The result is an image of the topography of the specimen The SEM has great

The result is an image of the topography of the

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The result is an image of the topography of the specimen. The SEM has great depth of field, resulting in an image that seems three-dimensional. Electron microscopes reveal organelles that are impossible to resolve with the light microscope. However, electron microscopes can only be used on dead cells. Light microscopes do not have as high a resolution, but they can be used to study live cells.
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Microscopes are major tools in cytology, the study of cell structures. Cytology combined with biochemistry, the study of molecules and chemical processes in metabolism, to produce modern cell biology. Cell biologists can isolate organelles to study their functions. The goal of cell fractionation is to separate the major organelles of the cells so their individual functions can be studied. This process is driven by an ultracentrifuge, a machine that can spin at up to 130,000 revolutions per minute and apply forces of more than 1 million times gravity (1,000,000 g). Fractionation begins with homogenization, gently disrupting the cell. The homogenate is spun in a centrifuge to separate heavier pieces into the pellet while lighter particles remain in the supernatant. As the process is repeated at higher speeds and for longer durations, smaller and smaller organelles can be collected in subsequent pellets. Cell fractionation prepares isolates of specific cell components. This enables the functions of these organelles to be determined, especially by the reactions or processes catalyzed by their proteins. For example, one cellular fraction was enriched in enzymes that function in cellular respiration. Electron microscopy revealed that this fraction is rich in mitochondria. This evidence helped cell biologists determine that mitochondria are the site of cellular respiration. Cytology and biochemistry complement each other in correlating cellular structure and function. Concept 6.2 Eukaryotic cells have internal membranes that compartmentalize their functions Prokaryotic and eukaryotic cells differ in size and complexity. All cells are surrounded by a plasma membrane. The semifluid substance within the membrane is the cytosol, containing the organelles. All cells contain chromosomes that have genes in the form of DNA. All cells also have ribosomes, tiny organelles that make proteins using the instructions contained in genes. A major difference between prokaryotic and eukaryotic cells is the location of chromosomes. In a eukaryotic cell, chromosomes are contained in a membrane-enclosed organelle, the nucleus. In a prokaryotic cell, the DNA is concentrated in the nucleoid without a membrane separating it from the rest of the cell. In eukaryote cells, the chromosomes are contained within a membranous nuclear envelope.
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