16.4 Electron Microscope

16.4 Electron Microscope

  • Experiments in 1925 showed that the electrons passing through the crystals form wavelike patterns with a configuration corresponding to the wavelength given.
  • The size of the smallest object observable by a microscope is about half the wavelength of the illuminating radiation.
  • The resolution of light microscopes is limited to 200 nm.
  • Because of the wave properties of electrons, it is possible to build microscopes that are 1000 times smaller than this value.
  • It is easy to accelerate electrons in an evacuated chamber to high speeds so that their wavelength is less than 10 m. The direction of motion of electrons can be changed by electric and magnetic fields.
    • The fields can act as a lens for the electrons.
  • The development of electron microscopes that can observe objects 1000 times smaller than are visible with light microscopes is a result of the short wavelength of electrons and the possibility of focusing them.
    • The electron and light microscope have the same basic configuration of two lens which produce two-stage magnification.
    • The electrons are accelerated and collimated into a beam.
    • The electrons are diffracted in the same way light is diffracted in an optical microscope when the beam passes through the thin sample.
  • There is a region of the myelin sheath surrounding the axon.
  • The electrons are focused by the objective lens.
  • The final image is projected onto film or a fluorescent screen with the help of the projector lens.
    • The theoretical optimum resolution implied by short wavelength electrons has not yet been realized.
    • The best resolution of electron microscopes is about 10 m.
  • The microscope needs to be in an evacuated chamber because electrons are scattered by air.
    • The samples must be dry and thin.
    • The samples need to be prepared for an electron microscope.
    • They have to be dry, thin, and coated.
  • The discovery of X-rays was made in 1895 by Conrad Roentgen.
    • He found that when high-energy electrons hit a material such as glass, they emit radiation that enters objects which are opaque to light.
  • The radiation X-rays were called by him.
    • It was shown that X-rays come from excited atoms.
  • X-rays can expose film and produce images of objects in containers.
  • A film shows the shadow cast by an object.
  • Two French physicians obtained X-rays of bones in a hand within three weeks of Roentgen's announcement.
    • X-rays have become one of the most important diagnostic tools.
  • It is possible to see internal body organs that are transparent to X-rays.
    • A fluid opaque to X-rays is injected into the organ.
    • By contrast, the walls of the organ show up clearly.
  • Valuable information about the structure of biologically important molecules has been provided by X-rays.
    • The wavelength of X-rays is about the same as the distance between atoms in a molecule or crystal.
    • If a beam of X-rays passes through a crystal, it will produce a pattern of light that contains information about the structure and composition of the crystal.
    • Spotting of varying brightness can be seen in the regions of high and low X-ray intensity.
  • There is a way to detect the X-rays by a crystal.
  • Diffraction studies can be done with molecules that can be formed into a periodic array.
    • The proper conditions can allow for the crystallization of many biological molecules.
    • The molecule in the crystal isn't a unique, unambiguous picture of it's shape.
    • The pattern is a representation of the collective effect of the arrayed molecules on the X-rays.
    • The structure of the molecule must be deduced from the indirect evidence.
  • The X-ray diffraction pattern is easy to interpret if the crystal has a simple structure.
    • Complicated crystals, such as those made from organic molecule, produce very complex patterns.
    • It is possible to get some information about the structure of the molecule forming the crystal.
    • Diffraction patterns must be formed from thousands of different angles to resolve the threedimensional features of the molecule.
    • The patterns are analyzed with the help of a computer.
    • Critical information for the determination of the structure of penicillin and many other biologically important molecules was provided by these types of studies.
  • The X-ray picture doesn't give depth information.
    • As the X-ray beam passes through the object, the image shows the total attenuation.
    • A conventional X-ray of the lung may show the existence of a tumor, but it won't show how deep the lung is.
  • X-ray computerized tomography (CT Scan) was developed in the 1960s.
    • A thin beam of X-rays passes through the plane we want to see and is detected by an opposing detector.
    • The process is repeated around the object after the angle is changed by a small amount.
  • While at each position, the detected signal carries integrated information about the full path, two paths that intersect contain common information about the one point of intersection.
    • Information about the X-ray transmission properties of each point within the plane of the object is contained in the multiple images obtained by translation and rotation.
    • A point by point image is constructed of the thin slice scanned within the body after these signals are stored.
  • The slices obtained in this way are typically about 2mm thick.
    • In the newer versions of the instrument, a fan rather than a beam of X-rays scans the object, and an array of multiple detectors is used to record the signal.
    • This method of data acquisition yields an image in a few seconds.