What you need to know
- 3.7(i)magnificationHow many times larger an image is than the real object: magnification = image size ÷ actual size. and resolutionThe smallest distance between two points that can still be seen as separate points. in light and electron microscopy.
- 3.7(ii)The importance of staining.
- 3.6Using electron images to recognise cell structures.
By the end, you should be able to…
- Explain why a bigger image is not always more detailed.
- Compare light microscopy, TEM and SEM.
- Calculate magnification and actual size with consistent units.
Build your understanding
Magnification is size; resolution is detail
magnificationHow many times larger an image is than the real object: magnification = image size ÷ actual size. is image size divided by actual object size. resolutionThe smallest distance between two points that can still be seen as separate points. is the ability to distinguish two close points as separate. Making a blurred image larger does not reveal features that were never resolved.
A conventional light microscope uses visible light and glass lenses. Its useful resolution is commonly around 0.2 µm under good conditions, not a universal limit for every modern optical technique. Cell outlines and nuclei can be visible, but individual ribosomeA small organelle made of rRNA and protein, where translation takes place. cannot be resolved by an ordinary school light microscope.
Why electrons reveal finer structure
An electron beam has a much shorter effective wavelength than visible light, allowing much higher resolving power under suitable conditions. Electromagnetic lenses control and focus the beam. Resolution also depends on the instrument and specimen preparation; high magnification alone does not guarantee a clear result.
Transmission electron microscopy (TEM) uses electrons passing through a thin specimen to reveal internal ultrastructureThe detailed structure of a cell as seen with an electron microscope. as a two-dimensional section. Scanning electron microscopy (SEM) scans the surface and detects signals that reveal surface form, producing an image with a three-dimensional appearance. It does not show a transparent view through the whole cell.
Conventional EM preparation and vacuum conditions prevent observing living cells in the normal school-level comparison. Light microscopy can be used with living specimens. EM images are originally greyscale; any added colour is a presentation choice, not the natural colour of the structures.
Staining adds contrast
Many cell components transmit light similarly and are hard to distinguish unstained. A stain binds preferentially to some components and increases contrast, making boundaries or structures such as nuclei easier to see. Staining does not itself turn a low-resolution microscope into a high-resolution one.
Electron-dense stains can increase contrast in TEM by scattering electrons differently in different regions. Fixation, sectioning and staining can introduce artefacts, so interpret images as prepared observations rather than perfect views of an untouched living cell.
Calculate with one set of units
Magnification = image size / actual size; actual size = image size / magnification. Convert lengths before dividing: 1 mm = 1,000 µm and 1 µm = 1,000 nm. Magnification is a ratio and has no length unit.
Calculate with one set of units (continued)
An image 45 mm across representing a cell 30 µm across has magnification 45,000/30 = ×1,500. For a scale bar, actual object length = measured object length / measured bar length × labelled bar value. If an object is four times the bar labelled 2 µm, it is 8 µm long.
An embedded scale bar stays valid when the image and bar are resized together. A printed magnification label may become wrong after resizing. Never measure a website image using an assumed physical screen size.
NOT: a larger image is not a more detailed image. Stain does not raise resolving power.
- Resolution is distinguishing two close points as separate
- School light microscopes do not resolve ribosomes
- TEM and SEM preparations are not views of untreated living cells
- Added colour on an EM image is not natural colour
Two traps this diagram blocks
- Do not treat magnification and resolution as the same measurement.
- Do not claim an electron micrograph of a prepared section shows the living cell in colour.
From the diagram alone
Question
Explain the difference between magnification and resolution, and state one structure a TEM can show that a school light microscope cannot. [3]
Show a model answer
Magnification is how many times larger the image is than the object. Resolution is the ability to tell two close points apart. A TEM can show ribosomes or membrane detail inside organelles that a school light microscope cannot resolve.
| Method | Useful view | Important limitation |
|---|---|---|
| Light | Cells, larger structures, some living processes | Lower conventional resolution |
| TEM | Internal detail in thin sections | Prepared specimen; a 2D slice |
| SEM | Surface topography | Does not reveal internal detail like a thin-section TEM |
Make your reasoning visible
Use the word “separate” for resolution
Explain that two nearby points can be distinguished, not just that the image is larger or clearer.
Convert before calculating
Changing mm to µm multiplies by 1,000. Keep image and actual lengths in the same units.
