Cell theory states that the cell is the basic building block of all life forms and that all living things, whether plants or animals, consist of one or more cells. It further states that new cells can only be made from existing ones, and that organisms can grow and reproduce because their cells are able to divide.
Although the cell is the basic unit of life, it is also the smallest part of a living organism, and it is therefore too tiny to be seen by the naked eye. Because of this, the notion of a cell did not exist until the seventeenth-century invention of the microscope. Before then, it was commonly believed that the basic units of life were things like fibers and vessels, and that new living things came about by a process simply known as "spontaneous generation" in which life developed suddenly from dead or decomposing matter. By the middle of the seventeenth century, however, some individuals were beginning to investigate the new, subvisible world that the recently invented microscope could bring them. One of these pioneers was the Dutch naturalist Jan Swammerdam (1637–1680), who examined blood and in 1658 became the first person to observe a red blood cell. Also in Holland at the same time, the Dutch naturalist Anton van Leeuwenhoek (1632–1723) developed a powerful, simple (single-lens) microscope and saw, among other things, sperm cells and one-celled animals known as protozoa.
In England at this time, the most famous microscopist was the physicist Robert Hooke (1635–1703), who published his book, Micrographia, in 1665. It was in this book that Hooke first used the word "cell" to describe the tiny structures he observed when examining a thin slice of cork under a microscope. He called the structures cells because they resembled a small room (as in "jail cell"). What he was really seeing were the dead remnants of structures that were filled with fluid when the cork was part of a living tree. Although Hooke did not discover living cells, he did coin the word "cell," which was eventually adopted by biologists.
Over the next 150 years, improvements in the microscope allowed observers to better study living tissue, and in 1831, the Scottish botanist Robert Brown (1773–1858) discovered that every plant cell had what he described as a "little nut" or nucleus in it. This discovery paved the way for the cell theory of German botanist Matthias Schleiden (1804–1881) and German physiologist Theodor Schwann (1810–1882). Although others had observed cells and even recognized that animal tissues contained cells, no one had as yet made the connection between cells and life. In 1838 however, Schleiden announced his findings that all vegetable matter is made up of cells and are the fundamental unit of plant life. The following year, Schwann took Schleiden's basic idea and expanded upon it by stating that the cell is the basic unit of all living matter, plants and animals. Schwann's clearly stated and well-summarized ideas were more elaborate than Schleiden's and Schwann usually gets most of the credit for establishing cell theory, which he also named. Schwann also suggested that eggs were actually cells and that all life starts as a single cell.
By the middle of the nineteenth century, the final major point was added to cell theory by the German pathologist Rudolph Virchow (1821–1902), who summed up his research with the Latin phrase, Omnis cellula e cellula, translated as "all cells arise from cells." Virchow correctly proposed that all cells originate from other cells (putting an end to notions of spontaneous generation), and further demonstrated that even diseased tissue comes from normal cells through the process of division. With this he founded cellular pathology or the study of diseased cells. Cell theory was essentially complete with the 1861 contribution of the Swiss anatomist and physiologist Rudolf Albert von Kolliker (1817–1905), who was the first scientist to study the developing embryo (a living organism in its early stages before birth) in terms of cell theory. Kolliker showed that eggs and sperm should be considered cells and argued that the cell nucleus
MATTHIAS JAKOB SCHLEIDEN AND THEODOR SCHWANN
German botanist Matthias Jakob Schleiden (1804–1881) and German physiologist Theodor Schwann (1810–1882) are credited with establishing cell theory as a basic, unifying theme of all biology. The cell theory states that all forms of life are made up of cells and that all living things grow and reproduce because these cells can divide. While it was Schleiden who first formulated the theory in regard to plants, it was Schwann who applied it to both animals and plants and concluded that biology was a single science. Although they were not collaborators, they discussed and compared each other's work, and together offered biology one of its most important concepts.
Matthias Schleiden was born in Hamburg, Germany and began his professional life as a lawyer. He found this field unsatisfying and returned to school to study medicine, later specializing in botany (the study of plants). Using his microscope to study plant tissue, Schleiden eventually concluded that what he was seeing was the most essential unit, or the basic physical unit, of the living plant. In 1838, therefore, he first offered the then-unknown idea that the cell is the fundamental unit of a living plant. As the first to recognize the importance of cells, Schleiden announced that all the various parts of a plant consisted of cells and that they were all created in the same manner. Schleiden was incorrect, however, about a few things, incorrectly stating that all cells developed from the nucleus (a cell's control center), which then disappeared after the cell was fully formed. Yet this does not take away from the overall importance of his work to the world of biology. First, his cell theory of plants focused attention on the basic unit of that organism and second, it laid the foundation for Schwann's broader, more comprehensive work on cell theory.
Theodore Schwann was a very different man than Schleiden. Six years younger, he was as gentle and quiet as Schleiden was impulsive and testy. Where Schleiden would publicly denounce his critics, Schwann would go out of his way to avoid controversy. Before his work on animal cells, Schwann had done solid work on digestion and was actually the first person to isolate an enzyme (a protein catalyst that speeds up chemical reactions in living things) from animal tissue. An enzyme is a protein catalyst that speeds up chemical reactions in living things. Schwann was able to isolate the enzyme he called pepsin from the lining of the stomach. It was also Schwann who coined the word "metabolism" to describe all of the chemical changes that take place within living tissue.
In 1839, Schwann was working on disproving, yet again, the ancient idea of spontaneous generation (that living things can be generated out of nonliving matter) when he arrived at his own cell theory. Schleiden of course had formulated his cell theory of plants the year before, and in October 1838, the two men got together for dinner and discussed each other's work. Schwann listened to Schleiden's description of what he had seen under his microscope, and said it sounded similar to what he was viewing under his. After dinner, the two went to Schwann's lab and discovered that the cell structure in the spinal cord of a fish was almost the same as that of plants. It was left to Schwann, therefore, to conclude that a cell structure was common to all living things. Having extended Schleiden's theory to animals, Schwann then formulated it in its best and clearest way. The cell theory would prove to unite animal and vegetable biology and show that it was fundamentally one science of biology.
Both men were not entirely correct about everything they proposed, and while their theories did have specific flaws, their work was responsible for formulating one of the most fundamental concepts in biology. The establishment of the cell theory was a landmark achievement in the history of biology. Besides its unifying importance, it also would lead to further research by the best minds who extended the theory and who began to investigate what went on inside the cell.
was the key to the transmittal of hereditary factors. The only significant modern exceptions added to the original cell theory are that viruses (disease-causing agents) are not composed of cells, although they contain some genetic (hereditary) material and can reproduce in a host cell. Also, mitochondria (produces energy in cells) and chloroplasts (contains chlorophyll to capture the sun's energy) are considered to be parts of cells but contain genetic material and can also reproduce in a cell. As one of the major theories in the life sciences, cell theory serves as the basis for all of today's breakthroughs in the important field of genetics.