ans Christian Joachim Gram (1853-1938) was ■1 a Danish physician working in a laboratory at JL JL. the morgue of the City Hospital in Berlin, microscopically examining the lungs of patients who had died of pneumonia. He was working under the direction of Dr. Carl Friedlander, who was trying to identify the cause of pneumonia by studying patients who had died of it. Gram's task was to stain the infected lung tissue to make the bacteria easier to see under the microscope. Strangely, one of the methods he developed did not stain all bacteria equally; some types retained the first dye applied in this multistep procedure, whereas others did not. Gram's staining method revealed that two different kinds of bacteria were causing pneumonia, and that these types retained the dye differently. We now recognize that this important staining method, called the Gram stain, efficiently identifies two large, distinct groups of bacteria: Gram-positive and Gram-negative. The variation in the staining outcome of these two groups reflects a fundamental difference in the structure and chemistry of their cell walls.
For a long time, historians thought that Gram did not appreciate the significance of his discovery. In more recent years, however, several letters show that Gram did not want to offend the famous Dr. Friedlander under whom he worked; therefore, he played down the importance of his staining method. In fact, the Gram stain has been used as a key test in the initial identification of bacterial species ever since the late 1880s.
—A Glimpse of History
IMAGINE THE ASTONISHMENT ANTONY VAN Leeuwenhoek must have felt in the 1600s when he first observed microorganisms with his handcrafted microscope, an instrument that could magnify an image 300-fold (300x). Even today, observing diverse microbes interacting in a sample of stagnant pond water can provide enormous education and entertainment.
Microscopic study of cells has revealed two fundamental types: prokaryotic and eukaryotic. The cells of all members of the Domains Bacteria and Archaea are prokaryotic. In contrast, cells of all animals, plants, protozoa, fungi, and algae are eukaryotic. The similarities and differences between these two basic cell types are important from a scientific standpoint and
Color-enhanced TEM of bacterial cells.
also have significant consequences to human health. For example, chemicals that interfere with processes unique to prokary-otic cells can be used to selectively destroy bacteria without harming humans. ■ prokaryotic cells, p. 9
Prokaryotic cells are much smaller than most eukaryotic cells—a trait that carries with it certain advantages as well as disadvantages. On one hand, their high surface area relative to their low volume makes it easier for these cells to take in nutrients and excrete waste products. Because of this, they can multiply much more rapidly than their eukaryotic counterparts. On the other hand, their small size makes them vulnerable to an array of threats. Predators, parasites, and competitors constantly surround them. Prokaryotic cells, although simple in structure, have developed many unique attributes that enhance their evolutionary success.
Eukaryotic cells are considerably more complex than prokaryotic cells. Not only are they larger, but many of their cellular processes take place within membrane-bound compartments. Eukaryotic cells are defined by the presence of a membrane-bound nucleus, which contains the chromosomes. Although eukaryotic cells share many of the same characteristics as prokaryotic cells, many of their structures and cellular processes are fundamentally different. ■ eukaryotic cells, p. 9
Chapter 3 Microscopy and Cell Structure
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