Microscopes

• Light microscopes use light, which has a longer wavelength and a lower resolution (than electrons).
• Electron microscopes use electrons, which have a shorter wavelength and therefore a higher resolution (than light).
• Transmission microscopes can distinguish between points 0.1nm apart.
• Electron microscopes use a beam of electrons through a vacuum to prevent any obstacles/deflections (electrons passing through air can break apart molecules).
• Electron microscopes cannot observe living organisms in a vacuum. Samples must be stained, thinly sliced, and dehydrated, so the electrons can be absorbed (complex procedure - results in artefacts).
• Images produced by electron microscopes are always in black and white.
• The image produced by an electron microscope may contain artefacts: things that result from the way the specimen is prepared. They appear on the final photomicrograph so it is difficult to determine whether the thing in the photo really is part of the specimen.
• Transmission microscopes produce 2D photomicrographs.
• Scanning microscopes produce 3D images.
• 𝙎𝘾𝘼𝙉𝙉𝙄𝙉𝙂 𝙀𝙇𝙀𝘾𝙏𝙍𝙊𝙉 𝙈𝙄𝘾𝙍𝙊𝙎𝘾𝙊𝙋𝙀 (𝙎𝙀𝙈):

  • SEM directs an electron beam across a sample.
  • SEM can produce 3D images of a specimen.
  • SEM can be used on thick specimens.
  • Limitation: SEM has a lower resolution.
• 𝙏𝙍𝘼𝙉𝙎𝙈𝙄𝙎𝙎𝙄𝙊𝙉 𝙀𝙇𝙀𝘾𝙏𝙍𝙊𝙉 𝙈𝙄𝘾𝙍𝙊𝙎𝘾𝙊𝙋𝙀 (𝙏𝙀𝙈):

  • TEM projects an electron beam through a sample.
  • Denser tissue appears darker in the micrograph.
  • A TEM produces very high-resolution images.
  • Limitation: TEM must be performed in a vacuum.
  • Limitation: TEM can only be used for thin tissues.
• 𝙇𝙄𝙂𝙃𝙏 𝙑𝙎 𝙀𝙇𝙀𝘾𝙏𝙍𝙊𝙉:

  • LMs can be used to visualise living specimens whereas EMs can only work with dead specimens.
  • Light microscopy is relatively quick whereas electron microscopy is time-consuming and complex, (e.g. specimen preparation).
  • Light microscopy is less expensive as a decent electron microscope can cost upwards of a million pounds and the reagents needed to prepare specimens for electron microscopy are much more expensive.
• 𝙀𝙇𝙀𝘾𝙏𝙍𝙊𝙉 𝙈𝙄𝘾𝙍𝙊𝙎𝘾𝙊𝙋𝙀𝙎:

  • Use electrons to form images because electrons are smaller than photons of light: EMs have a much greater resolution than optical microscopes.
  • The maximum resolution of an electron microscope is 0.002 micrometres: EMs can be used to produce very detailed images (electron micrographs) of tiny structures.
  • The maximum magnification is 500,000x because the resolution is so good.
  • Electron micrographs are typically black and white.
  • There are two major types of electron microscopy: TEM and SEM.
• Contrast how an optical microscope and a transmission electron microscope work and contrast the limitations of their use when studying cells.

  1. TEM use electrons and optical use light;
  2. TEM allows a greater resolution;
  3. (So with TEM) greater detail in organelles can be observed;
  4. TEM view only dead specimens and optical can view live specimens;
  5. TEM does not show colour and optical can;
  6. TEM requires thinner specimens;
  7. TEM requires more complex preparation;
• The cell-surface membrane can be seen with a transmission electron microscope but not with an optical microscope. Explain why.

  • the electron microscope has a higher resolution;
• Before the cell was examined using the electron microscope, it was stained. This stain caused parts of the structure of the cell-surface membrane to appear as two dark lines.

  Suggest an explanation for the appearance of the cell-surface membrane as two dark lines.

  • cell has a phospholipid bilayer;
  • the stain binds to phosphate;
  • on the inside and outside of the membrane;
• Name the parts of the chloroplast labelled A and B.

  • A = stroma
  • B = granum

  
• Name two structures in a eukaryotic cell that cannot be identified using an optical microscope.

  • mitochondrion;
  • ribosome;
• Describe how you could make a temporary mount of a piece of plant tissue to observe the position of starch grains in the cells when using an optical microscope.

  • add a drop of water to slide;
  • obtain a thin section of plant tissue and place it on the slide;
  • stain with iodine in potassium iodine;
  • lower the cover slip using a mounted needle;
• Give the name and function of the structures labelled W and Z

  • W = chloroplast - the site of photosynthesis
  • Z = nucleus - contains genetic information

  
• A transmission electron microscope was used to produce the image in the figure above. Explain why.

  • high resolution;
  • can see the internal structures of organelles;

  
• Explain why it is not possible to determine the identity of the structures labelled X using an optical microscope.

  • lower resolution;
  • because of the longer wavelength of light;

  
• Suggest why a nucleus is not visible in the above image.

  • nucleus is in a different part of the cell;
  • nucleus wasn't stained;

  
• Name the organelles labelled S and T in the image above.

  • S = vacuole
  • T = chloroplast

  
• Give one advantage of viewing a biological specimen using a transmission electron microscope compared with using a scanning electron microscope.

  • higher resolution;
  • so can see the internal structures of organelles;
• Identify structures labelled D and E

  • D = granum
  • E = starch grain

  
• The detail shown in the diagram above would not be seen using an optical microscope.

  • light has a longer wavelength;
  • so have a lower resolution;

  
• What type of microscope was used to obtain the image shown in the diagram above? Give one piece of evidence to support your answer

  • scanning electron microscope;
  • because the image is 3D;