3.1: Lab Foundations of Modern Cell Theory
- 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 \(\PageIndex{1}\)). At the time, Hooke was not aware that the cork cells were long dead and, therefore, lacked the internal structures found within living cells.
Despite Hooke’s early description of cells, their significance as the fundamental unit of life was not yet recognized. Nearly 200 years later, in 1838, Matthias Schleiden (1804–1881), a German botanist who made 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.
Theodor Schwann (1810–1882), a noted German physiologist, made similar microscopic observations of animal tissue. In 1839, after a conversation with Schleiden, Schwann realized that similarities existed between plant and animal tissues. This laid the foundation for the idea that cells are the fundamental components of plants and animals.
In the 1850s, two Polish scientists living in Germany pushed this idea further, culminating in what we recognize today as the modern cell theory. In 1852, Robert Remak (1815–1865), a prominent neurologist and embryologist, published convincing evidence that cells are derived from other cells as a result of cell division. However, this idea was questioned by many in the scientific community. Three years later, Rudolf Virchow (1821–1902), a well-respected pathologist, published an editorial essay entitled “Cellular Pathology,” which popularized the concept of cell theory using the Latin phrase omnis cellula a cellula (“all cells arise from cells”), which is essentially the second tenet of modern cell theory. 1 Given the similarity of Virchow’s work to Remak’s, there is some controversy as to which scientist should receive credit for articulating cell theory. See the following Eye on Ethics feature for more about this controversy.
Rudolf Virchow, a prominent, Polish-born, German scientist, is often remembered as the “Father of Pathology.” Well known for innovative approaches, he was one of the first to determine the causes of various diseases by examining their effects on tissues and organs. He was also among the first to use animals in his research and, as a result of his work, he was the first to name numerous diseases and created many other medical terms. Over the course of his career, he published more than 2,000 papers and headed various important medical facilities, including the Charité – Universitätsmedizin Berlin, a prominent Berlin hospital and medical school. But he is, perhaps, best remembered for his 1855 editorial essay titled “Cellular Pathology,” published in Archiv für Pathologische Anatomie und Physiologie , a journal that Virchow himself cofounded and still exists today.
Despite his significant scientific legacy, there is some controversy regarding this essay, in which Virchow proposed the central tenet of modern cell theory—that all cells arise from other cells. Robert Remak, a former colleague who worked in the same laboratory as Virchow at the University of Berlin, had published the same idea 3 years before. Though it appears Virchow was familiar with Remak’s work, he neglected to credit Remak’s ideas in his essay. When Remak wrote a letter to Virchow pointing out similarities between Virchow’s ideas and his own, Virchow was dismissive. In 1858, in the preface to one of his books, Virchow wrote that his 1855 publication was just an editorial piece, not a scientific paper, and thus there was no need to cite Remak’s work.
By today’s standards, Virchow’s editorial piece would certainly be considered an act of plagiarism, since he presented Remak’s ideas as his own. However, in the 19 th century, standards for academic integrity were much less clear. Virchow’s strong reputation, coupled with the fact that Remak was a Jew in a somewhat anti-Semitic political climate, shielded him from any significant repercussions. Today, the process of peer review and the ease of access to the scientific literature help discourage plagiarism. Although scientists are still motivated to publish original ideas that advance scientific knowledge, those who would consider plagiarizing are well aware of the serious consequences.
In academia, plagiarism represents the theft of both individual thought and research—an offense that can destroy reputations and end careers. 2 3 4 5
- What are the key points of the cell theory?
- What contributions did Rudolf Virchow and Robert Remak make to the development of the cell theory?
Endosymbiotic Theory
As scientists were making progress toward understanding the role of cells in plant and animal tissues, others were examining the structures within the cells themselves. In 1831, Scottish botanist Robert Brown (1773–1858) was the first to describe observations of nuclei, which he observed in plant cells. Then, in the early 1880s, German botanist Andreas Schimper (1856–1901) was the first to describe the chloroplasts of plant cells, identifying their role in starch formation during photosynthesis and noting that they divided independent of the nucleus.
