Developments in microscopySpec A2.2.3
In short
Electron microscopes use a beam of electrons, which has a much shorter wavelength than light, so they have far higher resolution and reveal cell ultrastructure. Freeze fracture splits frozen membranes to show the proteins inside them, cryogenic electron microscopy reveals the 3D structure of proteins, and fluorescent stains and immunofluorescence show where specific molecules are inside cells.
Resolution is the ability to distinguish two points that are close together as separate. Increasing magnification without increasing resolution only gives a bigger, blurred image.
| Feature | Light microscope | Electron microscope |
|---|---|---|
| Radiation used | Light | A beam of electrons |
| Resolution | About 0.2 µm (200 nm) | About 1–2 nm for biological specimens |
| Useful magnification | Up to about ×1500 | Over ×100 000 |
| Specimens | Living or dead; natural colour can be seen | Dead only, because the specimen is in a vacuum; images are black and white |
| What it shows | Cells, nuclei, chloroplasts | Ultrastructure: ribosomes, membranes and the inside of organelles |
The advantage of electron microscopy is its much higher resolution, because electrons have a far shorter wavelength than light. Transmission electron microscopes pass electrons through thin sections to show internal structure. Scanning electron microscopes scan the surface to give 3D images.
Freeze fracture
A sample is frozen rapidly and then cracked with a blade. The fracture often runs along the middle of membranes, between the two layers of lipid. A thin metal replica of the fractured surface is examined with an electron microscope. Freeze fracture revealed proteins spanning the membrane, which supported the fluid mosaic model.
Cryogenic electron microscopy (cryo-EM)
Samples are frozen so fast that water forms glass-like ice rather than crystals, which preserves molecules in their natural shape. Thousands of images of individual molecules are combined by computer to build a 3D model of a protein, often at near-atomic resolution, without the need to crystallise it. Cryo-EM is used to find the structures of membrane proteins and virus capsids.
Fluorescence in light microscopy
- Fluorescent stains bind to specific structures, such as DNA, and emit visible light when illuminated with light of a particular wavelength (often ultraviolet). The stained structures glow against a dark background.
- Immunofluorescence uses antibodies carrying a fluorescent marker. Each antibody binds one specific protein (its antigen), so the glow shows exactly where that protein is in the cell. Several proteins can be labelled in different colours at once.
Give a specific advantage: 'higher resolution, so ribosomes and membranes can be seen' scores; 'a clearer picture' does not.
Written and checked against the IB Biology SL specification · Updated October 2026