Cell theory

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Human cancer cells with nuclei (specifically the DNA) stained blue. The central and rightmost cell are in interphase, so the entire nuclei are labeled. The cell on the left is going through mitosis and its DNA has condensed.

In biology, cell theory is a scientific theory which describes the properties of cells. These cells are found to be the basic unit of structure in all organisms and also the basic unit of reproduction. With continual improvements made to microscopes, in the 17th century, magnification became strong enough to discover cells. This discovery is largely attributed to Robert Hooke, thus beginning the study of cells, also known as cell biology. Over a century later, many debates about cells began amongst scientist. Most of these being how regeneration happens and connecting cells to being the fundamental unit of life. During the mid-19th century, in 1838, cell theory was formulated. This is usually credited to Matthias Schleiden and Theodor Schwann. However, many other scientist like Rudolf Virchow contributed to the theory.

Overtime, cell theory has become the foundation of biology and is a widely accepted explanation of the relationship between cells and living things. The cell theory holds true for all living things, no matter how big or small. Since according to research, cells are common to all living things, they can provide information about all life. And because all cells come from other cells, scientists can study cells to learn about growth, reproduction, and all other functions that living things perform. By learning about cells and how they function, you can learn things like diseases in living things.

The three tenets to the cell theory are as described below:

  1. All living organisms are composed of one or more cells
  2. The cell is the most basic unit of life.
  3. All cells arise from pre-existing, living cells.


Anton van Leeuwenhoek's Microscope from the 17th century with a magnification of 270x.
Robert Hooke's microscope

The discovery of the cell was made possible through the invention of the microscope. In the first century BC, Romans were able to make glass, discovering that objects appeared to be larger under the glass. In Italy during the 12th century, Salvino D’Armate made a piece of glass to fit over one eye, allowing for a magnification effect to that eye. It was not until the 1590’s when a Dutch spectacle maker Zacharias Jansen began to test lenses that progress had been made to microscopes. Jansen was able to obtain about 9x magnification, but the objects appeared to be blurry. In 1595, Jansen and his father built the first compound microscope. While simple glasses were able to magnify objects, they were not considered to be a microscope. A compound microscope was defined by having two or more lenses in a hollow tube. [1] In 1665, Robert Hooke used a microscope about six inches long with two convex lenses inside and examined specimens under reflected light for the observations in his book Micrographia. Hooke also used a simpler microscope with a single lens for examining specimens with direct transmitted light, because this allowed for a clearer image. [2]

However, the first real invention and use of a microscope was by Anton van Leeuwenhoek. He was a Dutch draper that took interest in microscopes after seeing one while on an apprenticeship in Amsterdam in 1648. At some point in his life before 1668, he was able to learn how to grind lenses. This eventually led to Leeuwenhoek making his own microscope. His were instead simple powerful magnifying glasses, rather than a compound microscope. This was because he was able to use a single lens that was a small glass sphere but allowed for a magnification of 270x. This was a large progression since the magnification before was only a maximum of 50x. After Leeuwenhoek, there was not much progress for the microscopes until the 1850’s, two hundred years later. Carl Zeiss, a German engineer who manufactured microscopes, began to make changes to the lenses used. But the optical quality did not improve until the 1880’s when he hired Otto Schott and eventually Ernst Abbe.[3]

These microscopes could focus on objects the size of a wavelength or larger, giving restrictions still to advancement in discoveries with objects smaller than a wavelength. Later in the 1920’s, the electron microscope was developed, making it possible to view objects that are smaller than a wavelength, once again, changing the possibilities in science. [3]

Discovering Cells

Drawing of the structure of cork by Robert Hooke that appeared in Micrographia.

