In this activity, you will observe and compare the structure of plant cells (onion peel) and animal cells (cheek cells)
under a microscope. You'll discover the key structural differences between these two types of
cells.
Aim
To prepare temporary slides of onion peel and human cheek cells, observe them under a microscope, and compare their
structural differences.
Materials Required
For Onion Peel Slide:
Fresh onion (one bulb)
Forceps
Safranin stain (or iodine solution)
Glass slides and coverslips
Dropper
Water
For Cheek Cell Slide:
Cotton swab or toothpick (flat end)
Methylene blue stain
Glass slides and coverslips
Dropper
Water
Compound microscope
For Part 3: 20% sugar solution
Procedure
Part 1: Preparing Onion Peel Slide
Peel the membrane: Cut an onion into pieces. Using forceps, carefully peel off a thin, transparent
layer from the inner side of an onion scale (the thin skin between layers).
Prepare the slide: Place the onion peel flat on a clean glass slide.
Add water: Put 1-2 drops of water on the peel to prevent drying.
Add stain: Add 1-2 drops of safranin stain. Wait for 2-3 minutes to allow the stain to penetrate.
Place coverslip: Gently lower a coverslip over the peel at a 45° angle to avoid air bubbles.
Remove excess stain: Blot excess stain and water with blotting paper or tissue.
Observe under microscope: First use low power (10×), then switch to high power (40×) for detailed view.
Part 2: Preparing Cheek Cell Slide
Collect cells: Gently scrape the inside of your cheek with a clean cotton swab or flat end of
a toothpick. Do not press hard — gentle scraping is enough.
Spread on slide: Smear the material collected on a clean glass slide by making a thin film.
Add water: Put 1-2 drops of water on the smear.
Add stain: Add 1-2 drops of methylene blue stain. Wait for 1-2 minutes.
Place coverslip: Carefully place a coverslip over the stained cells.
Remove excess: Blot excess liquid from the edges.
Observe under microscope: Start with low power, then use high power to see nucleus and
cell membrane clearly.
Part 3: Effect of Sugar Solution (Demonstrating Plasmolysis)
Prepare fresh slides: Make fresh onion peel and cheek cell slides as above.
Add sugar solution: Instead of water, mount both specimens in 20% sugar solution.
Wait and observe: Leave for 30 minutes, then observe under microscope.
Compare: Note the differences from slides prepared with plain water.
Observations
Comparison of Onion Peel Cells vs Cheek Cells
Feature
Onion Peel Cells (Plant)
Cheek Cells (Animal)
Shape
Box-shaped, rectangular, regular
Irregular, rounded, no fixed shape
Cell Wall
Present (rigid, clearly visible)
Absent
Cell Membrane
Present (just inside cell wall)
Present (outermost boundary)
Nucleus
Present (stained pink/red, peripheral)
Present (stained blue, central)
Vacuole
Large, prominent, central
Small or absent
Arrangement
Arranged in neat rows and columns
Scattered, not in fixed pattern
Effect of Sugar Solution
Cell Type
In Water
In Sugar Solution
Onion Peel
Cell wall intact, cell contents fill the cell
Cell wall remains same, but cell contents shrink away from wall (plasmolysis)
Cheek Cell
Normal rounded shape
Entire cell shrinks considerably, becomes smaller
Result
Plant cells (onion peel):
Have a definite box-like shape due to rigid cell wall
Arranged in regular rows and columns
Clearly visible cell wall, nucleus, and large central vacuole
Animal cells (cheek cells):
Have irregular shape with no cell wall
Scattered arrangement
Only cell membrane visible as boundary, prominent nucleus
Inference
Plant cells have a rigid cell wall
in addition to the cell membrane, which gives them a fixed box-like shape.
Animal cells have only a cell membrane, so they have irregular, flexible shapes.
The cell wall is permeable and allows sugar solution to enter, but the
cell membrane is selectively permeable.
When water leaves plant cells in sugar solution, the cell wall maintains its shape while the cell contents shrink
(plasmolysis).
Animal cells shrink completely in sugar solution because they lack a rigid cell wall.
Why Different Shapes? — Scientific Explanation
Plant Cells Have Cell Walls
Plant cells need cell walls for support because plants
don't have a skeleton. The cell wall:
Is made of cellulose (a tough polysaccharide)
Provides structural support and rigidity
Maintains definite box-like shape
Protects the delicate cell membrane inside
Is fully permeable — allows water, minerals, and dissolved substances to pass through
Animal Cells Have Only Cell Membranes
Animal cells don't need cell walls because:
Animals have skeletal systems for support
Cells need to be flexible for movement (muscle cells, blood cells)
Flexibility allows cells to change shape when needed
Why Different Stains?
Safranin (red/pink): Stains plant cell walls and nuclei well because it binds to cellulose and DNA. Methylene blue (blue): Stains animal cell nuclei clearly and makes the cell membrane more visible.
Understanding Plasmolysis
What is Plasmolysis?
