The Cell The Common Denominator Of Living Things

Cells were discovered by Robert Hooke of England in the mid-seventeenth century, and named by him after the small dormitory-style rooms inhabited by monks. He first saw cell wall remains in thin slices of cork, using a microscope. Around the same time, Anton van Leeuwenhoek of Holland advanced the art of building microscopes, achieving magnification up to 500 times. With these he was able to make detailed studies of living cells. Further study led to the development in the nineteenth century of cell theory:

• All living things are composed of one or more cells.

• Cells are the basic units of living things and are the site for the reactions of life.

• Under today's conditions, all cells come from preexisting cells.

The first tenet encompasses everything from single-celled bacteria to large animals and trees that can have trillions of cells. The second tenet recognizes that individual parts of cells are not by themselves viable. This tenet also excludes viruses from being classified as living things, since they do not metabolize. The third tenet leaves open the possibility of cells arising spontaneously under the conditions of the primitive Earth.

The light microscope opened a new world to examination, literally under our noses. Typical cell sizes are about 1 mm for bacteria to 10 mm for most human cells. The human eye can resolve down to only about 100 mm (0.1mm). The light microscope extends resolution down to 0.2 mm [200 nanometers (nm)], which is half the wavelength of violet light. An advance similar in magnitude to Hooke and Leeuwenhoek's microscopes occurred in the 1960s with the development of electron microscopes. These can magnify by 30,000 to 100,000 times, yielding resolution down to 2nm. This is enough to resolve some of the larger macromolecules such as proteins and nucleic acids. Another

Environmental Biology for Engineers and Scientists, by David A. Vaccari, Peter F. Strom, and James E. Alleman Copyright © 2006 John Wiley & Sons, Inc.

leap was made in 1986, with the development of the atomic force microscope, which can resolve individual atoms and has been used to detect the shape of the DNA helix. These imaging tools, together with biochemical techniques, have led to continual advances in cytology, the study of cells.

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