Science 7–10 · Year 8

Plant and animal cells: onion epidermis and cheek cells side by side

Science understanding: Biological sciences

PracticalMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

The idea

Plant cells have a wall and a regular shape while animal cells do not, and both have a nucleus and membrane visible after staining.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

Human tissue on buds and slides, where a school allows learners to sample their own cheek cells
Iodine and methylene blue stain skin and clothing
Sharp scalpel and glass

Controls

The commercially prepared slide is the default, because Science ASSIST does not recommend the use of human tissue or body fluids in school science practical classes and records that some school jurisdictions have established policies prohibiting cheek sampling; no NSW Department of Education source on sampling human cells is cited here, so cheek sampling goes ahead only where the school's jurisdiction and its own risk assessment allow it, and then each learner samples only their own cheek and handles only their own bud and slide, the condition set by the Queensland CARA biological activities guideline
Used buds and slides go into disinfectant, then to the technician
Gloves for stains; wipe spills at once
Cut onion on a board with the blade away from the hand

Note

Chemicals: follow the NSW Department of Education Chemical Safety in Schools (CSIS) package (section 1, general information for all staff) and complete a RiskAssess risk assessment before the lesson. Confirm the CSIS category of each stain (methylene blue, and iodine in potassium iodide solution) in CSIS Appendix A or D before learners use it: under CSIS Section 1 (Figure 1.7-3) a chemical not listed in Appendix A or D is treated as category N, for demonstration by approved teachers only, until an approved (Code 1) user completes a site-specific risk assessment.

What you need

Brown onion, forceps, scalpel or scissors
Iodine in potassium iodide solution (Lugol's) in a dropper bottle
Methylene blue stain, a dilute aqueous solution from a school supplier, in a dropper bottle
Sterile cotton buds, one per learner, and a beaker of disinfectant (for example 1 % bleach) for used buds and slides, needed only where a school allows learners to sample their own cheek cells
Slides, coverslips, mounted needle, paper towel, compound microscope
A commercially prepared slide of human cheek cells, one per pair: the alternative Science ASSIST recommends, and the default here

How to do it

Snap an onion scale and peel a single transparent layer of epidermis from its inner face with forceps; lay it flat in a drop of water on a slide.
Add one drop of iodine solution beside the tissue, lower a coverslip with the needle, and blot excess liquid.
View at 40x, then 100x, then 400x. Draw two or three cells; label wall, membrane, nucleus and cytoplasm.
For the animal cell, view the commercially prepared slide of cheek cells. Science ASSIST does not recommend the use of human tissue or body fluids in school science practical classes because of the risk of disease transmission, and records that some school jurisdictions have policies prohibiting the activity, so the prepared slide is the default. Where a school's jurisdiction and its own risk assessment allow learners to sample their own cheek cells, each learner rubs the inside of their own cheek with a fresh cotton bud, smears it on a clean slide, adds one drop of methylene blue and a coverslip, and puts the bud straight into the disinfectant.
View the animal cells at 100x then 400x. Draw two or three cells; label membrane, nucleus and cytoplasm.
Estimate the size of one cell of each type from the field of view, and list the structures present in one and absent in the other.
Slides go into the disinfectant beaker at the end; wash hands.

What you should see

Onion epidermis appears as a pavement of brick-shaped cells with a clear wall outline and a stained nucleus in each; there are no chloroplasts because the bulb scale is not a photosynthetic organ. Cheek cells appear as scattered, irregular, flat cells with a stained nucleus and no wall. Each size comes from the field of view: a cheek cell that spans one-eighth of the 0.45 mm field at 400x is about 0.06 mm across. The learner knows it worked when both nuclei are visible and the drawings show a wall on one cell type only.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

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

All plant cells contain chloroplasts.
The cell wall is the same thing as the cell membrane.
Animal cells are smaller because they are simpler; size and complexity are unrelated.

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-04
Australian Curriculum v9AC9S8U01AC9S8I03

Sources

The pages the author read to write this activity.

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