Based upon the chloroplasts’ ability to reproduce independently, Russian botanist Konstantin Mereschkowski (1855–1921) suggested in 1905 that chloroplasts may have originated from ancestral photosynthetic bacteria living symbiotically inside a eukaryotic cell. He proposed a similar origin for the nucleus of plant cells. This was the first articulation of the endosymbiotic hypothesis, and would explain how eukaryotic cells evolved from ancestral bacteria.
Mereschkowski’s endosymbiotic hypothesis was furthered by American anatomist Ivan Wallin (1883–1969), who began to experimentally examine the similarities between mitochondria, chloroplasts, and bacteria—in other words, to put the endosymbiotic hypothesis to the test using objective investigation. Wallin published a series of papers in the 1920s supporting the endosymbiotic hypothesis, including a 1926 publication co-authored with Mereschkowski. Wallin claimed he could culture mitochondria outside of their eukaryotic host cells. Many scientists dismissed his cultures of mitochondria as resulting from bacterial contamination. Modern genome sequencing work supports the dissenting scientists by showing that much of the genome of mitochondria had been transferred to the host cell’s nucleus, preventing the mitochondria from being able to live on their own. 6 7
Wallin’s ideas regarding the endosymbiotic hypothesis were largely ignored for the next 50 years because scientists were unaware that these organelles contained their own DNA. However, with the discovery of mitochondrial and chloroplast DNA in the 1960s, the endosymbiotic hypothesis was resurrected. Lynn Margulis (1938–2011), an American geneticist, published her ideas regarding the endosymbiotic hypothesis of the origins of mitochondria and chloroplasts in 1967. 8 In the decade leading up to her publication, advances in microscopy had allowed scientists to differentiate prokaryotic cells from eukaryotic cells. In her publication, Margulis reviewed the literature and argued that the eukaryotic organelles such as mitochondria and chloroplasts are of prokaryotic origin. She presented a growing body of microscopic, genetic, molecular biology, fossil, and geological data to support her claims.
Again, this hypothesis was not initially popular, but mounting genetic evidence due to the advent of DNA sequencing supported the endosymbiotic theory, which is now defined as the theory that mitochondria and chloroplasts arose as a result of prokaryotic cells establishing a symbiotic relationship within a eukaryotic host (Figure \(\PageIndex{3}\)). With Margulis’ initial endosymbiotic theory gaining wide acceptance, she expanded on the theory in her 1981 book Symbiosis in Cell Evolution . In it, she explains how endosymbiosis is a major driving factor in the evolution of organisms. More recent genetic sequencing and phylogenetic analysis show that mitochondrial DNA and chloroplast DNA are highly related to their bacterial counterparts, both in DNA sequence and chromosome structure. However, mitochondrial DNA and chloroplast DNA are reduced compared with nuclear DNA because many of the genes have moved from the organelles into the host cell’s nucleus. Additionally, mitochondrial and chloroplast ribosomes are structurally similar to bacterial ribosomes, rather than to the eukaryotic ribosomes of their hosts. Last, the binary fission of these organelles strongly resembles the binary fission of bacteria, as compared with mitosis performed by eukaryotic cells. Since Margulis’ original proposal, scientists have observed several examples of bacterial endosymbionts in modern-day eukaryotic cells. Examples include the endosymbiotic bacteria found within the guts of certain insects, such as cockroaches, 9 and photosynthetic bacteria-like organelles found in protists. 10
- What does the modern endosymbiotic theory state?
- What evidence supports the endosymbiotic theory?
The Germ Theory of Disease
Prior to the discovery of microbes during the 17 th century, other theories circulated about the origins of disease. For example, the ancient Greeks proposed the miasma theory, which held that disease originated from particles emanating from decomposing matter, such as that in sewage or cesspits. Such particles infected humans in close proximity to the rotting material. Diseases including the Black Death, which ravaged Europe’s population during the Middle Ages, were thought to have originated in this way.
