端木·宇 2008-6-19 22:27
The Discovery of Cells
Most cells are too small to be observed with the naked eye. For thisreason, even the existence of cells escaped notice until scientistsfirst learned to harness the magnifying power of lenses in the secondhalf of the seventeenth century. At that time a Dutch clothing dealernamed [b]Antonie van Leeuwenhoek[/b] (1632–1723) fashionedextraordinarily accurate single-lens microscopes. Gazing into the lensof these microscopes, he discovered single-celled organisms, which hecalled “animalcules” and which, today, we call bacteria and protists.
Englishman Robert Hooke (1635–1703) expandedon Leeuwenhoek’s observations with the newly developed compoundmicroscope, which uses two or more aligned lenses to increasemagnification while reducing blurring. When Hooke turned the microscopeon a piece of cork, he noticed that the tiny, boxlike compartments ofthe wood resembled the cells of a monastery. The term “cell” was born.
[b] Cell Theory Emerges[/b]
As microscope technology improved,scientists were able to study cells in ever-greater detail. Hooke hadno way to tell if cells were living things, but later researchers whocould see the nucleus and the swirling motion of the cytoplasm wereconvinced that cells were indeed alive. By 1839, enough evidence hadaccumulated for German biologists Matthias Schleiden and TheodoreSchwann to proclaim that cells are “the elementary particles oforganisms.” But many researchers still did not believe that cellsarose from other cells until 1855, when famous German pathologistRudolph Virchow pronounced, “All cells come from cells.” Nearly 200years after the discovery of cells, the observations of Virchow,Schleiden, and Schwann established the cell theory:
[list][*]All living things are made of cells.[*]All cells arise from preexisting cells.[/list]
These two tenets made clear that the cell is the fundamental unit of life.
[b] Cell Size[/b]
Cells could not be studied until themicroscope was developed because they are very small. This fact raisestwo questions: why are cells so small, and why are living things madeup of millions of tiny cells?
Cells are small because their surface areaand volume must be balanced. In order to stay alive, cells with alarger volume need to carry out more chemical activity than smallercells do. However, for metabolic activity to take place, the cell mustalso have enough surface area to allow an adequate supply of nutrientsand waste products to move in and out of the cell. Because surface areaincreases at a slower rate than volume as objects get bigger, thesurface area-to-volume ratio in a cell decreases dramatically as thecell gets larger. It turns out that a size of 10 [img]http://www.24en.com/d/file/sat/sat2/biology/2008-01-24/a634b006f84f9f2bd136fdc6f0e5c280.gif[/img] provides the surface area-to-volume ratio necessary for the survival of most cells. ([img]http://www.24en.com/d/file/sat/sat2/biology/2008-01-24/7ba07e3a77934e86a8551001bc5d6e52.gif[/img] the micrometer, is one thousandth of a millimeter.)
[b] Microscopes[/b]
Two major types of microscopes allow scientists to study the miniature world of the cell.
[b] The Light Microscope[/b]
Light microscopes use light and lenses tomagnify their subjects. The most common of these used in the laboratoryis the compound microscope, which creates high magnification bycombining two relatively low-power lenses. The total power of acompound microscope is the power of the ocular lens, located in theeyepiece, multiplied by the power of the objective lens, located nearthe slide. For example, an ocular lens of 10x and an objective lens of11x yield a total magnification of 110x. Typical high schoolmicroscopes offer magnifications of up to about 430x.
From time to time the SAT II Biology testsyour knowledge of the various parts of the compound microscope, usuallyby showing you an image and asking you to identify the parts.
[align=center][img]http://www.24en.com/d/file/sat/sat2/biology/2008-01-24/547884b77e07b7244c8c930ebaeaabb4.gif[/img][/align]
Many parts of the cell are hard to see undermicroscopes because they are colorless. In order to view them,scientists sometimes employ stains that mark various cell partsdifferently. One alternative to staining is a technique called phasecontrast microscopy, which uses filters to emphasize the contrastbetween different parts of the cell.
[b] The Electron Microscope[/b]
At high magnifications, light microscopesproduce blurry images. In the 1950s, scientists invented a new type ofmicroscope called the electron microscope, which offers increased imageclarity, or resolving power. Electron microscopes are powerful enoughto resolve individual fats and proteins. Light microscopes are stillwidely used, however, because electron microscopes are expensive andcan only be used to view matter that is not living.
