🔬 Transmission Electron Microscopy (TEM) — Detailed Summary

This lecture explains how electron microscopes work, why they are powerful for structural biology, how they compare to light microscopes, and what technological breakthroughs enabled the modern “resolution revolution” in cryo-EM.

⚙️ 1. Basic Principle of the Electron Microscope

There are two main types of electron microscopes:

• Scanning Electron Microscope (SEM) → scans the surface
• Transmission Electron Microscope (TEM) → electrons pass through the sample

👉 For high-resolution structural biology, TEM is used because the transmitted electrons can form detailed images of internal structure.

🧠 How TEM Works — Step by Step

1. Electron gun emits electrons
2. High voltage (~300 kV) accelerates electrons
3. Electromagnetic lenses focus the beam
4. Beam passes through the specimen
5. Scattered electrons form an image
6. Image is magnified by additional lenses
7. Recorded on a fluorescent screen or digital camera

Key lens systems:

• Condenser lenses → focus beam on sample
• Objective lens → forms first image
• Intermediate + projection lenses → further magnification

🔍 2. Comparison: Light Microscope vs Electron Microscope

✅ Similarities

Both have:

• Illumination system
• Condenser lens
• Specimen holder
• Objective lens
• Imaging/detection system

❗ Important Differences

In EM, changing lens current → changes focal length and magnification continuously (very flexible).

❄️ 3. Why Samples Must Be Frozen (Cryo-EM Concept)

TEM requires ultra-high vacuum — otherwise electrons scatter and the beam becomes unstable.

But vacuum causes:

• Liquid samples to evaporate instantly
• Biological structures to collapse

Solution → Vitrified ice

• Sample is rapidly frozen
• Water forms amorphous (vitreous) ice
• Maintains native molecular structure
• Stable at liquid nitrogen temperature

This allows proteins and complexes to be imaged without dehydration artifacts.

☢️ 4. Radiation Damage — A Fundamental Limitation

Electron beams destroy biological samples.

Therefore:

• Must use low electron dose
• Trade-off:
  • Low dose → low signal & contrast
  • High dose → high resolution but sample destruction

This balance is one of the central challenges in cryo-EM.

📏 5. Resolution — Why Electrons Are Powerful

Resolution depends on wavelength:

• Visible light → ~400–700 nm
• Electrons (300 kV) → ~2 picometers

This is orders of magnitude smaller, meaning angstrom-level resolution is theoretically possible.

However in biology, practical limits come from:

• Radiation damage
• Poor contrast
• Beam-induced motion

🧬 6. Object Size Range Observable by EM

Electron microscopy spans a huge size range, for example:

• Cells → micrometers
• Bacteria (~E. coli) → few µm
• Viruses → ~50 nm
• Proteins → ~10–15 nm
• α-helices → ~3 nm
• Amino acids → ~5 Å
• Atoms → ~1 Å

Thus TEM can bridge cell biology → molecular → atomic structure.

📚 7. Historical Development of Electron Microscopy

Key milestones:

• 1897 — Thomson discovers electron
• 1920s — de Broglie proposes wave nature of electrons
• 1926 — Busch shows magnetic/electric fields can act as lenses
• 1929 — Ruska develops magnetic lens systems
• First TEM prototype built
• Rapid improvements → resolution surpasses light microscopy
• Commercial TEM produced by Siemens

Modern designs still use many principles invented in the 1920–30s.

💰 8. Modern TEM Instruments and Cost

Typical accelerating voltages:

• 120 kV → smaller and cheaper (~2–3 million DKK)
• 200 kV → mid-range
• 300 kV → large high-end instruments (~40–50 million DKK)

Higher voltage:

• Better penetration
• Higher resolution potential
• Much higher cost and size

⚛️ 9. Why Use Electrons vs X-rays vs Light vs Neutrons

🌈 Visible Light

✔ Non-damaging ✔ Easy to focus ❌ Low resolution

☢️ X-rays

✔ Atomic wavelength ✔ Good penetration ❌ Hard to focus (need diffraction + math reconstruction) ❌ Radiation damage

⚡ Electrons

✔ Even smaller wavelength ✔ Can be focused with magnetic lenses ❌ Poor penetration → sample must be very thin (<100 nm ideal ~30 nm) ❌ Radiation damage

🧊 Neutrons

✔ Very low damage ✔ Small wavelength ❌ Hard to produce ❌ Few facilities

Example: ESS neutron source in Sweden.

🚀 10. The Cryo-EM “Resolution Revolution” (≈2013)

Major breakthroughs:

🎥 Direct Electron Detectors

• Ultra-fast, ultra-sensitive cameras
• Record movies instead of single exposures
• Allows correction of beam-induced motion
• Frames can be aligned → sharper image

Huge improvement in resolution.

🌊 Phase Plates

• Increase phase contrast
• Makes weakly scattering biological particles easier to see
• Improves signal without increasing radiation dose

Essential for visualizing proteins.

🔧 Spherical Aberration (Cs) Correctors

Problem:

• Objective lens imperfections cause electrons to focus incorrectly

Solution:

• Add quadrupole/hexapole correctors
• Sharpen image quality

Major contribution to high-resolution cryo-EM.

💻 Advanced Image Processing

• Powerful computational algorithms
• Automation of data collection
• Single-particle reconstruction

These allowed routine near-atomic structures of proteins.

🧠 Key Take-Home Messages

⭐ TEM enables atomic-level structural biology ⭐ Resolution is limited by radiation damage + contrast, not wavelength ⭐ Samples must be vitrified and extremely thin ⭐ Cryo-EM revolution came from detectors + computation + aberration correction ⭐ Electrons can be focused → unlike X-rays ⭐ Cryo-EM bridges the gap between cell biology and atomic structure