Ch07WordLectureOutline

Ch07WordLectureOutline - CHAPTER 7 A TOUR OF THE CELL A....

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CHAPTER 7 A TOUR OF THE CELL A. How We Study Cells 1. Microscopes provide windows to the world of the cell The discovery and early study of cells progressed with the invention and improvement of microscopes in the 17th century. In a light microscope (LM) visible light passes through the specimen and then through glass lenses. The lenses refract light such that the image is magnified into the eye or onto a video screen. Microscopes vary in magnification and resolving power. Magnification is the ratio of an object’s image to its real size. Resolving power is a measure of image clarity. It is the minimum distance two points can be separated by and still be viewed as two separate points. Resolution is limited by the shortest wavelength of the source, in this case light. The minimum resolution of a light microscope is about 2 microns, the size of a small bacterium. Light microscopes can magnify effectively to about 1,000 times the size of the actual specimen. At higher magnifications, the image blurs. Techniques developed in the 20th century have enhanced contrast and enabled particular cell components to be labeled so that they stand out. While a light microscope can resolve individual cells, it cannot resolve much of the internal anatomy, especially the organelles . To resolve smaller structures we use an electron microscope (EM),
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which focuses a beam of electrons through the specimen or onto its surface. Because resolution is inversely related to wavelength used, electron microscopes with shorter wavelengths than visible light have finer resolution. Theoretically, the resolution of a modern EM could reach 0.1 nanometer (nm), but the practical limit is closer to about 2 nm. Transmission electron microscopes (TEMs) are used mainly to study the internal ultrastructure of cells. A TEM aims an electron beam through a thin section of the specimen. The image is focused and magnified by electromagnets. To enhance contrast, the thin sections are stained with atoms of heavy metals. Scanning electron microscopes (SEMs) are useful for studying surface structures. The sample surface is covered with a thin film of gold. The beam excites electrons on the surface. These secondary electrons are collected and focused on a screen. The SEM has great depth of field, resulting in an image that seems three- dimensional. Electron microscopes reveal organelles, but they can only be used on dead cells and they may introduce some artifacts. Light microscopes do not have as high a resolution, but they can be used to study live cells. Microscopes are major tools in cytology , the study of cell structures. Cytology coupled with
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Ch07WordLectureOutline - CHAPTER 7 A TOUR OF THE CELL A....

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