Biology 11–12 · Year 11

Prokaryotic and eukaryotic cells under the light microscope, with calibrated scale bars

Module 1: Cells as the Basis of Life (Cell Structure)

Practical, model not builtLow risk

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The idea

Cells differ in size and internal organisation, and a measured field of view turns a drawing into a scaled record of what is there.

What you need

Compound light microscope with 4x, 10x and 40x objectives and a 10x eyepiece
Stage micrometer (1 mm ruled in 0.01 mm divisions) or a clear plastic ruler with millimetre marks for the 4x objective
Prepared stained slides: bacteria (for example Bacillus subtilis or a mixed bacterial smear from an educational supplier), yeast, human cheek epithelium, onion epidermis
Fresh red onion, 1 bulb; forceps; scalpel on a white tile; slides and coverslips, 6 of each
Pond water or a hay infusion, 50 mL, for unicellular and colonial organisms
Iodine solution in a dropper bottle, 10 mL; distilled water in a dropper bottle
Sharp pencil and unlined paper for scaled drawings

How to do it

Place the millimetre ruler or stage micrometer on the stage under the 4x objective (40x total) and count how many millimetres span the field of view; record the field diameter in millimetres.
Calculate the field diameter at 100x and 400x: diameter at higher power = diameter at 40x multiplied by (40 divided by the higher total magnification).
Mount a single layer of red onion epidermis in a drop of water (or iodine solution to stain the nuclei), add a coverslip, and view at 100x then 400x; count how many cells fit across the field diameter and estimate one cell length as field diameter divided by that count.
View the prepared bacterial slide at 400x and note that individual cells are close to the resolution limit; estimate their length against the field diameter in the same way.
View the cheek epithelium and yeast slides and a drop of pond water; classify each organism seen as prokaryotic or eukaryotic and note whether it is unicellular, colonial or multicellular.
Draw one cell of each type in pencil, filling at least half a page, with a title, the total magnification, a scale bar labelled in micrometres derived from your field measurement, and labels for the structures you can see (cell wall, membrane, nucleus, cytoplasm, vacuole).
Repeat the size estimate on three different cells of each type and report the mean and range.

What you should see

With a 4.5 mm field at 40x the field at 400x is 0.45 mm (450 micrometres); eight onion cells across that field gives an estimated cell size of about 56 micrometres (computed). Onion epidermal cells appear as elongated brick-shaped cells, longer than they are wide, cheek cells as flattened irregular cells of a few tens of micrometres, yeast as ovals of about 5 to 10 micrometres, and bacteria as rods or spheres of about 1 to 5 micrometres that show no visible nucleus. The learner knows the scale bar is right when the estimated onion cell size is consistent between fields and between the 100x and 400x views.

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.

A higher magnification shows a larger area; in fact the field of view shrinks as magnification rises.
Bacteria have a nucleus that is too small to see; they have no nucleus at all.
All cells are about the same size; onion, cheek, yeast and bacterial cells differ by more than an order of magnitude.

Safety card

Low riskLearners carry it out

Hazards

Scalpel cuts when peeling onion epidermis
Broken slides and coverslips
Onion vapour irritating the eyes
Pond water may carry microorganisms

Controls

Cut on a white tile away from the body; carry scalpels in a tray
Dispose of broken glass in the sharps bin
Cover cuts and wash hands after handling pond water; do not eat or drink in the laboratory
Prepared bacterial slides are fixed and stained, so no live bacteria are handled

Note

Iodine solution is a low-hazard irritant: see RiskAssess (https://www.riskassess.com.au/) and the NSW Department of Education Chemical Safety in Schools package (https://education.nsw.gov.au/content/dam/main-education/asset-management/chemical-safety/1._Section_1_-_General_information_for_all_staff.pdf).

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 BIO11/12-4
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 BI-11WS-04
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

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