echoesof.science · Ages 15–16
Cells: size, magnification and surface area
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Prokaryotic cells (bacteria and archaea) have no nucleus and no membrane-bound organelles; their DNA lies in the nucleoid. They are 0.1–5.0 µm across. Eukaryotic cells, such as plant and animal cells, have a nucleus and are typically 10–100 µm across.
Magnification is how much bigger an object looks; resolution (resolving power) is the ability to see two close structures as separate. Electron microscopes give higher magnification and resolution than light microscopes.
For a light microscope, total magnification = eyepiece magnification × objective magnification: a 10× eyepiece and a 40× objective give 400×. So an 8 µm red blood cell appears 400 × 8 µm = 3.2 mm wide. Rearranged: actual size = image size ÷ magnification.
Experiment: How big is an onion cell?
- Prepare a wet mount of onion epidermis with one drop of dilute iodine solution; lower the coverslip at an angle to avoid bubbles.
- At the lowest total magnification (10× × 4× = 40×) place the clear ruler on the stage and measure the diameter of the field of view in mm.
- Replace the ruler with your slide. Count how many cells lie end to end across the diameter of the field of view.
- Estimate one cell's length: field-of-view diameter ÷ number of cells. Convert to µm (1 mm = 1000 µm).
- Repeat the count in two other places on the slide, take the mean, and state the uncertainty (half the range of your three results).
| Count 1 | Count 2 | Count 3 | Mean | |
|---|---|---|---|---|
| Field of view diameter (mm) | Predict: will an onion cell be closer to 1 µm, 10 µm or 100 µm long? | |||
| Cells across the field | Predict: will an onion cell be closer to 1 µm, 10 µm or 100 µm long? | |||
| Estimated cell length (µm) | Predict: will an onion cell be closer to 1 µm, 10 µm or 100 µm long? |
Questions
State the equation linking image size, actual size and magnification, and use it: an image is 6 mm wide at 400×. Find the actual size in µm.
Calculate A/V for spherical cells of radius 1 µm and 10 µm. Explain what your answers mean for diffusion.
Give your estimated onion cell length with its uncertainty. Name two sources of error in the method.
Explain the difference between magnification and resolution, and why an electron microscope shows more detail.
Sources for every fact on this sheet: echoesof.science/school/cells/sources/