Teacher sheet · Ages 15–16 · about 45–50 minutes
Cells: size, magnification and surface area
Lesson page: echoesof.science/school/cells/ages-15-16/ · Pupil worksheet: echoesof.science/school/cells/ages-15-16/print/
Learning aims
- Compare prokaryotic and eukaryotic cells, including size ranges.
- Use total magnification = eyepiece × objective and actual size = image size ÷ magnification, with unit conversion.
- Explain the limit on cell size using surface-area-to-volume ratio.
- Estimate cell size with uncertainty and evaluate the method.
Lesson plan
| Time | Activity |
|---|---|
| 5 min | Starter: why is there no bacterium the size of a football? |
| 10 min | Read the text; work two magnification examples on the board. |
| 25 min | Field-of-view practical in pairs. |
| 5 min | A/V calculations and the sorter game. |
| 5 min | Quiz; worksheet for homework. |
Materials for the experiment
- Onion epidermis (prepared as a wet mount)
- Microscope slides, coverslips, tweezers, dropper
- Dilute iodine solution from the school stain kit
- Light microscope with 10× eyepiece and 4×, 10× and 40× objectives
- Clear plastic ruler with mm markings
- Gloves and eye protection
Safety: Wear eye protection and gloves when using iodine solution; it stains skin and clothing and irritates eyes, so rinse any splashes with water and tell the teacher. Handle slides and coverslips by the edges and report breakages. Never use direct sunlight as the light source.
Common misconceptions
| Pupils may think… | Actually… |
|---|---|
| Higher magnification always shows more detail. | Above a certain magnification the image gets bigger but not sharper; detail is set by resolution. |
| Bacteria are just small animal cells. | Bacteria are prokaryotes: no nucleus and no membrane-bound organelles. |
| Big cells are better because they hold more. | As cells grow, A/V falls, so the membrane cannot supply the volume fast enough by diffusion. |
Worksheet answers
- Magnification = image size ÷ actual size, so actual size = 6 mm ÷ 400 = 0.015 mm = 15 µm.
- r = 1 µm: A/V = 3 µm⁻¹; r = 10 µm: A/V = 0.3 µm⁻¹. The larger cell has ten times less membrane per unit volume, so diffusion supplies its contents much more slowly.
- Answers vary with the onion and the method; compare them with the textbook range of 10–100 µm for eukaryotic cells and discuss any difference. Errors: cells are not all the same size or lined up; part-cells at the edge of the field; reading the ruler edge.
- Magnification is how much bigger the image is; resolution is the ability to tell two close points apart. Electron microscopes have much higher resolution, so they show smaller structures.
Quiz answers
- A 10× eyepiece and a 40× objective give a total magnification of… 400×. Total magnification is the product: 10 × 40 = 400.[4]
- A cell's image is 4 mm wide at 400×. What is its actual width? 10 µm. 4 mm ÷ 400 = 0.01 mm = 10 µm.[4]
- For a spherical cell, A/V = 3/r. What is A/V for r = 2 µm? 1.5 µm⁻¹. 3 ÷ 2 µm = 1.5 per micrometre.[2]
- Which statement describes a prokaryotic cell? No nucleus; DNA in a nucleoid. Prokaryotes lack a nucleus and membrane-bound organelles.[2]
- Resolution means… the ability to see two close structures as separate. Resolving power distinguishes adjacent structures; magnification only enlarges.[1]
- Roughly how many human cells does an adult have? About 30 trillion. NIH gives an estimate of about 30 trillion.[5]
Going further
Repeat the count at 100× (10× objective). The field of view is smaller: predict its diameter from your 40× value, then check by calculating cell length both ways.
Sources used in this lesson
- [1] Biology 2e, 4.1 Studying Cells (OpenStax, Rice University): reliability High
- [2] Biology 2e, 4.2 Prokaryotic Cells (OpenStax, Rice University): reliability High
- [3] Biology 2e, 4.3 Eukaryotic Cells (OpenStax, Rice University): reliability High
- [4] Microbiology, 2.3 Instruments of Microscopy (OpenStax, Rice University): reliability High
- [5] Cells by the Numbers (National Institute of General Medical Sciences (NIH), Biomedical Beat, 2024): reliability High
Full list with links and a note on each source's reliability: echoesof.science/school/cells/sources/
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