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.
As biologists came to realise these limitations they understood that the only solution was to find a way of employing a shorter wavelength. The electro-magnetic spectrum shows us that either ultraviolet light or X rays could be used. Both were tried, but neither proved to be satisfactory. It became apparent that a more effective solution would be to use electrons. Electrons are negatively charged particles that form an orbit around the nucleus of an atom. Electrons can break form this orbit when the atoms are heated to very high temperatures. Free electrons behave similarly to electromagnetic radiation, they have a very short wavelength and the greater their energy the shorter the wavelength.
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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.
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.
Response to 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.
