Explain how the electron microscope has affected our knowledge of cell form and structure Essay Example

Explain how the electron microscope has affected our knowledge of cell form and structure Essay Example Explain how the electron microscope has affected our knowledge of cell form and structure Essay Explain how the electron microscope has affected our knowledge of cell form and structure Essay Electrons are generated in an electron gun, which applies a high voltage of about 100,000 volts through a filament called a tungsten filament. The filament is heated to above 3000 degrees Kelvin, which is roughly 3273 degrees Celsius. Applying an increasing negative voltage to a cathode assembly, which is located just above an anode plate, accelerates the electrons. The anode plate has a tiny hole in its centre, the electron beam is sent through this hole creating a very concentrated beam of electrons. This beam is focused using magnetic coils that act like the condenser lenses that you find on a light microscope. The specimen is on a plate just above a second magnetic coil, which acts as an objective lens. The objective lens resolves the structure and magnifies it slightly. Focusing the specimen can be achieved by altering the electric current through the lens. More magnetic coils act as projector lenses, which enlarge the image. All specimens in electron microscopy are placed in a vacuum, this means that all specimens must be dead. All specimens must be in a vacuum chamber because the electrons would be deflected by particles in the air and so would not give a clear picture. (A sheet has been attached at the back, which shows the structure of an electron microscope). The transmission electron microscope takes a section of a specimen and passes electrons through it. But first the specimen must go through 5 stages of preparation. 1. Fixation and dehydration using alcohol. 2. Embedding in resin which is hardened in an oven. 3. Sectioning using an ultramicrotome and a glass knife. 4. Mounting on a copper grid to give support (electrons cannot pass through glass). 5. Staining using heavy metal stains to improve contrast. Another way of preparing slides is to use the freeze fracture technique. The specimen is frozen using liquid Nitrogen. The specimen is then hit with e chisel, which causes the specimen to break along the line of least resistance. This way allows surface detail to be seen. Not all specimens need to be sectioned, viruses and large molecules are thin enough to be examined without needing to be sectioned in any way. These stages may induce artefacts to be present in the electronmicrograph. Artefacts are features which can be observed in cells prepared for microscopy which do not appear in real life, they can be caused by disruption in the cell. The image can be viewed on a fluorescent screen. The image is black and white unless the specimen has been stained to produce a colour picture. Micrographs are prepared by allowing the electrons to fall on photographic paper. Scanning electron microscopes only produce a scan of the surface of a cell, it cannot penetrate the interior of the cell. The electrons are bounced of the specimen instead of passing through it. This procedure will give a 3D image of the specimen. This can be useful when looking at virus or bacterial cells. If we wish to study a particular organelle, we do not have to study the entire cell under a microscope to do so, using cell fractionation and centrifugation, we can separate the different organelles from each other and so we can study them separately. Cell fractionation allows us to view the activities of organelles without interference from all other reactions taking place in the cell. First the tissue is finely chopped up and then it is placed in a cold isotonic buffer so that the cells and organelles are distorted as little as possible. The cells are then broken open using a homogeniser. A homogeniser is a mini blender that is able to fit down a boiling tube. The finished product once the tissue has been homogenised is called homogenate. The homogenate is then filtered to remove any cells, which have not been broken open. The homogenate is transferred to a centrifuge. Centrifugation is used to separate different organelles from each other. The cell homogenate is spun at different speeds and times. As the homogenate is spun, the parts of the cell begin to separate out to produce a pellet of components in the bottom of the tube. The homogenate, which does not become part of the pellet is called the supernatant. The supernatant can then be poured off and the contents spun again at increasing speeds and times to cause more organelles and cell components to separate from each other. The nuclei will separate out first, followed by mitochondria, Lysosomes, Peroxisomes, Microsomes, ER and ribosomes. These specimens can be prepared as normal and studied using electron microscopy When studying cells and their components, electron microscopes are preferred to light microscopes for many different reasons. Light microscopes can only be used if the magnification is under x1500 whereas an electron microscope can magnify images will above x1500. Electron microscopes have a resolution power of 2nm, light microscopes can only distinguish two objects apart if they are 2?m apart. If you were to increase the magnification of an image on an electron microscope, the image would become clearer, but if you did the same with a light microscope, the image would blur. Because of these three facts, the only things visible with a light microscope are nuclei and cell walls and membranes, electron microscopes give us the ability of seeing all organelles, which make up a cell. Without these advances in microscopy, we would not be able to see the ultra structure of cells, or even know if it existed. Nor would we know what purpose they had within the cell. We would not know that mitochondria have a double membrane, with the inner one folded into cristae. Or that it is the site of ATP synthesis and contains its own circular strands of DNA. Granted we may have been able to come to the conclusion that the nucleus controlled the cell reactions, but we would not know what carried out those reactions. If we didnt have electron microscopes we would not know that bacteria and single celled organisms are different from other cells. Without electron microscopy, we would not know what happened to obsolete cells, they would appear to vanish! Because of this technology we now know a great deal about what happens within a cell, and what role cells play in our lives.