In the 1820s the French physiologist Henri Dutrochet watched water pass through animal membranes and coined the terms "endosmose" and "exosmose" for flow in and out. Our word osmosis comes from the same Greek root, meaning a push. Today you can see the same push with nothing more than a potato, some sugar solutions and a balance.
This lesson covers the osmosis required practical for AQA GCSE Biology Topic 1 (Cell biology). It's also useful for Combined Science.
What osmosis is
Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane. The membrane lets water molecules through but not larger solute molecules such as sugar. It's passive: it needs no energy from the cell.
What the practical asks you to do
Investigate the effect of a range of concentrations of salt or sugar solution on the mass of plant tissue.
Method
- Use a cork borer to cut potato cylinders, then trim them to the same length with a scalpel on a tile. Cut away from your fingers.
- Blot each cylinder and weigh it. Record the initial mass.
- Put each cylinder in a labelled tube with the same volume of solution. Use a range of concentrations, for example 0, 0.2, 0.4, 0.6, 0.8 and 1.0 mol/dm³ sugar solution.
- Leave them for the same length of time at the same temperature.
- Remove each cylinder, blot it gently and weigh it again.
- Calculate the percentage change in mass.
Blotting removes solution from the surface. Without it you'd weigh the liquid clinging to the potato as well as the potato.
Variables
| Type | In this practical |
|---|---|
| Independent | Concentration of the solution |
| Dependent | Percentage change in mass |
| Control | Cylinder size and surface area, volume of solution, time, temperature, same potato |
Percentage change in mass
Percentage change = (final mass − initial mass) ÷ initial mass × 100.
Use percentage change because the cylinders start at slightly different masses. A cylinder that goes from 2.50 g to 2.00 g has changed by −20%. A negative answer means it lost mass.
Reading the graph
Plot concentration on the x-axis and percentage change in mass on the y-axis, with zero in the middle of the y-axis.
- In pure water and weak solutions, the cylinders gain mass. The solution outside is more dilute than the cell contents, so water moves in.
- In strong solutions, the cylinders lose mass. Water moves out of the cells.
- Where the line crosses zero, there's no net movement of water. At that concentration the solution matches the concentration of the potato cells' contents.
In pure water, the cells don't burst. The cell wall stops them swelling further, and they become turgid. In a strong solution, the cells become flaccid as they lose water.
Common exam mistakes
- Saying sugar moves into the potato. Osmosis is about water moving, not the solute.
- Dividing by the final mass. Percentage change always divides by the initial mass.
- Saying osmosis needs energy. It's passive. Active transport is the process that needs energy.
- "Water moves from a high to a low concentration." Say which concentration you mean. Water moves from a dilute solution to a more concentrated one.
- Forgetting the minus sign when the potato loses mass.
Frequently asked questions
Why do you blot the potato before weighing it?
To remove surface solution, so the mass you record is the potato's mass alone.
Why does the potato gain mass in pure water?
The water outside is more dilute than the cell contents, so water moves into the cells by osmosis.
What does it mean where the line crosses the x-axis?
There's no change in mass, so the solution has the same concentration as the potato cells' contents.
Why use percentage change in mass?
The cylinders don't start at exactly the same mass. Percentage change lets you compare them fairly.
Try it
Say each answer before you flip the card. Then put yourself in Dutrochet's place: you've seen water push through a membrane, but you can't yet see why. What would you have tested next?
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