"The Living Cell" Essay

The Living Cell Term paper

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THE LIVING CELL The cell is the smallest unit of living matter that can exist by itself. Some plants and animals consist of only a single cell. Others are composed of billions of cells. Cells exist in a variety of shapes and sizes. They may, for example, be cube- shaped or flat. Scientists who study cells have determined that a single cell may be as large as a tennis ball, or so small that thousands would fit on the period at the end of a sentence on your paper. The yolk of a hen's egg is actually a large cell. By contrast, bacteria- each one of which is a tiny cell- are among the smallest cells. Regardless of its shape or size, every cell contains the "machinery" to maintain life. While normally cells function with great efficiency, they are subject to various disorders that result in disease. The size of the cell is usually measured in microns. A micron is a millionth of a meter, and about 25, 000 microns equal one inch. The smallest bacteria are about 0.2 micron in diameter. The average cell in the human body- about ten microns in diameter- is a speck barely visible without the aid of a microscope. The study of cells is the branch of biology called cytology. The science that deals with cells on the smallest structural and functional level is called molecular biology. A cell consists of protoplasm, the "living jelly." The protoplasm of a typical cell forms three vital parts: The cell membrane, the cytoplasm, and the nucleus. The membrane encloses the other cell structures. Much of the chemical work of the cell is done in the cytoplasm, which surrounds the nucleus. The nucleus, enclosed by its own membrane, is the control center of the cell. Cells were first described by the English scientist Robert Hooke, who in 1665 published a book about his findings. Hooke had sliced off thin sections of cork. With a microscope of his own design he was able to see the minute, boxlike units of which the cork was made up. Hooke called these structures "Cells" because he thought the "little Boxes" looked like monastery cells. The first description of living cells was provided by the Dutch scientist Anthony van Leeuwenhoek in 1683. More detailed investigations became possible with the development of improved compound microscopes. The Scottish botanist Robert Brown discovered the cell nucleus in 1831. In the 1830's two German scientists, Matthias J. Schleiden and Theodor Schwann, concluded independently that cells were the basis of all life, a view called the cell theory. Rudolph Virchow, another German scientist, stated in 1858 that all cells develop from previously existing cells. During the late 19th century, techniques of fixing and staining tissues to preserve cells in as lifelike as state as possible has opened the way for intensive cell research. In most laboratory microscopes, the background is brightly lighted, the objects are dark, and the power of magnification is about 1,000. In some instruments the background is dark, and the objects are examined bright. Ultraviolet microscopes achieve magnifications two to three times greater than those obtained with these light microscopes. Phase-contrast instruments use special equipment to reveal the refraction, or bending, of light passing through objects, enabling the viewer to see cell details not visible with other microscopes. The electron microscope uses waves of electrons, rather than light, and magnetic fields, rather than lenses, to get an image. With the aid of electron microscopes 200 times more powerful than the best light instruments, molecular biologists have learned a great deal more about the tiny structures within cells.

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