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bio chapter 4 outline

bio chapter 4 outline - I Cell Theory A Robert Hooke was...

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Chapter Four: Cell Structure I. Cell Theory A. Robert Hooke was the first to observe cells in 1665, naming the shapes he saw in cork cellulae. This comes down to us as cells. B. Cell theory is the unifying foundation of cell biology 1. The cell theory was proposed to explain the observation that all organisms are composed of cells. 2. In its modern form, cell theory includes the following three principals: a. All organisms are composed of one or more cells, and the life processes of metabolism and heredity occur within these cells. b. Cells are the smallest living things, the basic units of organization of all organisms. c. Cells arise only by division of a previously existing cell. C. Cell size is limited 1. The rate of diffusion is affected by a number of variables, including surface area available for diffusion, temperature, concentration gradient of diffusing substance, and the distance over which diffusion must occur. 2. Larger cells need to synthesize more macromolecules, have a correspondingly higher energy requirements, and produce a greater quantity of waste. 3. The rate at which this transport occurs depends on both the distance to the membrane, as well as the area of membrane available. For this reason, an organism made up of many relatively small cells has an advantage over one compound of fewer, larger cells. 4. The advantage of small cell size is readily apparent in terms of the surface area-to-volume ratio . As a cell’s size increases, its volume increases much more rapidly than its surface area. 5. The cell surface provides the only opportunity for interaction with the environment, because all substances enter and exit a cell via this surface. D. Microscopes allow visualization of cells and components 1. The reason we can’t see such small objects is the limited resolution of the human eye. Resolution is defined as the minimum distance two points can be apart and still be distinguished as two separated points. 2. Modern light microscopes , which operate with visible light, use two magnifying lenses to achieve very high magnification and clarity. 3. Microscopes that magnify in stages using several lenses are called compound microscopes . They can resolve structures that are separated by at least 200 nanometers (nm). 4. Electrons have a much shorter wavelength, and an electron microscope , employing electron beams, has 1000 times the resolving power of a light microscope. 5. Transmission electron microscopes , so called because the electrons used to visualize the specimens are transmitted through the material, are capable of resolving objects only 0.2 nm apart – just twice the diameter of a hydrogen atom! 6. A second kind of electron microscope, the scanning electron microscope , beams the electrons onto the surface of the specimen. The electrons reflected back from the surface, together with other electrons that the specimen itself emits as a result of the bombardment, are amplified and transmitted to a screen, where the image can be viewed and photographed.
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