AQA Combined Science (8464) 4.1.1.5 · Quantitative
Microscopy
A school microscope shows you a cell’s nucleus but never its ribosomes. What does a microscope need in order to show smaller things, and how do you work out how big a cell really is?
Combined Science · Foundation tier
Start here
Your cells are full of ribosomes. For nearly 300 years, nobody saw one.
Robert Hooke looked at cork through a microscope in 1665 and named the tiny boxes he saw cells. Better and better light microscopes followed. Ribosomes were there all along, and none of those microscopes ever showed one.
If you had to guess, to see a ribosome, what would you need most: a microscope that makes things even bigger, or one that shows finer detail?
A light microscope passes light through a thin specimen and magnifies it with glass lenses, up to about ×2000. It can show living cells, but it cannot separate two points closer together than about 200 nm. The electron microscope, invented in the 1930s, uses a beam of electrons instead. It magnifies up to about ×2 000 000 and separates points about 0.1 nm apart, but the specimen sits in a vacuum, so it must be dead.
Magnification is how many times bigger the image is than the real object. Resolution is the ability to distinguish two points that are close together as separate points.
Resolve it · light microscope, ×1500
Two mitochondria, 2 µm apart
0 of 4 called
Commit first. What will the view show?
Required practical · RP1 Combined Science · RP1 Biology
Prepare a slide. Focus. Draw what you see.
- Peel a thin layer of epidermis from a piece of onion with forceps. Lay it flat in a drop of iodine solution on a microscope slide.
- Lower a coverslip at an angle with a mounted needle, so no air bubbles are trapped.
- Put the slide on the stage and start with the lowest-power objective.
- Watching from the side, raise the stage until the slide is just below the lens. Then, looking through the eyepiece, lower it with the coarse focus until the cells appear. Sharpen them with the fine focus.
- Switch to a higher-power objective and refocus with the fine focus only.
- Draw what you see with a sharp pencil: clear, continuous lines and no shading. Label with ruled lines, and add a title and the magnification or a scale bar.
Risks. Iodine solution irritates the eyes and stains skin: wear eye protection. Slides and coverslips are thin glass: hold them by the edges.
Run the practical · onion epidermis in iodine
Turn up the magnification.
Eyepiece ×10 · objective ×4
Check the drawing
IdentifyA student drew onion epidermis cells seen at ×400. Which label should come off?
A drawing or a photograph also tells you a real size, if you know its magnification. Cells are measured in micrometres (µm), and the structures inside them in nanometres (nm). Very large and very small answers go in standard form: ×2 000 000 is 2 × 10⁶, and 0.00015 mm is 1.5 × 10⁻⁴ mm.
Equations
Learn it · no biology equation sheetM = I ÷ A
magnification = size of image ÷ size of real object
total = eyepiece × objective
The magnifications multiply: ×10 with ×40 is ×400.
mm ×1000 → µm
µm ×1000 → nm
Image size and real size in the same unit before you divide. Magnification has no unit.
Think again
“The image is 12 mm and the cell is 40 µm, so the magnification is 12 ÷ 40 = ×0.3.”
Spot the flaw
CalculateWhat should the magnification be?
Command words in this lesson
- State
- A short answer with no reason: for resolution, “distinguish two points close together”, not “clearer”.
- Calculate
- Every line written: units matched first, the equation, the numbers, the answer with its unit, or none for magnification.
- Explain
- Link each step to the next: resolution, then what can be distinguished, then what was discovered.
- Describe
- For a method: the steps in order, with the equipment, so someone else could follow it.
Key fact
More magnification without more resolution only makes a bigger blur. The electron microscope’s far higher resolution is what let biologists see ribosomes and the inside of mitochondria.
Key note · AQA 4.1.1.5 (8464)
Microscopy
- M = image size ÷ real size, both in the same unit: 1 mm = 1000 µm, 1 µm = 1000 nm.
- Total magnification = eyepiece × objective.
- Required practical (RP1 Combined / RP1 Biology): onion epidermis in iodine, lowest power first, a clear pencil drawing with a title and a magnification or scale bar.
Photograph this card. Tomorrow, cover it and say each line first.
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Ask MrBadmus
Stuck on magnification, or on converting between mm, µm and nm?
The views are models. In Resolve it the blur is drawn to show the idea, not calculated from the optics. On the bench the cells are flat outlines laid out like bricks, the field of view is set at 0.45 mm across at ×400, and each field is chosen to give a whole number of cells along the line. Real cells, stains and microscopes vary.