In 1546, Italian physician Girolamo Fracastoro proposed, in his essay De Contagione et Contagiosis Morbis , that seed-like spores may be transferred between individuals through direct contact, exposure to contaminated clothing, or through the air. We now recognize Fracastoro as an early proponent of the germ theory of disease, which states that diseases may result from microbial infection. However, in the 16th century, Fracastoro’s ideas were not widely accepted and would be largely forgotten until the 19th century.
In 1847, Hungarian obstetrician Ignaz Semmelweis (Figure \(\PageIndex{4}\)) observed that mothers who gave birth in hospital wards staffed by physicians and medical students were more likely to suffer and die from puerperal fever after childbirth (10%–20% mortality rate) than were mothers in wards staffed by midwives (1% mortality rate). Semmelweis observed medical students performing autopsies and then subsequently carrying out vaginal examinations on living patients without washing their hands in between. He suspected that the students carried disease from the autopsies to the patients they examined. His suspicions were supported by the untimely death of a friend, a physician who contracted a fatal wound infection after a postmortem examination of a woman who had died of a puerperal infection. The dead physician’s wound had been caused by a scalpel used during the examination, and his subsequent illness and death closely paralleled that of the dead patient.
Although Semmelweis did not know the true cause of puerperal fever, he proposed that physicians were somehow transferring the causative agent to their patients. He suggested that the number of puerperal fever cases could be reduced if physicians and medical students simply washed their hands with chlorinated lime water before and after examining every patient. When this practice was implemented, the maternal mortality rate in mothers cared for by physicians dropped to the same 1% mortality rate observed among mothers cared for by midwives. This demonstrated that handwashing was a very effective method for preventing disease transmission. Despite this great success, many discounted Semmelweis’s work at the time, and physicians were slow to adopt the simple procedure of handwashing to prevent infections in their patients because it contradicted established norms for that time period.
Around the same time Semmelweis was promoting handwashing, in 1848, British physician John Snow conducted studies to track the source of cholera outbreaks in London. By tracing the outbreaks to two specific water sources, both of which were contaminated by sewage, Snow ultimately demonstrated that cholera bacteria were transmitted via drinking water. Snow’s work is influential in that it represents the first known epidemiological study, and it resulted in the first known public health response to an epidemic. The work of both Semmelweis and Snow clearly refuted the prevailing miasma theory of the day, showing that disease is not only transmitted through the air but also through contaminated items.
Although the work of Semmelweis and Snow successfully showed the role of sanitation in preventing infectious disease, the cause of disease was not fully understood. The subsequent work of Louis Pasteur, Robert Koch, and Joseph Listerwould further substantiate the germ theory of disease.
While studying the causes of beer and wine spoilage in 1856, Pasteur discovered properties of fermentation by microorganisms. He had demonstrated with his swan-neck flask experiments (link) that airborne microbes, not spontaneous generation, were the cause of food spoilage, and he suggested that if microbes were responsible for food spoilage and fermentation, they could also be responsible for causing infection. This was the foundation for the germ theory of disease.
Meanwhile, British surgeon Joseph Lister (Figure \(\PageIndex{5}\)) was trying to determine the causes of postsurgical infections. Many physicians did not give credence to the idea that microbes on their hands, on their clothes, or in the air could infect patients’ surgical wounds, despite the fact that 50% of surgical patients, on average, were dying of postsurgical infections. 11 Lister, however, was familiar with the work of Semmelweis and Pasteur; therefore, he insisted on handwashing and extreme cleanliness during surgery. In 1867, to further decrease the incidence of postsurgical wound infections, Lister began using carbolic acid (phenol) spray disinfectant/antiseptic during surgery. His extremely successful efforts to reduce postsurgical infection caused his techniques to become a standard medical practice.
