Ever wondered how cells control what goes in and out?...
Biology397 views·Updated 18 Aug 2026·4 pagesAQA A Level Biology - Osmosis Practical Notes
Milkshakemi@milkshakemi
1 / 4
1
of 4
Setting Up Your Dilution Series
You'll be creating different sucrose concentrations to test osmosis in potato tissue. Think of it like making squash - you start with the strongest solution and add water to make weaker ones.
The key formula here is M₁V₁ = M₂V₂, which helps you work out exactly how much concentrated solution you need. For example, to make 20cm³ of 0.2M solution from 1.0M stock, you'd need 4cm³ of the concentrated stuff plus 16cm³ of distilled water.
Your independent variable is the sucrose concentration (what you're changing), whilst the dependent variable is the mass change of the potato (what you're measuring). Everything else - temperature, time, potato type - stays constant as controlled variables.
Top Tip: Label your test tubes clearly with concentrations (0M, 0.2M, 0.4M, 0.6M, 0.8M, 1.0M) before you start - trust me, it gets confusing quickly!
2
of 4
Preparing Your Potato Samples
Fresh potatoes are absolutely crucial here - dried-out spuds will mess up your results because their solute concentration changes as water evaporates. You want consistent starting conditions across all samples.
Use a cork borer to cut six uniform cylinders from the same potato, then carefully remove the skin with a scalpel. The skin acts like a waterproof barrier, so leaving it on would prevent proper osmosis from occurring.
Blot each cylinder dry before weighing - any surface water will throw off your initial mass readings. You don't need perfectly identical lengths, but try to keep them fairly uniform.
Key Point: Always zero your balance before weighing, and record masses immediately in a pre-prepared table to avoid mixing up your data.
3
of 4
Running the Experiment
Pour exactly 20cm³ of each solution into separate test tubes, adding the most concentrated solution last to avoid contamination. Pop one potato cylinder into each tube and place them all in a water bath simultaneously.
After 20 minutes, remove the cylinders and place each one on labelled paper towels according to their concentration. This timing is critical - too short and osmosis won't be complete, too long and other factors might interfere.
Blot away surface water before the final weighing - you're measuring water that's moved into or out of the cells, not water clinging to the outside. Any excess moisture will appear as a false mass increase.
Remember: Handle each cylinder gently and work systematically through the concentrations to avoid mix-ups.
4
of 4
Analysing Your Results
Calculate the percentage change in mass using the formula: (change in mass ÷ initial mass) × 100. This standardises your results, making comparisons meaningful even if your potato cylinders weren't identical sizes.
When you plot concentration against percentage change, you'll get a lovely negative correlation - as sucrose concentration increases, mass change decreases. Positive values mean water moved in (hypotonic solution), negative values mean water moved out (hypertonic solution).
The magic happens where the line crosses zero - this is the isotonic point, roughly around 0.3M in most cases. Here, no net water movement occurs because the solute concentration inside the potato cells matches the external solution.
Analysis Tip: This zero-crossing point tells you the actual solute concentration inside potato cells - pretty neat for a simple practical!
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Biology397 views·Updated 18 Aug 2026·4 pagesAQA A Level Biology - Osmosis Practical Notes
Milkshakemi@milkshakemi
Ever wondered how cells control what goes in and out? This osmosis practical uses potato chips and sugar solutions to demonstrate how water moves across cell membranes - and it's actually quite brilliant for understanding this fundamental biological process.
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Setting Up Your Dilution Series
You'll be creating different sucrose concentrations to test osmosis in potato tissue. Think of it like making squash - you start with the strongest solution and add water to make weaker ones.
The key formula here is M₁V₁ = M₂V₂, which helps you work out exactly how much concentrated solution you need. For example, to make 20cm³ of 0.2M solution from 1.0M stock, you'd need 4cm³ of the concentrated stuff plus 16cm³ of distilled water.
Your independent variable is the sucrose concentration (what you're changing), whilst the dependent variable is the mass change of the potato (what you're measuring). Everything else - temperature, time, potato type - stays constant as controlled variables.
Top Tip: Label your test tubes clearly with concentrations (0M, 0.2M, 0.4M, 0.6M, 0.8M, 1.0M) before you start - trust me, it gets confusing quickly!
2
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Preparing Your Potato Samples
Fresh potatoes are absolutely crucial here - dried-out spuds will mess up your results because their solute concentration changes as water evaporates. You want consistent starting conditions across all samples.
Use a cork borer to cut six uniform cylinders from the same potato, then carefully remove the skin with a scalpel. The skin acts like a waterproof barrier, so leaving it on would prevent proper osmosis from occurring.
Blot each cylinder dry before weighing - any surface water will throw off your initial mass readings. You don't need perfectly identical lengths, but try to keep them fairly uniform.
Key Point: Always zero your balance before weighing, and record masses immediately in a pre-prepared table to avoid mixing up your data.
3
of 4Sign up to see the content. It's free!
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By signing up you accept Terms of Service and Privacy Policy
Running the Experiment
Pour exactly 20cm³ of each solution into separate test tubes, adding the most concentrated solution last to avoid contamination. Pop one potato cylinder into each tube and place them all in a water bath simultaneously.
After 20 minutes, remove the cylinders and place each one on labelled paper towels according to their concentration. This timing is critical - too short and osmosis won't be complete, too long and other factors might interfere.
Blot away surface water before the final weighing - you're measuring water that's moved into or out of the cells, not water clinging to the outside. Any excess moisture will appear as a false mass increase.
Remember: Handle each cylinder gently and work systematically through the concentrations to avoid mix-ups.
4
of 4Sign up to see the content. It's free!
- Access to all documents
- Improve your grades
- Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Analysing Your Results
Calculate the percentage change in mass using the formula: (change in mass ÷ initial mass) × 100. This standardises your results, making comparisons meaningful even if your potato cylinders weren't identical sizes.
When you plot concentration against percentage change, you'll get a lovely negative correlation - as sucrose concentration increases, mass change decreases. Positive values mean water moved in (hypotonic solution), negative values mean water moved out (hypertonic solution).
The magic happens where the line crosses zero - this is the isotonic point, roughly around 0.3M in most cases. Here, no net water movement occurs because the solute concentration inside the potato cells matches the external solution.
Analysis Tip: This zero-crossing point tells you the actual solute concentration inside potato cells - pretty neat for a simple practical!
We thought you’d never ask...
Our AI Companion is a student-focused AI tool that offers more than just answers. Built on millions of Knowunity resources, it provides relevant information, personalised study plans, quizzes, and content directly in the chat, adapting to your individual learning journey.
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