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Chapter 06 - Chapter 06 A Tour of the Cell Chapter 6 A Tour...

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Chapter 06 - A Tour of the Cell Chapter 6 A Tour of the Cell Lecture Outline Overview: The Importance of Cells All organisms are made of cells. o Many organisms are single-celled. o Even in multicellular organisms, the cell is the basic unit of structure and function. The cell is the simplest collection of matter that can live. All cells are related by their descent from earlier cells. Concept 6.1 To study cells, biologists use microscopes and the tools of biochemistry The discovery and early study of cells progressed with the invention of microscopes in 1590 and their improvement in the 17th century. In a light microscope (LM), visible light passes through the specimen and then through glass lenses. o 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. o Magnification is the ratio of an object’s image to its real size. o Resolving power is a measure of image clarity. It is the minimum distance two points can be separated and still be distinguished as two separate points. Resolution is limited by the shortest wavelength of the radiation used for imaging. The minimum resolution of a light microscope is about 200 nanometers (nm), the size of a small bacterium. Light microscopes can magnify effectively to about 1,000 times the size of the actual specimen. o At higher magnifications, the image blurs. Techniques developed in the 20th century have enhanced contrast and enabled particular cell components to be stained or labeled so 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), which focuses a beam of electrons through the specimen or onto its surface. o Because resolution is inversely related to wavelength used, electron microscopes (whose electron beams have shorter wavelengths than visible light) have finer resolution.
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o Theoretically, the resolution of a modern EM could reach 0.002 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. o A TEM aims an electron beam through a thin section of the specimen. o The image is focused and magnified by electromagnets. o To enhance contrast, the thin sections are stained with atoms of heavy metals. Scanning electron microscopes (SEMs) are useful for studying surface structures. o The sample surface is covered with a thin film of gold. o The beam excites electrons on the surface of the sample. o These secondary electrons are collected and focused on a screen.
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