This simple experiment demonstrates osmosis —
the movement of water through a selectively permeable cell membrane.
You'll see how potato pieces react differently when placed in plain water versus salt solution.
Aim
To demonstrate osmosis by observing water movement through the cell membrane of potato cells.
Materials Required
Two fresh potatoes (similar size)
Kitchen knife or blade
Weighing balance (digital or manual)
Two beakers (250 ml capacity)
Plain water (distilled or tap water)
20% salt solution (or 20% sugar solution)
Labels/marker pen
Paper towels
How to Make 20% Salt Solution
Dissolve 20 grams of salt in 100 ml of water. Stir well until salt dissolves completely. This gives you a
20% concentrated solution.
Procedure
Follow these steps carefully:
Step-by-Step Instructions
Cut potato pieces: Take two fresh potatoes and cut them into roughly equal-sized pieces
(approximately 5 cm × 3 cm × 2 cm). Try to make them as similar as possible.
Weigh the pieces: Use a weighing balance to measure the initial weight of each potato piece.
Record these weights carefully in your notebook. Label them as "Piece A" and "Piece B".
Prepare the beakers: Take two clean beakers and label them as "Beaker A" and "Beaker B".
Add solutions:
Beaker A: Fill with plain water (about 200 ml)
Beaker B: Fill with 20% salt solution (about 200 ml)
Immerse potato pieces: Place Piece A in Beaker A (plain water) and Piece B in Beaker B
(salt solution). Make sure both pieces are completely submerged.
Wait for 1 hour: Leave the setup undisturbed for about 60 minutes. Do not shake or move the beakers.
Remove and weigh again: After 1 hour, carefully remove both potato pieces. Gently pat them dry
with paper towels and weigh them again. Record the final weights.
Observations
Record your observations in this table format:
Beaker
Solution
Initial Weight
Final Weight
Change
Appearance
A
Plain water
50g (example)
55g
+5g
Swells, firm, turgid
B
20% salt solution
50g (example)
45g
-5g
Shrinks, limp, flaccid
What you'll notice:
Beaker A (Plain Water): Potato piece becomes heavier, swells up, feels firm and rigid to touch
Beaker B (Salt Solution): Potato piece becomes lighter, shrinks in size, feels soft and limp
Result
Beaker A (Plain Water):
Weight increases (approximately 5-10% gain)
Potato swells and becomes turgid (firm)
Beaker B (Salt Solution):
Weight decreases (approximately 5-10% loss)
Potato shrinks and becomes flaccid (limp)
Inference
This experiment proves that water moves through the selectively permeable
cell membrane from a region of
higher water concentration to a region of lower water concentration.
This process is called osmosis.
Why This Happens — Scientific Explanation
Understanding Osmosis
Definition
Osmosis: The movement of water molecules through a selectively permeable membrane from a region of
higher water concentration (dilute solution) to a region of lower water concentration (concentrated solution).
What Happened in Beaker A (Plain Water)?
Plain water = Hypotonic Solution
Plain water has higher water concentration than potato cells
Water enters potato cells through
the cell membrane by osmosis
Cells become filled with water and swell
This condition is called turgid (firm and rigid)
Result: Potato gains weight and becomes firm
What Happened in Beaker B (Salt Solution)?
Salt solution = Hypertonic Solution
Salt solution has lower water concentration than potato cells
Water leaves potato cells through the cell membrane by osmosis
Cells lose water and shrink away from the cell wall
This condition is called plasmolysis (cell contents shrink)
Result: Potato loses weight and becomes soft and limp
Why is the Cell Membrane "Selectively Permeable"?
The cell membrane is called
selectively permeable because:
It allows small water molecules to pass through easily
It blocks larger molecules like salt and sugar from passing through
This selective nature controls what enters and exits the cell
Only water moves in this experiment — salt stays outside the cells
Real-Life Application
This is why vegetables become limp when you add salt to them. The salt creates a hypertonic solution that draws water
out of the vegetable cells through osmosis, making them soft. Similarly, dried fruits like raisins swell when placed
in water because water enters by osmosis.
Precautions and Pro Tips
Precautions:
Cut potato pieces of approximately equal size for fair comparison
Weigh accurately and record initial weights carefully
Ensure potato pieces are completely immersed in solutions
Do not disturb or shake the beakers during the experiment
Use fresh, firm potatoes (not old or sprouted ones)
Pat potato pieces dry before final weighing
Pro Tips:
Leave for 1-2 hours for better results (overnight is even better)
Use same variety of potato for both pieces
You can use sugar solution instead of salt solution
Take photos before and after for better comparison
Try different concentrations (10%, 20%, 30%) to see varying effects
Use peeled potato for faster osmosis
5 Important Viva Questions
Q1. Define osmosis. How is it different from diffusion?