A few years later, Robert Koch (Figure \(\PageIndex{5}\)) proposed a series of postulates (Koch’s postulates) based on the idea that the cause of a specific disease could be attributed to a specific microbe. Using these postulates, Koch and his colleagues were able to definitively identify the causative pathogens of specific diseases, including anthrax, tuberculosis, and cholera. Koch’s “one microbe, one disease” concept was the culmination of the 19th century’s paradigm shift away from miasma theory and toward the germ theory of disease. Koch’s postulates are discussed more thoroughly in How Pathogens Cause Disease .
- Compare and contrast the miasma theory of disease with the germ theory of disease.
- How did Joseph Lister’s work contribute to the debate between the miasma theory and germ theory and how did this increase the success of medical procedures?
After suffering a fever, congestion, cough, and increasing aches and pains for several days, Barbara suspects that she has a case of the flu. She decides to visit the health center at her university. The PA tells Barbara that her symptoms could be due to a range of diseases, such as influenza, bronchitis, pneumonia, or tuberculosis.
During her physical examination, the PA notes that Barbara’s heart rate is slightly elevated. Using a pulse oximeter, a small device that clips on her finger, he finds that Barbara has hypoxemia—a lower-than-normal level of oxygen in the blood. Using a stethoscope, the PA listens for abnormal sounds made by Barbara’s heart, lungs, and digestive system. As Barbara breathes, the PA hears a crackling sound and notes a slight shortness of breath. He collects a sputum sample, noting the greenish color of the mucus, and orders a chest radiograph, which shows a “shadow” in the left lung. All of these signs are suggestive of pneumonia, a condition in which the lungs fill with mucus (Figure \(\PageIndex{6}\)).
What kinds of infectious agents are known to cause pneumonia?
Lab: The Cell Theory Journal
BACKGROUND INFORMATION
Imagine life before the discovery of cells, when diseases attacked the human population, unseen and undetected by doctors and scientists of the time. Diseases like the Black Plague, caused by a bacterium, brought about one of the greatest pandemics in human history. The Irish Potato Famine was caused by a unicellular fungus that infected potatoes, thereby destroying one of the country's main food sources and resulting in mass starvation.
The invention of the microscope allowed for the discovery of cells and unicellular organisms, opening scientists' eyes to a whole other world - the world of microorganisms. The study of microorganisms is known as microbiology. The microscope opened the doors to many modern fields of science, including:
Microscopy - the study and use of microscopes
Bacteriology - the study of bacteria Mycology - the study of fungi Parasitology - the study of parasites Virology - the study of viruses Botany - the study of plants Cytology - the study of cells
In the nearly four hundred years since Robert Hooke identified the cell, the work of other scientists led to the development of the Cell Theory.
The Cell Theory states three points:
1. All living things are made up of cells.
2. Cells are the basic unit of structure and function in living things.
3. All cells come from pre-existing cells.
By studying these early scientists and their discoveries, you will be taking a scientific journey back in time. As you view each slide, imagine that what you see is what each of the scientists saw through the ocular lenses of their primitive microscopes!
This activity consists of 8 stations. Find the following materials at each station:
MATERIALS:
· Microscope
· Slide
PROCEDURE:
1. Go to the station for your group, as designated by the instructor.
2. Once at the station, find the write-up for that station number on your Lab Report.
At each station:
1. The historian should read the narrative for that station. Take turns reading so that each member of your group is the historian at least twice.
2. Listen carefully to the narrative, as it sets the stage for that scientist's work.
3. Write the scientist's name on the blank line, next to the date of the discovery.
4. Have each person in your group take a turn looking through the microscope at the station. When it is your turn, look through the microscope and then, in the circle for the appropriate station number on the worksheet, sketch what you see. In the space below the circle, record the
magnification used.
5. In your own words, describe what you see, under 'written observations'. In your description, be sure to note the name of the slide you are observing.
