Microscopy and measuring cells — Edexcel International AS Level Biology
Magnification and resolution in light and electron microscopes, why specimens are stained, and Core Practical 5 on drawing and measuring cells.
Revision notes
3 short notes, in the order of the specification. Each one in short:
Magnification is how many times larger an image is than the object. Resolution is the smallest distance between two points that can still be seen as separate. Light microscopes resolve about 200 nm because of the wavelength of light, whereas electron microscopes use electrons with a much shorter wavelength, so they resolve far smaller structures.
Most cell structures are transparent or have little contrast, so specimens are stained. In light microscopy, coloured dyes bind to particular structures and make them visible. In electron microscopy, salts of heavy metals scatter electrons strongly, giving contrast between structures. Different stains can show up different components, so they help to identify structures.
In Core Practical 5, animal cells such as stained cheek cells are viewed with a light microscope and drawn with clear, labelled outlines. An eyepiece graticule is calibrated against a stage micrometer at each objective lens, so that the actual size of a cell can be calculated from the number of graticule divisions it covers.
Exam questions
9 exam-style questions (23 marks), each with its mark scheme.
Answer the questionsFlashcards
11 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of cell structure, reproduction and development on one page, so you can see where this subtopic fits.
Open the mind mapPrint it
Free PDFs to print or save.
Quick check questions
Define resolution.
Show answer
The smallest distance between two points at which they can still be seen as separate.
Why do electron microscopes have a higher resolution than light microscopes?
Show answer
Electrons have a much shorter wavelength than light.
Why are specimens stained?
Show answer
To increase contrast so that structures are visible and can be told apart.
What is the formula for magnification?
Show answer
Magnification = size of image ÷ size of real object.
Why must an eyepiece graticule be calibrated?
Show answer
Its divisions have no fixed size; they depend on the objective lens, so they must be compared with a stage micrometer.
Frequently asked questions
What is the difference between magnification and resolution?
Magnification is how many times bigger the image is than the real object. Resolution is the smallest distance between two points that can still be seen as separate. A higher magnification does not show more detail unless the resolution is also high enough.
Why can an electron microscope show more detail than a light microscope?
An electron microscope uses a beam of electrons, which have a much shorter wavelength than visible light, so it can resolve points a fraction of a nanometre apart. A light microscope resolves only about 200 nm, so cannot show structures such as ribosomes or the inner membranes of mitochondria.
How do you calibrate an eyepiece graticule?
Line up the eyepiece graticule with a stage micrometer of known scale, count how many graticule divisions match a number of micrometer divisions, then divide the known length by the graticule divisions to find the size of one division. Repeat for each objective lens, because the value changes.
How do you calculate the actual size of a cell from a drawing?
Measure the cell in the drawing, convert it to the same units as the answer you want (usually µm), then use actual size = image size ÷ magnification. For example, 60 mm is 60 000 µm, and at ×4000 the actual size is 15 µm.
Written and checked against the Edexcel International AS Level Biology (XBI11) specification · Updated October 2026