Question

In Cellular Biology, Microscopy is a very useful tool. No matter how useful, however, in certain types of microscopy it is difficult to tell bumps from pits just by looking at a pattern of shadows. Consider the figure below, which shows a set of shaded circles. In Panel A, the circles appear to be bumps, however, when the picture is simply turned upside down (as in Panel B), the circles seem to be pits. This is a classic illusion. In utilizing metal shadowing one sees the same illusion (see electron micrographs in panels C & D). In one the membrane appears covered in bumps, in the other the membrane appears speckled with pits. Is it possible for an electron microscopist to be sure that one view is correct, or is it all arbitrary? Explain your reasoning

Answer 1

It is difficult to distinguish pits from bumps by merely observing shadow patterns.The circle form bumps and outs when image is turned upside down. This strategy can be compared with shadowing of metals.

However, Yes, it is possible for microscopist to be sure that one view is correct. Technique, known as Metal Shadowing can be used to ensure this. In this method heavy metals such as Platinum is evaporated on the specimen at an oblique angle so as to ensure that a thin film of metal is formed over the specimen. Thus the surface of the specimen towards the metal source will recieve heavy metal deposits as compared to surface away from source, this difference in metal deposition causes a shadow. This shadow can be avoided by rotating the specimen while depositing the metal thus ensuring equal deposition of metal. After stabilising metal deposition Transition Electron Microscope can be used to take pictures of metal replica.