Edexcel A (SNAB) A Level Biology:复习笔记3.1.7 Recognising Organelles

• Electron microscopes can produce highly detailed images of animal and plant cells
• This is because they have a higher resolution, or resolving power, than a light microscope
  • Resolution is the ability to distinguish between two separate points
  • If two separate points cannot be resolved they will be observed as one point, producing an unclear image
  • Magnification is limited by the resolution of a microscope; there is no point in magnifying an image many times if the resulting image is unclear
• The resolution of a light microscope is limited by the wavelength of light
  • The wavelength of light is too long to provide a high resolution
• Electron microscopes have a much higher resolution than a light microscope as electrons have a much smaller wavelength than visible light
  • This means that electron microscopes can distinguish between two separate points that are much closer together than the points that can be distinguished by a light microscope
• Electron microscopes are therefore capable of higher magnification before the image becomes unclear
• Some of the key cellular structures within animal and plant cells are visible within electron micrographs
  • Electron micrographs are the images produced by electron microscopes
• Electron microscopes, both scanning and transmission, are used for specimens larger than 0.5 nm
  • Electron microscopes fire a beam of electrons at or through a specimen
  • The electrons are picked up by an electromagnetic lens which then generates an image
  • Electron microscopes are useful for looking at organelles, viruses, and DNA, as well as looking at whole cells in more detail
  • Electron microscopy requires the specimen to be dead which can provide a snapshot in time of what is occurring in a cell, e.g. DNA can be seen replicating and chromosome position within the stages of mitosis are visible

Electron Microscopes Compared with Light Microscopes Table

Organelles under the electron microscope

• There are two types of electron microscope
  • Transmission electron microscopes (TEMs)
  • Scanning electron microscopes (SEMs)
• Transmission Electron Microscopes
  • TEMs use electromagnets to focus a beam of electrons
  • This beam of electrons is transmitted through a thin specimen
  • Denser parts of the specimen absorb more electrons
  • Denser parts appear darker on the final image, producing contrast between different parts of the object being observed
  • The internal structures within cells, or even within organelles, can be seen as an 2D image
  • The resolution of these images are very high
• Scanning Electron Microscopes
  • SEMs scan a beam of electrons across the specimen
  • This beam bounces off the surface of the specimen and the electrons are detected, forming an image
    • This means SEMs can produce 3D images that show the surface of specimens
    • Since they scan the outside surface it means that the specimens viewed does not have to be thin
  • The images they form are of a lower resolution than TEMs

A stained TEM microscope of the nucleus. It is clear this is a TEM micrograph as the image is 2D and in high resolution

A stained TEM micrograph of a mitochondrion. It is clear this is a TEM micrograph as the image is 2D and in high resolution

A SEM of a spiracle (part of an insect). You can tell this is a SEM micrograph as the image is 3D

A TEM of a prokaryote cell and its associated drawing

A TEM of a eukaryotic cell and its associated drawing