Your instructor will give you a designated amount of time at each station. When time is up, proceed to the next numbered station. For example, if your group began at station #6, your group should now proceed to station #7. The group at station #8 should proceed to station #1
Use this format for completing the write ups:
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1978 |
Scientist: Their name |
Magnification: 40x, 100x, or 400x |
|
Written Observations: |
Describe what you see here and be as detailed as possible |
Draw your slide here: Label as needed |
Station #1:
Narrative: An English scientist named made a primitive microscope and used it to look at a thin piece of cork (a piece of bark from a cork oak tree). He described the structures that he saw as cells, coming from the Latin word 'cella', meaning "small rooms". He wrote about what he saw in a book called Micrographia, published in 1665. In your sketch of the cork cells that you observe at this station, label one of the "small rooms" that you see "cell".
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1665 |
Scientist: |
Magnification: |
|
Written Observations: |
Draw your slide here: |
Station #2:
Narrative: was a fabric merchant from Holland. He was interested in lenses and used them to count threads in woven cloth. He became so interested in lenses that he learned how to make his own
by grinding and polishing pieces of glass. He noticed that if he made smaller lenses with greater arcs, the lenses produced greater magnification. Although he did not invent the microscope, he improved the microscopes of his day by increasing the magnification. His microscopes were able to magnify over 200 times what the unaided eye could observe and Leeuwenhoek was able to see things that no one had seen before. In 1674, he observed algae and many swimming 'animalcules' (tiny animals or as we know them, protists) in a drop of water. Because of his discoveries, he is often referred to as the father of microbiology. The long string of cells you will view at this station are called Spirogyra. Spirogyra are green algae that are often found in pond water. Count how many cells appear in your field of view and note that number in your observations. Hint: The individual cells may be easier to see in some strings than in others.
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1674 |
Scientist: |
Magnification: |
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Written Observations: |
Draw your slide here: |
Station #3:
Narrative: was a French botanist (a scientist who studies plants). In 1809, he observed that plants are made up of cells that form a continuous membrane. For this and other observations concerning plant and animal cells, he is known as the founder of cytology (the study of cells). The cells you will view at this station are generalized plant cells. Because these are plant cells, you will see a box- like structure surrounding each cell. This structure is known as the cell wall. In your sketch, label the "cell wall".
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1809 |
Scientist: |
Magnification: |
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Written Observations: |
Draw your slide here: |
Station #4:
Narrative: was a French soldier and professor. Lamarck observed that all living things (plant or animal) are made up of cellular tissue. Lamarck is much better known for his work in evolution, but is no doubt a contributor to the Cell Theory by making the above observation. In the slide of a general animal cell that you will observe at this station, notice that all of the cells that make up this tissue are very similar. In your sketch, label one of these cells as an "animal cell".
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1810 |
Scientist: |
Magnification: |
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Written Observations: |
Draw your slide here: |
Station #5:
Narrative: was a Scottish botanist. He named the cell nucleus while viewing orchid epidermal cells. The word "nucleus" is Latin for "kernel" or "little nut." He also discovered the movement of certain cells suspended in a liquid. This motion is called Brownian movement, named after him. In the allium or onion epidermal cells that you will view at this station, notice the small dark structures near the center of each cell. This is the nucleus of the cell. In your sketch, label one of these structures "nucleus".
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1831 |
Scientist: |
Magnification: |
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Written Observations: |
Draw your slide here: |
Station #6:
Narrative: was a German botanist. During his studies, he began to formulate the Cell Theory for plants. In his published paper of 1838, Schleiden proposed that all plants are composed of cells and that the cell is the most basic unit of plant life. He also forecasted that plant growth takes place by the generation of new cells somehow involving the nucleus. Although his idea of how cell division took place was not correct, he was definitely on the right track! In the slide of tilia (basswood) root cells that you will observe at this station, notice the cell wall surrounding each cell. In your sketch, label one of the cells "plant cell of plant tissue".
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1838 |
Scientist: |
Magnification: |
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Written Observations: |
Draw your slide here: |
Station #7:
Narrative: was a German physiologist (a scientist who studies living organisms). He discovered cells that surround and protect the axons of nerve cells. (An axon is the part of a nerve cell where impulses travel away from the cell body.) Today these cells are named after him. He took the work of Matthias Schleiden one step further and applied the Cell Theory to plants and animals. In his publication of 1839, he stated that cells are organisms, all organisms consist of one or more cells, and the cell is the basic unit of structure for all organisms (plants and animals). He further discussed cell origin, organization, and differentiation. At this station, you will be viewing a slide of frog skin. Notice that the skin is made of many similar epidermal cells. In your sketch, label one of the cells "animal cell of animal tissue".