Match the method to the question
Internal membrane detail suggests TEM; surface form suggests SEM; watching living cells usually suggests light microscopy.
Key explanations
- TEM passes electrons through a thin section and shows internal membranes as a greyscale image.
- SEM scans a surface. It shows surface shape and does not show a transparent view through the cell.
Avoid this misconception
The claim: Increasing digital zoom always improves resolutionThe smallest distance between two points that can still be seen as separate points..
The correction: Zoom enlarges existing pixels; it does not recover detail absent from the original image.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
A nucleus is 6 µm across and its image is 24 mm across. Calculate magnification.
Show suggested answer
24 mm = 24,000 µm. Magnification = 24,000/6 = ×4,000.
Practice question
Retrieval and explanation · suggested answer
A scale bar labelled 500 nm measures 10 mm on a printout. A structure measures 30 mm. Find its actual length in µm.
Show suggested answer
30/10 × 500 = 1,500 nm = 1.5 µm.
Practice question
Retrieval and explanation · suggested answer
Why stain a transparent animal-cell preparation?
Show suggested answer
Staining increases contrast between components, helping identify structures such as nuclei; it does not fundamentally improve the optical resolving limit.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A micrograph is enlarged by 200%, including its scale bar. Does an actual-size calculation using the ratio of object length to bar length change?
Show a hint
Both measured lengths are multiplied by the same factor.
Explore the reasoning
No. The factor cancels in the ratio, so the calculated actual size is unchanged. A separately printed magnification label would need updating.
Recall this lesson
Lesson recall questions
Question 1 of 5: Compare the appearance of rER and Golgi on an electron micrograph.
Recognising organelles on EM
Practice question
Retrieval and explanation · suggested answer
Compare the appearance of rER and Golgi on an electron micrograph.
Lesson: 3A 2: Eukaryotic Cells 1: Common Cellular Structures
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Beyond the conventional light microscope
3A · Observing CellsThe 2014 Chemistry Nobel Prize recognised super-resolved fluorescence microscopy. Specialised techniques can reveal detail beyond conventional light-microscopy limits. A research microscopist chooses a method according to the question:…Specification 3.7(i)Read & use the scienceClose article
Why this matters
The 2014 Chemistry Nobel Prize recognised super-resolved fluorescence microscopy. Specialised techniques can reveal detail beyond conventional light-microscopy limits. A research microscopist chooses a method according to the question: does the experiment need the position of a labelled molecule, the outline of an organelle, or a measured cell size? Fluorescent tags can reveal where selected proteins occur, but a bright spot is not every structure in the cell. Resolution describes whether two nearby features can be distinguished; magnification only makes the image larger. This is why a scale bar and the staining or labelling method belong beside a published micrograph.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.7(i) know how magnification and resolution can be achieved using light and electron microscopy
Use the science
Why does this not mean that digitally enlarging a school microscope image gives the same result?
Explore a possible response
The research methods collect or localise information differently. Zooming a finished image does not acquire new structural information.
Explore the source
- Nobel Prize in Chemistry 2014Nobel Prize Outreach
Freezing molecules to see their shape
3A · Observing CellsCryo-electron microscopy freezes biological molecules in a thin layer of ice and combines many electron images to determine structures. The 2017 Chemistry Nobel Prize recognised the researchers who…Specification 3.7(i)Read & use the scienceClose article
Why this matters
Cryo-electron microscopy freezes biological molecules in a thin layer of ice and combines many electron images to determine structures. The 2017 Chemistry Nobel Prize recognised the researchers who developed this approach. It answers a different question from a stained classroom slide: not merely where a cell is, but what a biomolecule looks like at high resolution. A structural biologist still has to identify what was imaged and justify how the images were combined. More detail depends on resolving new information, not enlarging a low-resolution picture. This extends the specification’s comparison of light and electron microscopy without making students learn a new microscope protocol.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.7(i) know how magnification and resolution can be achieved using light and electron microscopy
Use the science
Why would enlarging one low-resolution micrograph not reveal the molecular shape obtained by cryo-EM?
Explore a possible response
Enlargement increases image size but does not recover missing detail. Cryo-EM collects electron information from prepared molecules and combines many images to resolve structure.
Explore the source
- The Nobel Prize in Chemistry 2017Nobel Prize Outreach