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Biology Chapter 6 Objectives

Biology Chapter 6 Objectives - Chapter 6 A Tour of the Cell...

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Chapter 6 A Tour of the Cell How We Study Cells 1. Magnification in microscopy is the ratio of an object’s image to its real size. Resolving power, or resolution, is the minimum distance two points can be separated and still be distinguished as two separate points. 2. In light microscopes (LMs), visible light is passed through the specimen and then through glass lenses. The lenses refract the light so that the image of the specimen is magnified as it is projected. The resolving power of it is limited though. Microscopes can be designed to magnify objects as much as desired, but the light microscope cannot resolve detail finer than about 0.2 micrometer. This resolution is limited by the shortest wavelength of light used to illuminate the specimen; at great magnifications, the image becomes increasingly blurry. However, if can magnify effectively to about 1,000 times the size of the actual specimen. The electron microscope (EM) focuses a beam of electrons through the specimen or onto its surface, instead of using light. The resolution is inversely related to the wavelength of the radiation a microscope uses for imaging, and the electron beams have wavelengths much shorter than the wavelengths of visible light. The electron microscopes can generally only achieve a resolution of about 2 nm. There are two basic types of electron microscopes: the scanning electron microscope (SEM) and the transmission electron microscope (TEM). The SEM is useful for detailed study of the surface of a specimen because the electron beam scans the surface of the sample, which is usually coated with a thin film of gold. The beam excites electrons on the sample’s surface, and is detected by a device that translates the pattern of electrons into an image that appears three- dimensional. TEMs are mainly used to study the internal ultrastructure of cells. It aims and electron beam through a very thing section of the specimen. The specimen, stained with atoms of heavy metals which attach to certain cellular structure, enhances the electron density of some parts of the cell more than others. The image is created by the pattern of transmitted electrons. 3. The first step of cell fractionation is to homogenize the cells in a blender to break them up. Next, the resulting mixture, the homogenate is centrifuged at various speeds and durations using the centrifuge, the instrument used to fractionate cell components. This forms a series of pellets. The supernatant is poured into the next tube, where the process is repeated again. This is a useful technique because biologists have been able to assign various functions of the cell to the different organelles. A Panoramic View of the Cell 4. In a prokaryotic cell, the DNA is concentrated in a region called the nucleoid, but no membrane separates this region from the rest of the cell. The eukaryotic cell has a true nucleus, bounded by a membranous nuclear envelope. Prokaryotic cells
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also lack the entire region between the nucleus and the plasma membrane called the cytoplasm. Eukaryotic cells are generally bigger than prokaryotic cells.
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