Science 7–10 · Year 8

Plasmolysis in red onion cells: the membrane pulls away from the wall and returns

Science understanding: Biological sciences

PracticalLow risk

The idea

The cell membrane controls what passes into and out of a plant cell while the cell wall keeps its shape: in a strong salt solution water passes out of the cell, so the membrane and the purple vacuole shrink away from the wall, and in pure water they swell back.

What you need

Red onion (the purple pigment sits in the vacuole and makes the protoplast visible), forceps
Sodium chloride solution 1.0 mol/L (58.4 g dissolved and made up to 1 L with water), 0.3 mol/L sucrose solution (102.7 g made up to 1 L) and distilled water, in dropper bottles
Slides, coverslips, filter paper strips, compound microscope, stopwatch

How to do it

Peel a piece of purple epidermis from the inner face of a red onion scale and mount it in a drop of distilled water under a coverslip. View at 100x and draw one cell.
Place a drop of 1.0 mol/L sodium chloride at one edge of the coverslip and draw it through with filter paper at the opposite edge. Start the stopwatch.
Watch one cell at 100x or 400x; record the time at which the purple protoplast first pulls away from the wall and the time at which it has shrunk to a rounded mass.
Draw the plasmolysed cell.
Draw distilled water through in the same way and time the recovery until the protoplast again fills the cell.
Repeat on a fresh piece of epidermis with the 0.3 mol/L sucrose solution and record the fraction of cells that plasmolyse in each solution.

What you should see

In 1.0 mol/L sodium chloride the purple protoplast pulls away from the wall and rounds up while the wall keeps its shape; drawing distilled water through lets it swell back. Sodium chloride dissolves as two ions, sodium and chloride, so the 1.0 mol/L salt holds 2.0 mol/L of dissolved particles against 0.3 mol/L for the sucrose, and more cells are expected to plasmolyse in the salt; compare the fraction of cells plasmolysed in each. The learner knows it worked when the same cell is drawn before, during and after and the wall outline is unchanged in all three.

What changes

What you change: External solution and its concentration
What you measure: Time to plasmolysis and degree of protoplast shrinkage
What you keep the same: Tissue source (same onion, same scale)
Temperature
Method of drawing solution through
Cell observed

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

The cell wall shrinks during plasmolysis; only the protoplast does.
Salt moves into the cell and pushes the contents; water moves out.
The cell wall controls what enters the cell; the membrane does, and the wall holds the shape.

Safety card

Low riskLearners carry it out

Hazards

Glass slides and coverslips

Controls

Handle coverslips by the edge; report breakages

Note

No hazardous chemicals; record a RiskAssess risk assessment for the activity as school procedure requires.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science 7–10 Syllabus (2023)SC4-CLS-01 SC4-WS-01 SC4-WS-06
Australian Curriculum v9AC9S8U01AC9S8I03AC9S8I05

Sources

The pages the author read to write this activity.

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