Required practical - Investigating osmosis Revision Notes for AQA GCSE Biology Combined Science

Required practical - Investigating osmosis

What this practical is about

This practical lets you see how osmosis works in real life. You'll find out how different sugar solutions affect plant tissue by measuring changes in mass.

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This investigation demonstrates osmosis in action using everyday materials, helping you connect the theory you've learned to real biological processes happening in plant cells.

Aim of the experiment

The main goal is to see how different concentrations of sucrose (sugar) solution affect the mass of potato tissue. This helps you understand how osmosis moves water in and out of cells.

Equipment you'll need

You'll need these items for your experiment:

Fresh potato
Cork borer (a tool that cuts neat cylinders)
Cutting board and sharp knife
Ruler with 1mm markings
Electronic balance (accurate to 0.01g)
Different concentrations of sucrose solution
Test tubes or boiling tubes with bungs and labels
Paper towels for drying

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Concentration Range Tip

A good range of sucrose concentrations is 0 to 300 g/dm³. You can use distilled water as your 0 g/dm³ solution. This range will give you clear results showing water movement in both directions.

Method

Here's how to carry out the practical step by step:

Prepare your potato cylinders - Use the cork borer to cut neat potato cylinders. Trim them so they're all about 30mm long. This keeps your test fair.
Set up your solutions - Fill each test tube about two-thirds full with a different sucrose solution.
Weigh your potato - Pat each potato cylinder dry with paper towel, then weigh it carefully. Record this as your starting mass.
Add to solutions - Put one potato cylinder into each sucrose solution.
Wait and measure - After about 30 minutes, remove each potato cylinder. Dry it gently and weigh it again. Record this as your final mass.

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Critical for Fair Testing

Make sure all potato cylinders are the same length and that you dry them the same way each time. Any differences in preparation could affect your results and make your conclusions unreliable.

Recording your results

Create a table like this to record what happens:

Sucrose concentration (g/dm³)	Start mass (g)	End mass (g)	Change in mass (g)	% change
0
75
150

Working out percentage change

You need to calculate percentage change in mass using this formula:

$\text{Percentage change} = \frac{\text{final mass} - \text{initial mass}}{\text{initial mass}} \times 100$

If you get a positive number, the potato gained mass
If you get a negative number, the potato lost mass

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Worked Example: Calculating Percentage Change

Initial mass = 2.50g
Final mass = 2.75g

Step 1: Find the change in mass
Change = 2.75 - 2.50 = 0.25g

Step 2: Apply the formula
Percentage change = $\frac{0.25}{2.50} \times 100 = 10\%$

This positive result means the potato gained 10% of its original mass.

What your results mean

When you look at your results, you'll see a clear pattern:

Low concentrations - The potato cylinders gain mass because water moves into the cells by osmosis
High concentrations - The potato cylinders lose mass because water moves out of the cells by osmosis
Medium concentrations - You might find a concentration where there's no change - this shows the concentration inside and outside the cells is similar

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Understanding the Pattern

This pattern occurs because osmosis always moves water from areas of high water potential (low solute concentration) to areas of low water potential (high solute concentration). The potato cells act like selective barriers that allow water through but not the large sucrose molecules.

Analysis tips

After collecting your results:

Calculate the percentage change for each concentration
Plot a graph with concentration on the x-axis and percentage change on the y-axis
Look for the point where the line crosses zero - this tells you the concentration inside the potato cells

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Graph Analysis

The point where your line crosses the x-axis (zero percentage change) represents the concentration that matches the cell contents. This is called the isotonic point - where water movement in equals water movement out.

Summary

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Key Points to Remember:

Osmosis moves water from areas with lots of water to areas with less water
Higher sucrose concentrations have less water, so water moves out of potato cells
Always dry your potato cylinders the same way before weighing
The percentage change formula helps you compare results fairly
A negative percentage means the potato lost mass due to water leaving the cells
The isotonic point shows you the internal concentration of the potato cells

Required practical - Investigating osmosis (AQA GCSE Biology Combined Science): Revision Notes