Plasmolysis and recovery in red onion epidermis · Concept Studio

The idea

In a concentrated salt solution the cytoplasm loses water and the membrane pulls away from the wall; the change reverses when water is returned, showing that osmosis acts across the membrane, not the wall.

What you need

Red onion, 1; forceps; scalpel on a white tile
Sodium chloride solution 5 percent (5 g per 100 mL), 20 mL in a dropper bottle; distilled water in a dropper bottle
Optional quantitative series: sodium chloride 0, 1, 2, 3, 4 and 5 percent, 10 mL each
Microscope slides and coverslips, 4 each; filter paper strips; compound microscope

How to do it

Peel a single pigmented layer of 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, then 400x, and draw two or three cells: the red cell sap fills the whole cell and the membrane is pressed against the wall (turgid).
Place a drop of 5 percent sodium chloride at one edge of the coverslip and draw it through with filter paper at the opposite edge; watch the same cells for two to five minutes.
Draw the same cells again once the red contents have shrunk away from the wall (plasmolysed).
Draw distilled water through in the same way and record whether and how quickly the cells recover.
For a quantitative run, mount fresh strips in each salt concentration, count 50 cells after five minutes and record the percentage plasmolysed at each concentration.

What you should see

In 5 percent sodium chloride the red protoplast contracts into the centre of the cell within a few minutes, leaving a clear gap inside the unchanged cell wall; on returning water the protoplast re-expands and most cells recover. In the concentration series the percentage of plasmolysed cells rises with salt concentration, with about half plasmolysed at the concentration that matches the cell sap. The learner knows it worked when the same identified cells are seen turgid, then plasmolysed, then recovered.

What changes

What you change: sodium chloride concentration (percent)
What you measure: percentage of cells plasmolysed after 5 minutes
What you keep the same: onion
time before counting
temperature
number of cells counted

Common misconceptions

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

The cell wall shrinks during plasmolysis; the wall keeps its shape and only the membrane-bound contents contract.
Salt enters the cell and pushes the contents inward; water leaves the cell by osmosis.
Plasmolysed cells are dead; most recover when water is restored.

Safety card

Low riskLearners carry it out

Hazards

Onion vapour irritating the eyes
Scalpel cuts
Broken coverslips

Controls

Cut on a tile away from the body
Rinse eyes with water if irritated
Broken glass to the sharps bin

Note

Sodium chloride solution is low hazard: RiskAssess (https://www.riskassess.com.au/).

Curriculum references

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

Biology Stage 6 Syllabus (2017), current: Year 11 taught to the end of 2026 and Year 12 to Term 3 2027BIO11-8 BIO11/12-3
Biology 11–12 Syllabus (2025), Year 11 focus area Cells as the basis of life; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-11-01
Biology 11–12 Syllabus (2025), Year 11 Working scientifically; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-11WS-03
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.