3.2 Foundations of Modern Cell Theory
Learning Objectives
- Explain the key points of cell theory and the individual contributions of Hooke, Schleiden, Schwann, Remak,
and Virchow. - Explain the key points of endosymbiotic theory and cite the evidence that supports this concept.
- Explain the contributions of Semmelweis, Snow, Pasteur, Lister, and Koch to the development of germ theory.
While some scientists were arguing over the theory of spontaneous generation, other scientists were making
discoveries leading to a better understanding of what we now call the cell theory. Modern cell theory has two basic
tenets:
- All cells only come from other cells (the principle of biogenesis).
- Cells are the fundamental units of organisms.
Today, these tenets are fundamental to our understanding of life on earth. However, modern cell theory grew out of
the collective work of many scientists.
The Origins of Cell Theory
The English scientist Robert Hooke first used the term “cells” in 1665 to describe the small chambers within cork
that he observed under a microscope of his own design. To Hooke, thin sections of cork resembled “Honey-comb,” or
“small Boxes or Bladders of Air.” He noted that each “Cavern, Bubble, or Cell” was distinct from the others (Figure
3.5). At the time, Hooke was not aware that the cork cells were long dead and, therefore, lacked the internal structures
found within living cells.
Figure 3.5 Robert Hooke (1635–1703) was the first to describe cells based upon his microscopic observations of
cork. This illustration was published in his workMicrographia.
Despite Hooke’s early description of cells, their significance as the fundamental unit of life was not yet recognized.
Nearly 200yearslater,in1838,Matthias Schleiden (1804–1881),aGerman botanist whomade extensive microscopic
observations of plant tissues, described them as being composed of cells. Visualizing plant cells was relatively easy
because plant cells are clearly separated by their thick cell walls. Schleiden believed that cells formed through
crystallization, rather than cell division.
84 Chapter 3 | The Cell
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