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The advantages and limitations of electron microscopy.

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Introduction

The advantages and limitations of electron microscopy

        There are two main branches of microscopy that are pertinent to cell biology. These branches arise from the two types of microscope; the light microscope and the electron microscope. The basic principles of light microscopy have been known since circa 17th century, however improvements in lens manufacture in circa 19th century allowed the use of microscopy to be much more practically available and useful. This is increased ability inspired rapid research into both the design of microscopes and the preparation of specimens.

        However, the light microscope can only magnify objects bigger than 0.2 micrometres; due to its limited resolving powers. This is because it utilises a beam of light. Relatively, light has a long wavelength, this means that when there are two small points close together there is too much refraction and wave front overlap, the eye then only sees one point. This can also be considered in terms of objects “crossing the path” of the wavelength. The smallest wavelength of visible light is 400nm, the diameter of mitochondria is 1000nm, and therefore mitochondria cross the path of the light wave. However ribosomes have a diameter of 22nm, and do not cross the path of the light wave and are therefore not seen by the light microscope.  

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Middle

        The scanning electron microscope uses and interprets a reflected beam from the specimen. This microscope provides advantages in terms of being able to view surface structures and provides a large depth of field so large areas of the specimen can be viewed at once. However the SEM does not have the same resolving power as the TEM. The SEM is a lot simpler to use than the TEM as the preparation is a lot easier. Specimens usually do not need to be sectioned. However most specimens do need to be coated in gold to prevent them becoming negatively charged and this can affect the accuracy of the result.

        Electron beams do not stimulate the human eye, and so to view the image created by the electron microscope the beam must be projected onto fluorescent screens. This produces a “black and white” image. Stains are often used to improve the contrast of biological specimens. However these stains must be caused by very heavy metals such as lead because they are the only substances dense enough to stop the electron beam. This results in an X ray type image, where denser parts of the specimen appear darker. “False colour” images can now be created by computer.

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Conclusion

Staining. Electrons are not distorted by biological samples. Therefore it is necessary to stain the samples to differentiate between different structures. However electrons can only be prevented from passing through substances with very high densities, so very heavy metals are used.

It is impossible to say with precise accuracy how this process affects the specimen. However it is possible to conclude that the end result will not be exactly the same as the initial specimen. There are often artefacts in the specimens; frequently these are from the heavy metal staining, but they may be caused by a number of things, from chemical changes caused by the fixing chemicals to structures physically altering in the dehydrating and embedding phases. This has made it difficult to understand whether or not the conclusions found from electron micrographs are real, or if they are interpretations of artefacts.

Another limitation of the electron microscope is its cost and size. They are prohibitively expensive and as such are only usually found in universities and industrial laboratories. This obviously limits the amount of people who have access to them.

The advantages of electron microscopy are clear, in that it allows a far more detailed knowledge of the microscopic world than has ever been possible before. However it does bring many difficulties, and whilst ways have been found to overcome these the result is far from perfect.  

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Response to the question

The introduction was spot on and introduced the student's work well. It addressed the question and guides the reader well into the essay. The response is clearly set out and is set out logically. However, in assessing the advantages and ...

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Response to the question

The introduction was spot on and introduced the student's work well. It addressed the question and guides the reader well into the essay. The response is clearly set out and is set out logically. However, in assessing the advantages and disadvantages, I would say that more explicit points need to be made. Whilst most of these advantages and disadvantages are implied whilst the student explains the science behind the technology, I feel that more explicit reference to the question is needed. This reduces all doubts that the examiner should have about whether the student has explicitly answered the question. Nevertheless, the understanding of the topic is clearly shown in this piece of work.

Level of analysis

The analysis in this piece of work would be the knowledge of understanding how the microscopes work. The level of understanding is very high in this piece of work and the student clearly understands the topic. Yet, I feel that the wording of the explanations are tending towards a 'textbook' style. Whilst it is hard to avoid this assertion (which occurs a lot in textbooks), it is best to avoid copying the style of writing of a textbook as this means that the work is more likely to become just a paraphrased essay of the textbook. This said, I do not feel that this piece of work is plagarised at all. The appropriate conclusion was reached but it should be clearer such that the examiner can easily identify the advantages and disadvantages to award the marks.

Quality of writing

The spelling and grammar are sound, with a few places of questionable punctuation problems. Namely, I question the lack of the use of a comma after the word however (I was always taught this was imperative). The technical terms used in this piece of work is that which is expected of students at A level, however I would suggest putting in a glossary which would help further demonstrate the understanding of the topic and therefore, the question set. The student follows the general expectations of an A level student. To make this piece of work 'stand out from the crowd', extra explanation would be needed to demonstrate further understanding of the microscopes. Whilst such explanations are not required to obtain high marks, it will impress the examiner and arguably, will make them more likely to award the candidate a higher mark.


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