Cell Biology · Advanced: Introduction & Imaging
07 — Electron Microscopy
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Why this matters
Electron microscopy reveals cellular ultrastructure at resolutions impossible with light — organelles, membranes, ribosomes, and macromolecular complexes become directly visible. Understanding EM is essential for interpreting images of cellular architecture.
The college version
Core Explanation
Electron microscopy uses a beam of electrons rather than photons. Because electrons have much shorter wavelengths (~0.004 nm at 100 keV) than visible light (~500 nm), the diffraction limit is dramatically reduced, enabling sub-nanometer resolution. In practice, resolution is limited by sample preparation, lens aberrations, and contrast — not by the electron wavelength.
Transmission Electron Microscopy (TEM)
Electrons pass through a very thin specimen (typically 50–100 nm sections). Regions that scatter electrons appear dark (electron-dense); regions that transmit electrons appear bright.
Sample preparation:
- Fixation: Glutaraldehyde cross-links proteins; osmium tetroxide fixes lipids and adds electron density
- Dehydration: Ethanol series, then resin embedding
- Sectioning: Ultramicrotome with diamond knife → 50–100 nm slices
- Staining: Heavy metals (uranyl acetate, lead citrate) bind to cellular components, providing contrast
Key point: TEM shows internal ultrastructure — organelle membranes, ribosomes, cytoskeletal filaments, vesicles — at resolutions of ~1–2 nm in biological specimens. Cryo-EM (rapid freezing, no fixation/staining) can achieve near-atomic resolution for purified macromolecules.
Scanning Electron Microscopy (SEM)
Electrons scan across the specimen surface. Secondary electrons emitted from the surface are detected, producing a 3D-appearing topographical image. Resolution: typically ~1–10 nm for biological specimens.
Sample preparation:
- Fixation and dehydration (similar to TEM)
- Critical point drying — prevents surface tension damage during drying
- Sputter coating — thin layer of gold or platinum makes the surface conductive and increases secondary electron emission
Key point: SEM visualizes surface topology — the shapes of whole cells, tissues, and organisms.
Compare: TEM vs SEM
| Feature | TEM | SEM |
|---|---|---|
| What is imaged | Internal structure (thin section) | Surface topology |
| Electron path | Through specimen | Scanned across surface |
| Resolution | ~1–2 nm (biological) | ~1–10 nm |
| Sample | Ultrathin sections | Whole, coated specimens |
| Image appearance | 2D projection | 3D-like surface view |
| Typical use | Organelle ultrastructure | Cell/tissue surface morphology |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Light microscopes use light to see cells. Electron microscopes use a beam of electrons instead — electrons have much smaller waves, so they can see much tinier things, down to individual protein molecules.
TEM is like a slide projector: the electron beam shines through an incredibly thin slice of a cell. The dense parts block electrons and look dark; the empty parts let electrons through and look bright. You see the insides of organelles.
SEM is like taking a photo of the outside: the electron beam scans across the surface, and the microscope detects electrons bouncing off, creating a picture that looks 3D. It shows you the shape of the cell, not what is inside.
Key takeaways
- A researcher wants to determine the diameter of clathrin-coated vesicles in a cell. Which EM technique is appropriate, and why?
- Why must SEM specimens be sputter-coated with metal?
- TEM achieves sub-nanometer resolution for some specimens. Why is biological TEM typically limited to ~1–2 nm?
- TEM — vesicles are internal structures requiring thin sections to visualize. TEM provides the necessary resolution (~1–2 nm) to measure vesicle dimensions accurately.
- Biological specimens are non-conductive. The metal coating (gold/platinum) prevents charging from the electron beam (which would distort the image) and increases secondary electron emission for better signal.
- Biological sample preparation (chemical fixation, dehydration, resin embedding, heavy-metal staining) introduces artifacts and limits resolution. Beam damage and inherent contrast limitations in organic material also restrict achievable resolution compared to inorganic specimens. Cryo-EM avoids some of these issues for purified samples.
Study tools & related lessonsYou’ll learn to · Related
You’ll learn to
- Compare TEM and SEM principles and applications
- Explain why EM requires specialized sample preparation
- Describe the resolution regime of electron microscopy
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