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1839 |
Scientist: |
Magnification: |
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Written Observations: |
Draw your slide here: |
Station #8:
Narrative: was a German doctor. His many discoveries include being the first to recognize leukemia, He also recognized the cell as the basic unit of structure and function in all living things and is known for his theory of 1858, "Omnis cellula e cellula" (Latin for "every cell originates from another existing cell like it"). The slide that you will view at this station is an allium or onion root tip undergoing cell division. You may be able to distinguish other stages of the cell cycle. In your sketch, find an area where one cell is clearly dividing into two and label that area "cell division".
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1858 |
Scientist: |
Magnification: |
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Written Observations: |
Draw your slide here: |
CONCLUSIONS:
Referring to the instructions online, list the three points of the Cell Theory, below.
1.
2.
3.
After the name of each scientist, note which point(s) of the Cell Theory (1, 2 and or 3) is/are supported by that scientist's work.
|
Robert Hooke |
|
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Antony Leeuwenhoek |
|
|
Charles-Francois Brisseau de Mirbel |
|
|
Jean-Baptiste Lamarck |
|
|
Robert Brown |
|
|
Matthias Schleiden |
|
|
Theodor Schwann |
|
|
Rudolf Virchow |
EXTENSION ACTIVITY:
Create a time line to summarize the development of the Cell Theory.
Key Concepts and Summary
- Although cells were first observed in the 1660s by Robert Hooke, cell theory was not well accepted for another 200 years. The work of scientists such as Schleiden, Schwann, Remak, and Virchow contributed to its acceptance.
- Endosymbiotic theory states that mitochondria and chloroplasts, organelles found in many types of organisms, have their origins in bacteria. Significant structural and genetic information support this theory.
- The miasma theory of disease was widely accepted until the 19th century, when it was replaced by the germ theory of disease thanks to the work of Semmelweis, Snow, Pasteur, Lister, and Koch, and others.
Footnotes
- 1 M. Schultz. “Rudolph Virchow.” Emerging Infectious Diseases 14 no. 9 (2008):1480–1481.
- 2 B. Kisch. “Forgotten Leaders in Modern Medicine, Valentin, Gouby, Remak, Auerbach.” Transactions of the American Philosophical Society 44 (1954):139–317.
- 3 H. Harris. The Birth of the Cell . New Haven, CT: Yale University Press, 2000:133.
- 4 C. Webster (ed.). Biology, Medicine and Society 1840-1940 . Cambridge, UK; Cambridge University Press, 1981:118–119.
- 5 C. Zuchora-Walske. Key Discoveries in Life Science . Minneapolis, MN: Lerner Publishing, 2015:12–13.
- 6 T. Embley, W. Martin. “Eukaryotic Evolution, Changes, and Challenges.” Nature Vol. 440 (2006):623–630.
- 7 O.G. Berg, C.G. Kurland. “Why Mitochondrial Genes Are Most Often Found in Nuclei.” Molecular Biology and Evolution 17 no. 6 (2000):951–961.
- 8 L. Sagan. “On the Origin of Mitosing Cells.” Journal of Theoretical Biology 14 no. 3 (1967):225–274.
- 9 A.E. Douglas. “The Microbial Dimension in Insect Nutritional Ecology.” Functional Ecology 23 (2009):38–47.
- 10 J.M. Jaynes, L.P. Vernon. “The Cyanelle of Cyanophora paradoxa : Almost a Cyanobacterial Chloroplast.” Trends in Biochemical Sciences 7 no. 1 (1982):22–24.
- 11 Alexander, J. Wesley. “The Contributions of Infection Control to a Century of Progress” Annals of Surgery 201:423-428, 1985.