cell-structure-and-organisation recognising-organelles

Recognising Organelles from Electron Microscope Images

• 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