AS Module 1

The Electron Microscope

The electron microscope was first developed in the 1950’s. Unlike optical microscopes which rely on light to provide the resolving power, they use an electron beam. An image is formed by the electrons being scattered to different extremes by the biological material.

The advantage of using electrons

In order to be able to see two objects separately, there must be a minimum distance between them. If they are closer together than this minimum, then the images will merge. This distance is known as the microscope’s limit of resolution. The smaller the limit of resolution, the greater the resolving power. For example, your eyes can only see the letters on this page, not the individual pixels. This is because the gap between them has a smaller limit of resolution than the resolving power of your eyes.

Resolution differs from magnification by the fact that magnification simply refers to the amount an object has increased in size.

The limit of resolution depends on the wavelength of the radiation passing through it. This means that no matter how good a light microscope is, it will always be limited by the wavelength of light.

Electrons behave like light waves , but have a much smaller wavelength. Resolving power is increased as the wavelength gets smaller, and as a result the electron microscope can resolve detail many thousand times better than the light microscope.

This is the most common type of electron microscope. They are used to view extremely thin section of material (30 thousandths of a mm thick). It can magnify objects several hundred thousand times. The electrons produced by the microscope pass through the object with different degrees of penetration. This is due to the variable densities of the material under observation. The microscope then uses these emerging electrons to create an image.

Below is an example of a TEM micrograph. As you can see, it allows a cross-sectional view of the specimen. Inset into the picture is an actual TEM micrograph.

Limitations and benefits of electron microscopes

As can be seen from the pictures above, the amount of detail produced on electron micrographs (EM) is immense. This is the main advantage they have over optical microscopes. In fact, it was only the advent of the EM which allowed many biological structures to be discovered.

There are disadvantages to the EM. Due to the fact that air in the analysis chamber would distort the electron beam, experiments have to be performed in a vacuum. This prevents any living material to be studied. This also then leads to the question, How realistic a picture can be gained by such a processed tissue sample? This point is open to debate. The other problem with EM’s is that they are very expensive, and are mainly confined to research laboratories.

Cell fractionation

Live tissue is placed into an isotonic solution, this prevents distortion of cell organelles. The tissue is then ground up using a homogeniser. Sometimes ultrasound can be used to rupture cells. The resulting homogenate is then placed in a centrifuge, this is a device that spins the contents of a test tube at very high speeds, causing the cell fractions to settle out based on their mass.