3.5-3.6

What types of objects require an electron microscope for observation?

Objects smaller than about 0.2 nanometers, such as viruses or internal cellular structures.

What does an electron microscope use instead of light?

A beam of electrons.

Why do electrons provide better resolution than visible light?

Electrons have wavelengths about 100,000 times shorter than visible light, allowing much higher resolving power.

What is a key difference in imaging between electron microscopes and light microscopes?

EM images are always black and white but can be artificially colored.

What type of lenses do electron microscopes use?

Electromagnetic lenses to focus the electron beam.

What are the two main types of electron microscopes?

Transmission electron microscope (TEM) and scanning electron microscope (SEM).

How does a transmission electron microscope (TEM) produce an image?

A finely focused beam of electrons passes through a specially prepared, ultrathin section of the specimen.

What directs the electron beam onto the specimen in TEM?

An electromagnetic condenser lens.

What is the function of the condenser lens in TEM?

It focuses the electron beam in a straight line to illuminate a small area of the specimen, similar to a light microscope condenser.

What type of specimen preparation is required for TEM?

Specimens must be ultrathin so electrons can pass through them.

What is the typical magnification range of a TEM?

From 10,000× to 10,000,000×.

Why is contrast often weak in TEM images?

Because most specimens are very thin, making their ultrastructures hard to distinguish from the background.

How is contrast enhanced in TEM?

By staining specimens with heavy metal salts that absorb electrons.

What is positive staining in TEM?

Staining that fixes heavy metals onto the specimen to darken structures.

What is negative staining in TEM?

Staining that increases the electron opacity of the background, making small specimens stand out.

What is negative staining especially useful for?

Studying very small specimens like viruses, bacterial flagella, and protein molecules.

Which metals are commonly used in shadow casting?

Platinum or gold.

At what angle is the metal sprayed in shadow casting?

About 45 degrees.

How does shadow casting create contrast?

What is a key advantage of TEM?

It has high resolution and is valuable for examining different layers of specimens.

What is a major limitation of TEM regarding specimen thickness?

Only ultrathin sections (~100 nm) can be studied effectively.

Why does TEM lack a three-dimensional view?

Because it uses thin sections and electrons cannot penetrate deeply.

Why must TEM specimens be fixed, dehydrated, and viewed under high vacuum?

To prevent electron scattering and obtain clear images.

How do specimen preparation steps affect the specimen?

They kill it and may cause shrinkage or distortion.

What are artifacts in TEM?

Structures that appear due to specimen preparation methods rather than being part of the actual cell.

What problem does SEM overcome compared to TEM?

SEM overcomes the sectioning problems of a transmission electron microscope.

What type of images does SEM produce?

Three-dimensional images of specimens.

How does SEM create an image?

A primary electron beam scans the surface, knocking out secondary electrons that are collected, amplified, and displayed on a screen.

What is an SEM image called?

A scanning electron micrograph

What is SEM especially useful for studying?

Surface structures of intact cells and viruses.

What is the resolution of SEM?

About 0.5 nanometers.

How do scanned-probe microscopes examine specimens?

They use various kinds of probes and electric current to examine the surface.

Why are scanned-probe microscopes advantageous?

They do not modify the specimen or expose it to damaging, high-energy radiation.

What can scanned-probe microscopes be used for?

Map atomic and molecular shapes

• Characterize magnetic and chemical properties

• Determine temperature variations inside cells

Name two types of scanned-probe microscopes.

Scanning tunneling microscope (STM) and atomic force microscope (AFM).

What does STM stand for?

Scanning Tunneling Microscopy.

What type of probe does STM use?

A thin tungsten probe.

What does STM reveal about a specimen?

The bumps and depressions of the atoms on the surface.

How does the resolving power of STM compare to electron microscopes?

STM has much greater resolving power and can resolve features as small as an atom.

Does STM require special specimen preparation?

No, special preparation is not needed.

What can STM be used to view in detail?

Molecules such as DNA.

What does AFM stand for?

Atomic Force Microscopy.

What type of probe does AFM use?

A metal-and-diamond probe.

How does AFM produce an image?

The probe moves along the surface of the specimen, its movements are recorded, and a three-dimensional image is produced.

Does AFM require special specimen preparation?

No, special preparation is not needed.

What can AFM be used to image?

Biological substances in nearly atomic detail and molecular processes, such as the assembly of fibrin in blood clots.