The cell was first discovered by Robert Hooke in 1665, which can be found to be described in his book Micrographia. In this book, he gave 60 ‘observations’ in detail of various objects under a coarse, compound microscope. [2] One observation was from very thin slices of bottle cork. Hooke discovered a multitude of tiny pores that he named "cells". This came from the Latin word Cella, meaning ‘a small room’ like monks lived in and also Cellulae, which meant the six sided cell of a honeycomb. However, Hooke did not know their real structure or function.[4] What Hooke had thought were cells, were actually empty cell walls of plant tissues. With microscopes during this time having a low magnification, Hooke was unable to see that there were other internal components to the cells he was observing. Therefore, he did not think the "cellulae" was alive.[5] His cell observations gave no indication of the nucleus and other organelles found in most living cells. In Micrographia, Hooke also observed mould, bluish in color, found on leather. After studying it under his microscope, he was unable to observe “seeds” that would have indicated how the mould was multiplying in quantity. This led to Hooke suggesting that spontaneous generation, from either natural or artificial heat, was the cause. Since this was an old Aristotelian theory still accepted at the time, others did not reject it and was not disproved until Leeuwenhoek later discovers generation is achieved otherwise. [2]

Anton van Leeuwenhoek is another scientist who saw these cells soon after Hooke did. He made use of a microscope containing improved lenses that could magnify objects almost 300-fold, or 270x. [6] Under these microscopes, Leeuwenhoek found motile objects. In a letter to The Royal Society on October 9, 1676, he states that motility is a quality of life therefore these were living organisms. Over time, he wrote many more papers in which described many specific forms of microorganisms. Leeuwenhoek named these “animalcules,” which included protozoa and other unicellular organisms, like bacteria. [3] Though he had did not have much formal education, was able to identify the first accurate description of red blood cells and discovered bacteria after gaining interest in the sense of taste that resulted in Leeuwenhoek to observe the tongue of an ox, then leading him to study "pepper water" in 1676. He also found for the first time the sperm cells of animals and humans. Once discovering these types of cells, Leeuwenhoek saw that the fertilization process requires the sperm cell to enter the egg cell. This put an end to the previous theory of spontaneous generation. After reading letters by Leeuwenhoek, Hooke was the first to confirm his observations that were thought to be unlikely by other contemporaries. [2]

The cells in animal tissues were observed after plants were because the tissues were so fragile and susceptible to tearing, it was difficult for such thin slices to be prepared for studying. Biologists believed that there was a fundamental unit to life, but were unsure what this was. It would not be until over a hundred years later that this fundamental unit was connected to cellular structure and existence of cells in animals or plants. [7] This conclusion was not made until Henri Dutrochet. Besides stating “the cell is the fundamental element of organization,” [8] Dutrochet also claimed that cells were not just a structural unit, but also a physiological unit.

In 1804, Karl Rudolphi and J.H.F. Link were awarded the prize for "solving the problem of the nature of cells," meaning they were the first to prove that cells had independent cell walls by the Königliche Societät der Wissenschaft (Royal Society of Science), Göttingen. [9] Before, it had been thought that cells shared walls and the fluid passed between them this way.

Cell Theory

Matthias Jakob Schleiden (1804-1881)
Theodor Schwann (1810-1882)

Credit for developing cell theory is usually given to two scientists: Theodor Schwann and Matthias Jakob Schleiden. While Rudolf Virchow contributed to the theory, he is not as credited for his attributions toward it. In 1838, Schleiden suggested that every structural part of a plant was made up of cells or the result of cells. He also suggested that cells were made by a crystallization process either within other cells or from the outside. However, this was not an original idea of Schlieden. He claimed this theory as his own, though Barthelemy Dumortier had stated it years before him. This crystallization process is no longer accepted with modern cell theory. [10] In 1839, Theodor Schwann states that along with plants, animals are composed of cells or the product of cells in their structures. This was a major advancement in the field of biology since little was known about animal structure up to this point compared to plants. From these conclusions about plants and animals, two of the three tenets of cell theory were postulated. [7]

  1. All living organisms are composed of one or more cells
  2. The cell is the most basic unit of life

Schleiden's theory of free cell formation through crystallization was refuted in the 1850s by Robert Remak, Rudolf Virchow, and Albert Kolliker.[3] In 1855, Rudolf Virchow added the third tenet to cell theory. In greek, this tenet states Omnis cellula e cellula. [7] This translated to:

  1. All cells arise only from pre-existing cells

However, the idea that all cells come from pre-existing cells had in fact already been proposed by Robert Remak; it has been suggested that Virchow plagiarized Remak and did not give him credit. [11] Remak published observations in 1852 on cell division, claiming Schleiden and Schawnn were incorrect about generation schemes. He instead said that binary fission, which was first introduced by Dumortier, was how reproduction of new animal cells were made. Once this tenet was added, the classical cell theory was complete.

