Movement of Substances » Diffusion and Osmosis Practical

What you'll learn this session

Study time: 30 minutes

Understand the principles of diffusion and osmosis
Learn how to set up and conduct diffusion and osmosis practical experiments
Interpret experimental results and understand their biological significance
Apply knowledge to real-world biological processes
Develop practical skills for the IGCSE Biology examination

Introduction to Diffusion and Osmosis

Living organisms need to move substances in and out of their cells to survive. Two of the most important processes that make this possible are diffusion and osmosis. These are types of passive transport, which means they don't require energy from the cell.

Key Definitions:

Diffusion: The net movement of particles from an area of higher concentration to an area of lower concentration down a concentration gradient.
Osmosis: The net movement of water molecules from a region of higher water concentration to a region of lower water concentration through a partially permeable membrane.
Concentration gradient: The difference in the concentration of a substance between two areas.
Partially permeable membrane: A membrane that allows some molecules to pass through but not others (typically allows small molecules like water but not larger molecules).

💡 Diffusion in Action

Diffusion happens all around us! When you spray perfume in one corner of a room, the scent molecules gradually spread throughout the space. In our bodies, oxygen diffuses from our lungs into our bloodstream, while carbon dioxide diffuses in the opposite direction.

💧 Osmosis in Action

Osmosis is crucial for plants to absorb water from soil through their roots. It's also why your fingers go wrinkly in the bath - water moves into your skin cells by osmosis! In medical settings, doctors must carefully match the concentration of solutions given to patients to avoid harmful osmotic effects.

Diffusion Practical Experiments

Investigating Diffusion with Agar Cubes

This experiment helps us visualise how substances diffuse into cells and how the rate of diffusion is affected by surface area.

Experiment Materials

Agar jelly containing universal indicator
Knife or scalpel
Ruler
Dilute hydrochloric acid
Stopwatch
White tile

Method:

Cut the agar jelly into cubes of different sizes (e.g., 1cm³, 2cm³, 3cm³).
Place the cubes on a white tile.
Add a few drops of dilute hydrochloric acid to each cube.
Start the stopwatch.
Observe the colour change as the acid diffuses into the agar.
Measure how far the colour has changed after set time intervals (e.g., 5, 10, 15 minutes).

Results and Analysis:

The acid causes the universal indicator in the agar to change colour. The smaller cubes will show colour change throughout more quickly than larger cubes. This demonstrates that:

A larger surface area to volume ratio increases the rate of diffusion
Diffusion occurs more rapidly over shorter distances

This is why single-celled organisms are small and why larger organisms need specialised transport systems (like our circulatory system)!

Osmosis Practical Experiments

Investigating Osmosis with Potato Cylinders

This classic experiment demonstrates osmosis in plant tissues and helps us understand how water moves in and out of cells.

Experiment Materials

Fresh potato
Cork borer
Ruler
Scalpel
Electronic balance
Sugar solutions of different concentrations (0%, 0.2%, 0.4%, 0.6%, 0.8%, 1.0%)
Test tubes and rack
Paper towels

Method:

Use a cork borer to cut potato cylinders of equal diameter.
Cut the cylinders to the same length (about 3cm).
Weigh each cylinder and record the initial mass.
Place each cylinder in a different concentration of sugar solution.
Leave for 24 hours.
Remove the cylinders, blot dry with paper towel and reweigh.
Calculate the percentage change in mass for each cylinder.

Results and Analysis:

You'll observe that:

In low concentration solutions (hypotonic), the potato gains mass as water moves into the cells by osmosis
In high concentration solutions (hypertonic), the potato loses mass as water moves out of the cells
There will be a concentration where no change occurs (isotonic)

By plotting a graph of percentage change in mass against solution concentration, you can determine the concentration at which the potato cells are isotonic with the solution.

💧 Hypotonic Solution

Solution has a lower solute concentration than the cell. Water moves INTO the cell, causing it to swell or even burst (in animal cells).

⚖ Isotonic Solution

Solution has the same solute concentration as the cell. No net movement of water occurs. Cells maintain their normal size.

💦 Hypertonic Solution

Solution has a higher solute concentration than the cell. Water moves OUT of the cell, causing it to shrink (plasmolysis in plant cells).

Visually Observing Osmosis in Plant Cells

Plasmolysis Experiment

Plasmolysis is the shrinking of the cell contents away from the cell wall when a plant cell loses water by osmosis. This experiment allows you to directly observe this process under a microscope.

Experiment Materials

Onion epidermis
Microscope slides and coverslips
Forceps
Distilled water
Strong salt or sugar solution (e.g., 1M)
Microscope
Pipettes

Method:

Peel a thin layer of epidermis from an onion.
Place it on a microscope slide with a drop of water.
Add a coverslip and observe under the microscope.
Draw what you see.
Without moving the slide, add a drop of strong salt solution at one edge of the coverslip.
Draw water away from the opposite edge using filter paper to pull the salt solution across.
Observe what happens to the cells and draw your observations.

Results and Analysis:

Initially, the cells appear turgid with the cell membrane pressed against the cell wall. When the salt solution is added, you'll observe:

The cytoplasm and cell membrane shrink away from the cell wall
This is plasmolysis - caused by water leaving the cell by osmosis
The cell becomes flaccid and eventually plasmolysed

This experiment demonstrates why plants wilt when they don't have enough water - their cells lose water by osmosis and become plasmolysed!

Case Study Focus: Dialysis

Dialysis is a medical treatment that uses the principles of diffusion and osmosis to clean the blood when kidneys fail. A dialysis machine contains a partially permeable membrane that allows waste products to diffuse out of the blood while keeping essential blood components in. The dialysis fluid is carefully balanced to ensure that only unwanted substances diffuse out, while important substances like glucose and amino acids remain in the blood. This life-saving treatment demonstrates how understanding diffusion and osmosis has real-world applications in medicine.

Common Mistakes and Tips for Success

When conducting diffusion and osmosis experiments, watch out for these common issues:

Inconsistent sample sizes: Make sure all your potato cylinders or agar cubes are exactly the same size at the start.
Inaccurate measurements: Blot samples dry before weighing to avoid water on the surface affecting your results.
Poor temperature control: Keep all samples at the same temperature, as diffusion and osmosis rates increase with temperature.
Forgetting controls: Always include a control (e.g., a potato cylinder in distilled water).
Mixing up terms: Remember that osmosis is specifically about water movement through a partially permeable membrane, while diffusion applies to any substance.

Exam Tip 💡

In your IGCSE exam, you might be asked to design an experiment to investigate diffusion or osmosis. Remember to include:

A clear, testable hypothesis
Identification of independent, dependent and control variables
A step-by-step method that someone else could follow
Safety precautions
How you would record and analyse results

🧠 Test Your Knowledge!