When a plant cell is placed in a hypertonic solution (like 20% sugar solution), water moves out of the cell by
osmosis. The cell membrane and cytoplasm
shrink away from the rigid cell wall, creating a gap. This is called plasmolysis.
Why plant cells show plasmolysis but animal cells don't:
Plant Cells
Animal Cells
Cell wall is rigid and doesn't shrink
Only cell membrane shrinks inward
Creates visible gap between wall and membrane
Cell maintains external shape
No cell wall to maintain shape
Entire cell shrinks uniformly
Cell becomes smaller overall
No gap formation
Real-Life Connection
This is similar to what you observed in Activity 2.2
(Potato Osmosis). Potato cells also showed plasmolysis in salt solution — the cell contents shrank but the cell walls
remained intact, making the potato limp.
Precautions and Pro Tips
Precautions:
Peel very thin onion membrane (single layer only)
Don't press coverslip too hard (cells will rupture)
Scrape cheek gently — hard scraping causes bleeding
Clean glass slides thoroughly before use
Avoid air bubbles under coverslip
Don't use too much stain (obscures view)
Pro Tips:
Lower coverslip at 45° angle to prevent air bubbles
Wait 2-3 minutes for stain to penetrate cells
Use fresh onion for better results
Rinse mouth before collecting cheek cells
If cells are too dark, rinse slide and re-stain
Compare both slides side by side for clear differences
5 Important Viva Questions
Q1. Why is safranin used for plant cells and methylene blue for animal cells?
Different stains are used because they bind to different cellular structures and provide better contrast.
Note: You can use iodine solution for onion cells (stains brown) or methylene blue for both types,
but safranin and methylene blue give the clearest results for their respective cell types.
Q2. Why do onion peel cells appear box-shaped while cheek cells are irregular?
The difference in shape is due to the presence or absence of a cell wall.
Not pressed together in layers, so they appear rounded or irregular
This flexibility is important for animal cells to perform various functions (like blood cells squeezing through
narrow blood vessels)
Q3. What is plasmolysis? Why do cheek cells not show plasmolysis?
Plasmolysis is the shrinking of cell contents (cytoplasm and cell membrane) away from the cell wall
when a plant cell loses water in a hypertonic solution.
How plasmolysis occurs:
Plant cell is placed in hypertonic solution (like 20% sugar solution)
Plasmolysis specifically refers to the separation of cell membrane from cell wall
Without a cell wall, there's nothing for the membrane to separate from
When cheek cells lose water in hypertonic solution, the entire cell shrinks uniformly
There's no gap formation — just overall reduction in cell size
This process is called crenation in animal cells, not plasmolysis
Q4. What does this experiment tell us about the cell wall? Is it permeable or selectively permeable?
This experiment shows that the cell wall is
fully permeable (not selectively permeable).
Evidence from the experiment:
When we added 20% sugar solution to onion cells, the sugar solution passed through the cell wall
If the cell wall were selectively permeable, sugar molecules would be blocked
But the sugar solution reached the cell membrane and caused water to move out (plasmolysis occurred)
This proves the cell wall allows all substances to pass through freely
Key differences:
Cell Wall
Cell Membrane
Fully permeable
Allows everything to pass (water, sugars, salts, minerals)
Selectively permeable
Allows only certain substances (like water) to pass
Important: The cell membrane (not the cell wall) controls what enters and leaves the cell.
The cell wall only provides support and protection. Learn more about
cell membrane functions.
Q5. List three major differences you observed between plant cells and animal cells in this activity.
Three major differences observed:
1. Shape and Arrangement
Plant cells (onion): Regular box-like shape, arranged in neat rows and columns like tiles
Animal cells (cheek): Irregular, rounded shape, scattered arrangement with no fixed pattern
2. Cell Wall Presence
Plant cells: Have a thick, rigid cell wall
clearly visible as the outer boundary (stained by safranin)
Animal cells: No cell wall present, only a thin
cell membrane as the boundary
3. Behavior in Sugar Solution
Plant cells: Showed plasmolysis — cell wall remained same but cell contents shrank away,
creating a visible gap
Animal cells: No plasmolysis — entire cell shrank uniformly because there's no rigid wall
to maintain shape
Additional observations: Onion cells had large central vacuoles (appeared as empty spaces), while
cheek cells had little to no visible vacuoles. For a complete comparison, see
cell structure comparison.
Key Terms to Remember
Plasmolysis
Shrinking of cell contents away from cell wall when plant cell loses water in hypertonic solution.
Safranin
Red/pink stain used to color plant cell walls and nuclei for better visibility under microscope.
Methylene Blue
Blue stain used to color animal cell nuclei and make cell membranes more visible.
Turgid
Firm and swollen condition of plant cells when they contain maximum water.
Flaccid
Limp and soft condition of plant cells when they lose water.
Crenation
Shrinkage of animal cells in hypertonic solution (similar to plasmolysis but without cell wall).
Temporary Slide
Microscope slide prepared for immediate observation, not preserved for long-term use.
Coverslip
Thin glass piece placed over specimen on slide to protect it and keep it flat for viewing.
