions

Term	Definition
Osmosis	The diffusion of water molecules from a dilute solution to a more concentrated solution through a partially permeable membrane.
Partially permeable membrane	A membrane that allows some molecules (e.g. water) to pass through but blocks larger molecules (e.g. sugar, starch).
Dilute solution	A solution with a high proportion of water molecules and a low proportion of solute molecules.
Concentrated solution	A solution with a low proportion of water molecules and a high proportion of solute molecules.
Water potential	A measure of the tendency of water to move from one place to another. Water moves from higher water potential to lower water potential. [H]
Turgor pressure	The pressure exerted by the cell contents pushing against the cell wall in a plant cell when water enters by osmosis.

Exam Tip: The AQA specification defines osmosis as the diffusion of water from a dilute to a more concentrated solution through a partially permeable membrane. Make sure you include ALL parts of this definition — movement of water, dilute to concentrated, AND through a partially permeable membrane. Missing any part will lose marks.

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How Osmosis Works

Water molecules are small enough to pass through the partially permeable membrane. Solute molecules (e.g. sugar, salt) are too large to pass through.

• In a dilute solution, there is a higher concentration of water molecules.
• In a concentrated solution, there is a lower concentration of water molecules (because more space is taken up by solute molecules).
• Water molecules move from the dilute side to the concentrated side through the membrane, down the water concentration gradient.

flowchart LR
    A["DILUTE Solution<br/>High water concentration<br/>Low solute concentration"] -->|"Water molecules move by OSMOSIS"| B["Partially Permeable<br/>Membrane"]
    B --> C["CONCENTRATED Solution<br/>Low water concentration<br/>High solute concentration"]

    style A fill:#3498db,color:#fff
    style B fill:#95a5a6,color:#fff
    style C fill:#e74c3c,color:#fff

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Effects of Osmosis on Cells

The effect of osmosis on a cell depends on the concentration of the solution surrounding the cell:

Animal Cells

Solution	Water Movement	Effect on Cell
Dilute (hypotonic) solution — more dilute than the cell's cytoplasm	Water moves INTO the cell by osmosis	The cell swells and may burst (lysis). Animal cells have no cell wall to prevent this.
Isotonic solution — same concentration as the cell's cytoplasm	No net movement of water	The cell stays the same size — equilibrium.
Concentrated (hypertonic) solution — more concentrated than the cell's cytoplasm	Water moves OUT of the cell by osmosis	The cell shrinks and becomes crenated (wrinkled/shrivelled).

Plant Cells

Solution	Water Movement	Effect on Cell
Dilute (hypotonic) solution	Water moves INTO the cell by osmosis	The cell becomes turgid (swollen and firm). The cell wall prevents the cell from bursting. Turgor pressure supports the plant.
Isotonic solution	No net movement of water	The cell stays the same — flaccid (neither turgid nor plasmolysed).
Concentrated (hypertonic) solution	Water moves OUT of the cell by osmosis	The cell becomes plasmolysed — the cell membrane pulls away from the cell wall as the cytoplasm and vacuole shrink. The plant wilts.

Exam Tip: Know the difference between turgid (firm, full of water — healthy plant cell), flaccid (limp — plant cell that has lost some water), and plasmolysed (cell membrane pulled away from cell wall — severe water loss). Also remember that animal cells can BURST in dilute solutions because they lack a cell wall, but plant cells do not burst — they become turgid.

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Investigating Osmosis: Required Practical

You can investigate the effect of different concentrations of sugar (sucrose) solution on plant tissue using potato cylinders. This is a common required practical.

Method

1. Cut several potato cylinders of equal length (e.g. 5 cm) and diameter using a cork borer.
2. Measure and record the initial mass (and/or length) of each cylinder.
3. Prepare a range of 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).
4. Place one potato cylinder in each solution and leave for a set time (e.g. 24 hours).
5. Remove the cylinders, blot them dry gently with paper towel, and measure the final mass (and/or length).
6. Calculate the percentage change in mass for each cylinder.

Calculating Percentage Change

percentage change = ((final mass - initial mass) / initial mass) x 100

Expected Results

Solution Concentration	Expected Result	Explanation
Distilled water (0.0 M)	Mass increases	Water enters the potato cells by osmosis (the potato cell is more concentrated than pure water).
Low concentration	Mass increases slightly	Some water enters by osmosis.
Isotonic point (around 0.4–0.5 M for potato)	No change in mass	The solution has the same water concentration as the potato cells — no net osmosis.
High concentration	Mass decreases	Water leaves the potato cells by osmosis (the solution is more concentrated than the cells).

Graph

Plot a graph of percentage change in mass (y-axis) against sucrose concentration (x-axis). The point where the line crosses the x-axis (0% change) indicates the concentration at which the solution is isotonic with the potato cells.

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What Is Active Transport?

Active transport is the movement of substances from a region of lower concentration to a region of higher concentration — that is, against the concentration gradient. This requires energy from cellular respiration.

Key Differences: Diffusion vs Osmosis vs Active Transport

Feature	Diffusion	Osmosis	Active Transport
What moves	Any particles (molecules, ions)	Water molecules only	Specific molecules or ions
Direction	High to low concentration (down the gradient)	Dilute to concentrated (down the water gradient)	Low to high concentration (against the gradient)
Energy required?	No (passive)	No (passive)	Yes — from cellular respiration (ATP)
Membrane required?	No (but can cross a membrane)	Yes — partially permeable membrane	Yes — requires carrier proteins in the membrane
Examples	O2 in lungs, CO2 out of cells	Water into root hair cells, water into red blood cells	Mineral ions into root hair cells, glucose absorption in the gut

flowchart TD
    A["Transport Across Cell Membranes"] --> B["PASSIVE (no energy needed)"]
    A --> C["ACTIVE (energy required)"]
    B --> D["Diffusion<br/>High → Low concentration"]
    B --> E["Osmosis<br/>Dilute → Concentrated<br/>(water only, through PPM)"]
    C --> F["Active Transport<br/>Low → High concentration<br/>(against the gradient)"]

    style A fill:#8e44ad,color:#fff
    style B fill:#3498db,color:#fff
    style C fill:#e74c3c,color:#fff