Modern interpretation

The generally accepted parts of modern cell theory include:

  1. All known living things are made up of one or more cells[12]
  2. All living cells arise from pre-existing cells by division.
  3. The cell is the fundamental unit of structure and function in all living organisms.[13]
  4. The activity of an organism depends on the total activity of independent cells.[citation needed]
  5. Energy flow (metabolism and biochemistry) occurs within cells.[citation needed]
  6. Cells contain DNA which is found specifically in the chromosome and RNA found in the cell nucleus and cytoplasm.[14]
  7. All cells are basically the same in chemical composition in organisms of similar species .[citation needed]

Types of cells

Prokaryote which are plant cells
Eukaryote which are animal cells

Cells can be subdivided into the following subcategories:

  1. Prokaryotes: Prokaryotes are relatively small cells surrounded by the plasma membrane, with a characteristic cell wall that may differ in composition depending on the particular organism.[15] Prokaryotes lack a nucleus (although they do have circular or linear DNA) and other membrane-bound organelles (though they do contain ribosomes). The protoplasm of a prokaryote contains the chromosomal region that appears as fibrous deposits under the microscope, and the cytoplasm.[15] Bacteria and Archaea are the two domains of prokaryotes.
  2. Eukaryotes: Eukaryotic cells are also surrounded by the plasma membrane, but on the other hand,they have distinct nuclei bound by a nuclear membrane or envelope. Eukaryotic cells also contain membrane-bound organelles, such as (mitochondria, chloroplasts, lysosomes, rough and smooth endoplasmic reticulum, vacuoles).[16] In addition, they possess organized chromosomes which store genetic material.[citation needed]

See also


  1. ^ "History of the Microscope". History-of-the-microscope.org, United Kingdom. Retrieved 24 April 2014. 
  2. ^ a b c d Gest, H (2004). "The discovery of microorganisms by Robert Hooke and Antoni Van Leeuwenhoek, fellows of the Royal Society". Notes and records of the Royal Society of London 58 (2): 187–201. PMID 15209075.  edit
  3. ^ a b c d Mazzarello, P. (1999). "A unifying concept: the history of cell theory". Nature Cell Biology 1 (1): E13–5. doi:10.1038/8964. PMID 10559875.  edit
  4. ^ Inwood, Stephen (2003). The man who knew too much: the strange and inventive life of Robert Hooke, 1635–1703. London: Pan. p. 72. ISBN 0-330-48829-5. 
  5. ^ Becker, Wayne M.; Kleinsmith, Lewis J. and Hardin, Jeff (2003). The World of the Cell. Benjamin/Cummings Publishing Company. p. 1. ISBN 978-0-8053-4854-5. 
  6. ^ Becker, Wayne M.; Kleinsmith, Lewis J. and Hardin, Jeff (2003). The World of the Cell. Benjamin/Cummings Publishing Company. p. 1. ISBN 978-0-8053-4854-5. 
  7. ^ a b c Robinson, Richard. "History of Biology: Cell Theory and Cell Structure". Advameg, Inc. Retrieved 17 March 2014. 
  8. ^ Dutrochet, Henri (1824) "Recherches anatomiques et physiologiques sur la structure intime des animaux et des vegetaux, et sur leur motilite, par M.H. Dutrochet, avec deux planches"
  9. ^ http://www.mathnat.uni-rostock.de/geschichte/kalenderblatt/kalenderblatt-dezember-2013/
  10. ^ Schleiden, Matthias Jakob (1839) "Contributions to Phytogenesis"
  11. ^ Silver, GA (1987). "Virchow, the heroic model in medicine: health policy by accolade". American Journal of Public Health 77 (1): 82–8. doi:10.2105/AJPH.77.1.82. PMC 1646803. PMID 3538915. 
  12. ^ Wolfe
  13. ^ Wolfe, p. 5
  14. ^ Wolfe, p. 8
  15. ^ a b Wolfe, p. 11
  16. ^ Wolfe, p. 13


Further reading

External links