Osmosis: The movement of water molecules through a selectively permeable membrane from a region of
higher water concentration (dilute solution) to a region of lower water concentration (concentrated solution).
Difference between Osmosis and Diffusion:
Osmosis
Diffusion
Movement of water/solvent only
Movement of any particles (solute or solvent)
Requires selectively permeable membrane
No membrane required
Only water moves
All particles move
Simple way to remember: Osmosis is a special type of
diffusion that involves only water
moving through a membrane.
Q2. What is a selectively permeable membrane? Give an example.
A selectively permeable membrane (also called semi-permeable membrane) is a membrane that allows
only certain substances to pass through while blocking others.
How it works:
It has tiny pores that act like filters
Small molecules like water can pass through easily
Large molecules like salt, sugar, proteins cannot pass through
This selective nature controls what enters and leaves the cell
Example:
The cell membrane
is the best example of a selectively permeable membrane
In this potato experiment:
Water molecules (small) passed through the cell membrane
Salt molecules (larger) were blocked by the membrane
This is why only water moved in or out, not salt
Q3. What would happen if both beakers had the same salt concentration? Explain.
If both beakers had the same salt concentration (or if potato cells and surrounding solution had equal water concentration),
no osmosis would occur.
What would happen:
No net movement of water: Water molecules would still move in and out of cells, but at equal rates
No weight change: Potato pieces would maintain their original weight
No change in appearance: Potatoes would remain firm and unchanged
This is called an isotonic solution — equal concentration inside and outside cells
Why osmosis requires concentration difference:
Osmosis is driven by the concentration gradient (difference in water concentration)
Water always moves from higher concentration to lower concentration
If concentrations are equal, there's no gradient, so no net movement
Example: If you place potato in solution matching its cell concentration, nothing happens
Q4. What is plasmolysis? In which beaker did it occur?
Plasmolysis is the process in which the cell contents (cytoplasm and cell membrane) shrink away
from the cell wall when a plant cell loses water.
Where it occurred:
Plasmolysis occurred in Beaker B (containing salt solution).
How plasmolysis happens:
Potato cell is placed in concentrated salt solution (hypertonic)
Salt solution has less water than inside the cell
Water moves out of the cell by osmosis
Cell contents shrink and pull away from the rigid cell wall
Cell becomes limp and flaccid
Important note:
The cell wall remains intact (it's rigid and doesn't shrink)
Only the cell membrane and cytoplasm shrink inward
This creates a gap between the cell wall and cell membrane
Plasmolysis can be reversed if the cell is placed back in plain water
Q5. Why did we use potato for this experiment? Can we use any other material?
Why potato is ideal for osmosis experiments:
Large cells: Potato cells are large enough to show visible changes
High water content: Fresh potatoes contain about 80% water, making osmosis effects clear
Firm structure: Easy to cut into uniform pieces and handle
Easily available: Found in every kitchen, inexpensive
Living cells: Potato cells are alive and have selectively permeable membranes
Visible results: Changes in weight and texture are easy to observe
Other materials you can use:
Material
Why it works
Raisins
Swell dramatically in water, easy to see osmosis
Carrot pieces
Similar to potato, shows clear weight changes
Beetroot slices
Color change makes osmosis visible
Onion pieces
Large cells, shows plasmolysis well
Note: You cannot use materials like plastic, metal, or dead cells because they don't have
selectively permeable membranes. Only living plant tissues work for this demonstration.
Key Terms to Remember
Osmosis
Movement of water through a selectively permeable membrane from higher to lower water concentration.
Selectively Permeable Membrane
Membrane that allows only certain substances (like water) to pass through while blocking others.
Hypotonic Solution
Solution with higher water concentration than the cell; causes water to enter cell, making it turgid.
Hypertonic Solution
Solution with lower water concentration than the cell; causes water to leave cell, leading to plasmolysis.
Isotonic Solution
Solution with equal water concentration as the cell; no net water movement occurs.
Turgid
Condition when plant cells are swollen and firm due to water entering by osmosis.
Flaccid
Condition when plant cells are limp and soft due to water loss.
Plasmolysis
Shrinking of cell contents away from cell wall when water leaves the cell in hypertonic solution.
