Biology · Simulation and virtual lab Free
Osmosis: potato cylinders in sugar solutions
Put potato cylinders in sugar solutions, weigh them and find the concentration that matches the inside of the cells.
- You will learn to
- Measure the change in mass of potato cylinders in sucrose solutions.
- Calculate the percentage change in mass and plot it against concentration.
- Find the concentration with no net movement of water.
- You will measure
- Mass before and after (g); percentage change; isotonic concentration.
- Time needed
- About 25–35 min
- Equipment
- Nothing extra. Real version: potato, cork borer, sucrose solutions, balance, beakers.
- Price
- Free: no payment, no account.
Virtual lab
Follow the steps with the simulation above (go back to the simulation ↑). Each time you tap Record, a row is added to the table and a point to the graph.
Aim
To investigate the effect of sucrose concentration on the mass of potato tissue and to estimate the water potential of potato cells.
What to do
- Set the sucrose concentration to 0.0 mol/dm³ (pure water) and the time to 30 min. The cylinder is first weighed on the electronic balance.
- Tap Put the potato cylinder in. When it is taken out and weighed, tap Record.
- Repeat for 0.2, 0.4, 0.6, 0.8 and 1.0 mol/dm³.
- Look at the microscope view each time: is the cell turgid or plasmolysed?
- Read the concentration where the graph crosses 0 %: there is no net gain or loss of water.
- Extra: at 0.6 mol/dm³ try 10, 30 and 60 minutes. Why do we leave the cylinders for at least 30 minutes?
Results
Questions
Define osmosis.
Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) through a partially permeable membrane.
Why did the cylinder in pure water gain mass?
The water outside had a higher water potential than the cell sap, so water entered the cells by osmosis. The cells became turgid.
Why did the cylinders in 0.6 mol/dm³ and above lose mass?
The solution had a lower water potential than the cells, so water left the cells by osmosis. The cells became flaccid, then plasmolysed.
Why is the percentage change in mass calculated, not just the change in mass?
The cylinders did not have exactly the same starting mass. Percentage change allows a fair comparison.
Why must the cylinders be blotted dry before weighing?
Water on the surface would add to the mass and give a false result.
Why does salting fish or making jam help to preserve food?
The very concentrated salt or sugar solution draws water out of bacterial and fungal cells by osmosis, so they cannot grow.
For teachers: expected results and how the model works
The model gives about +12 % in water, 0 % at 0.30 mol/dm³ and about −17 % at 0.8 mol/dm³, with ±0.6 % natural variation between cylinders, which matches typical school results. Equilibrium is reached after about 40 minutes. For S5, convert the isotonic concentration to water potential with ψ = −cRT (about −740 kPa).
Exam practice Exam set
National-exam style questions on this simulation (14 marks). Try each one on paper first, then open the marking scheme.
Define osmosis. (2 marks)
Marking scheme: The net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) (1) through a partially permeable membrane (1).
Potato cylinders were weighed, left in sucrose solutions for 30 minutes and weighed again. 0.0 mol/dm³: 2.00 g → 2.24 g; 0.4 mol/dm³: 2.00 g → 1.88 g. (a) Calculate the percentage change in mass for each. (b) Explain each result. (6 marks)
Marking scheme: (a) +12 % (1); −6 % (1). (b) In water the solution has a higher water potential than the cells (1), so water enters by osmosis and the cells become turgid (1); in 0.4 mol/dm³ sucrose the solution has a lower water potential (1), so water leaves the cells and they become flaccid / plasmolysed (1).
Explain why the percentage change in mass is calculated instead of the change in mass. (2 marks)
Marking scheme: The cylinders do not all have exactly the same starting mass (1); the percentage change allows a fair comparison between them (1).
How is the concentration of sucrose that has the same water potential as the potato cells found from the graph? (2 marks)
Marking scheme: Plot the percentage change in mass against concentration (1); read the concentration where the line crosses 0 % change (no net movement of water) (1).
State two variables that must be kept constant in this experiment. (2 marks)
Marking scheme: Size / surface area of the cylinders (1); time in the solution / temperature / volume of solution / type of potato (1).
Donat Sciences and MathsBiology · Virtual lab worksheet
School: ........................................ Name: ................................ Class: ........ Date: ............
Osmosis: potato cylinders in sugar solutions
Aim: To investigate the effect of sucrose concentration on the mass of potato tissue and to estimate the water potential of potato cells.
Apparatus (real laboratory): potato, cork borer, sucrose solutions, balance, beakers.
