Investigating osmosis (AQA GCSE Biology): Revision Notes
Investigating osmosis
What is this practical about?
This is a core practical that helps you understand how osmosis works. You'll investigate how different sugar concentrations affect the mass of plant tissue (potato). This shows you how water moves in and out of cells.
This practical is particularly important because it demonstrates osmosis in action using easily observable changes in mass, making it easier to understand this fundamental biological process.
Aim
The goal is to find out how different concentrations of sucrose solution affect the mass of potato tissue. This helps you see osmosis in action.
Equipment you need
- Potato
- Cork borer, cutting board and knife
- Ruler with 1mm measurements
- Balance (accurate to ±0.01g)
- Different concentrations of sucrose solution
- Boiling tubes with bungs and labels
Alternative Materials: You can use sodium chloride solution instead of sucrose if needed. The principle remains the same as both create solutions with different water concentrations.
Method
Step 1: Use the cork borer to cut potato cylinders. Trim them so they're all about 30mm long.
Step 2: Fill each boiling tube two-thirds full with a different sucrose solution.
Step 3: Dry each potato cylinder with a paper towel. Weigh it and record the mass. Add it to one of the solutions.
Step 4: Do step 3 for each different solution you're testing.
Step 5: After about 30 minutes, remove each cylinder. Dry them and record their new mass.
Critical for Valid Results:
- Make sure all potato cylinders are the same length for fair comparison
- You could also measure and record length changes for additional data
- Some people wait up to 24 hours for bigger changes, but 30 minutes is usually sufficient for clear results
Recording results
Make a table like this to record your data:
| Sucrose concentration (g/dm³) | Start mass (g) | End mass (g) | Change (g) | % change |
|---|---|---|---|---|
| 0 (distilled water) | 2.00 | 2.53 | +0.53 | +26.5 |
| 75 | 2.15 | 2.28 | ||
| 150 | 2.05 | 1.86 | ||
| 225 | 1.96 | 1.58 | ||
| 300 | 2.08 | 1.53 | -0.55 | -26.4 |
Notice how the mass change becomes more negative as the sucrose concentration increases. This pattern is crucial for understanding osmosis.
Calculating percentage change
This is the key calculation you need to know:
Worked Example: Calculating Percentage Change
Formula:
Using the 75 g/dm³ data:
- Initial mass = 2.15g
- Final mass = 2.28g
- Change = 2.28 - 2.15 = +0.13g
Calculation:
- A positive value means the mass increased
- A negative value means the mass decreased
Analysis
Step 1: Calculate the change in mass and percentage change for each cylinder.
Step 2: Plot a graph with:
- Percentage change in mass on the vertical axis
- Concentration of sucrose on the horizontal axis
Step 3: Use your graph to estimate the concentration of the potato tissue (this is where the percentage change is zero).
The point where your line crosses the x-axis (0% change) represents the concentration inside the potato cells. At this point, there's no net movement of water because the water concentration inside and outside the cells is equal.
What happens and why?
- In distilled water: Potato gains mass because water moves into the cells by osmosis
- In strong sucrose solution: Potato loses mass because water moves out of the cells
- At the right concentration: No change in mass because water movement is balanced
The stronger the sucrose solution, the more water moves out of the potato cells. This happens because higher sugar concentration means lower water concentration.
Understanding Water Movement: Remember that osmosis involves water moving from areas of high water concentration to low water concentration. When you increase the sugar concentration outside the cells, you decrease the water concentration, creating a concentration gradient that drives osmosis.
Analysis
Key Points to Remember:
- Osmosis is the movement of water from high water concentration to low water concentration
- Higher sugar concentration means lower water concentration
- Always calculate percentage change to compare results properly
- The point where percentage change is zero tells you the concentration inside the potato cells
- This practical shows how osmosis affects plant cells in different solutions