What to do (with the simulation on www.donatstem.co.rw)
- Set the sucrose concentration to 0.0 mol/dm³ (pure water) and the time to 30 min. The cylinder is first weighed on the electronic balance.
- Tap Put the potato cylinder in. When it is taken out and weighed, tap Record.
- Repeat for 0.2, 0.4, 0.6, 0.8 and 1.0 mol/dm³.
- Look at the microscope view each time: is the cell turgid or plasmolysed?
- Read the concentration where the graph crosses 0 %: there is no net gain or loss of water.
- Extra: at 0.6 mol/dm³ try 10, 30 and 60 minutes. Why do we leave the cylinders for at least 30 minutes?
Results
| # | Sucrose (mol/dm³) | Mass before (g) | Mass after (g) | Change (g) | Change (%) |
|---|
| 1 | | | | | |
| 2 | | | | | |
| 3 | | | | | |
| 4 | | | | | |
| 5 | | | | | |
| 6 | | | | | |
| 7 | | | | | |
Graph
Plot the percentage change in mass (vertical axis, positive and negative) against the sucrose concentration. Read the concentration where the line crosses 0 %.
Questions
- Define osmosis. [2]
- Why did the cylinder in pure water gain mass? [2]
- Why did the cylinders in 0.6 mol/dm³ and above lose mass? [2]
- Why is the percentage change in mass calculated, not just the change in mass? [2]
- Why must the cylinders be blotted dry before weighing? [2]
- Why does salting fish or making jam help to preserve food? [2]
- Define osmosis. [2]
- Potato cylinders were weighed, left in sucrose solutions for 30 minutes and weighed again. 0.0 mol/dm³: 2.00 g → 2.24 g; 0.4 mol/dm³: 2.00 g → 1.88 g. (a) Calculate the percentage change in mass for each. (b) Explain each result. [6]
- Explain why the percentage change in mass is calculated instead of the change in mass. [2]
- How is the concentration of sucrose that has the same water potential as the potato cells found from the graph? [2]
- State two variables that must be kept constant in this experiment. [2]
Donat Sciences and MathsBiology · Virtual lab worksheet
School: ........................................ Name: ................................ Class: ........ Date: ............
Marking scheme: Osmosis: potato cylinders in sugar solutions
Expected results
The model gives about +12 % in water, 0 % at 0.30 mol/dm³ and about −17 % at 0.8 mol/dm³, with ±0.6 % natural variation between cylinders, which matches typical school results. Equilibrium is reached after about 40 minutes. For S5, convert the isotonic concentration to water potential with ψ = −cRT (about −740 kPa).
Answers
- Define osmosis. [2]
Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) through a partially permeable membrane.
- Why did the cylinder in pure water gain mass? [2]
The water outside had a higher water potential than the cell sap, so water entered the cells by osmosis. The cells became turgid.
- Why did the cylinders in 0.6 mol/dm³ and above lose mass? [2]
The solution had a lower water potential than the cells, so water left the cells by osmosis. The cells became flaccid, then plasmolysed.
- Why is the percentage change in mass calculated, not just the change in mass? [2]
The cylinders did not have exactly the same starting mass. Percentage change allows a fair comparison.
- Why must the cylinders be blotted dry before weighing? [2]
Water on the surface would add to the mass and give a false result.
- Why does salting fish or making jam help to preserve food? [2]
The very concentrated salt or sugar solution draws water out of bacterial and fungal cells by osmosis, so they cannot grow.
- Define osmosis. [2]
The net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) (1) through a partially permeable membrane (1).
- Potato cylinders were weighed, left in sucrose solutions for 30 minutes and weighed again. 0.0 mol/dm³: 2.00 g → 2.24 g; 0.4 mol/dm³: 2.00 g → 1.88 g. (a) Calculate the percentage change in mass for each. (b) Explain each result. [6]
(a) +12 % (1); −6 % (1). (b) In water the solution has a higher water potential than the cells (1), so water enters by osmosis and the cells become turgid (1); in 0.4 mol/dm³ sucrose the solution has a lower water potential (1), so water leaves the cells and they become flaccid / plasmolysed (1).
- Explain why the percentage change in mass is calculated instead of the change in mass. [2]
The cylinders do not all have exactly the same starting mass (1); the percentage change allows a fair comparison between them (1).
- How is the concentration of sucrose that has the same water potential as the potato cells found from the graph? [2]
Plot the percentage change in mass against concentration (1); read the concentration where the line crosses 0 % change (no net movement of water) (1).
- State two variables that must be kept constant in this experiment. [2]
Size / surface area of the cylinders (1); time in the solution / temperature / volume of solution / type of potato (1).