cell theory homework

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Course: biology library   >   unit 8.

• Scale of cells

Cell theory

• Intro to cells
• Introduction to cells

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2.18: Cell Theory

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What are you made of?

Cells make up all living things, including your own body. This picture shows a typical group of cells, but not all cells look alike. Cells can differ in shape and sizes, and the different shapes usually indicate different functions.

Introduction to Cells

A cell is the smallest structural and functional unit of an organism. Some organisms, like bacteria , consist of only one cell. Big organisms, like humans, consist of trillions of cells. Compare a human to a banana. On the outside, they look very different, but if you look close enough you’ll see that their cells are actually very similar.

Observing Cells

Most cells are so small that you cannot see them without the help of a microscope . It was not until 1665 that English scientist Robert Hooke invented a basic light microscope and observed cells for the first time, by looking at a piece of cork. You may use light microscopes in the classroom. You can use a light microscope to see cells (Figure below). But many structures in the cell are too small to see with a light microscope . So, what do you do if you want to see the tiny structures inside of cells?

In the 1950s, scientists developed more powerful microscopes. A light microscope sends a beam of light through a specimen, or the object you are studying. A more powerful microscope, called an electron microscope , passes a beam of electrons through the specimen. Sending electrons through a cell allows us to see its smallest parts, even the parts inside the cell (Figure below). Without electron microscopes, we would not know what the inside of a cell looked like.

Cell Theory

In 1858, after using microscopes much better than Hooke’s first microscope, Rudolf Virchow developed the hypothesis that cells only come from other cells. For example, bacteria , which are single-celled organisms, divide in half (after they grow some) to make new bacteria. In the same way, your body makes new cells by dividing the cells you already have. In all cases, cells only come from cells that have existed before. This idea led to the development of one of the most important theories in biology, the cell theory .

Cell theory states that:

• All organisms are composed of cells.
• Cells are alive and the basic living units of organization in all organisms.
• All cells come from other cells.

As with other scientific theories, many hundreds, if not thousands, of experiments support the cell theory. Since Virchow created the theory, no evidence has ever been identified to contradict it.

Specialized Cells

Although cells share many of the same features and structures, they also can be very different (Figure below). Each cell in your body is designed for a specific task. In other words, the cell's function is partly based on the cell's structure. For example:

• Red blood cells are shaped like flat discs to move easily and quickly through blood vessels and deliver oxygen all over the body.
• Nerve cells are long and stringy in order to form a line of communication with other nerve cells, like a wire. Because of this shape, they can quickly send signals, such as the feeling of touching a hot stove, to your brain.
• Skin cells are flat and fit tightly together to protect your body.

As you can see, cells are shaped in ways that help them do their jobs. Multicellular (many-celled) organisms have many types of specialized cells in their bodies.

Levels of Organization

While cells are the basic units of an organism, groups of cells can perform a job together. These cells are called specialized because they have a special job. Specialized cells can be organized into tissues . For example, your liver cells are organized into liver tissue. Your liver tissue is further organized into an organ, your liver. Organs are formed from two or more specialized tissues working together to perform a job. All organs, from your heart to your liver, are made up of an organized group of tissues.

These organs are part of a larger system, the organ systems . For example, your brain works together with your spinal cord and other nerves to form the nervous system. This organ system must be organized with other organ systems, such as the circulatory system and the digestive system, for your body to work. Organ systems work together to form the entire organism. There are many levels of organization in living things (Figure below).

• Cells were first observed under a light microscope, but today's electron microscopes allow scientists to take a closer look at the inside of cells.
• Cells are organized into tissues, which are organized into organs, which are organized into organ systems, which are organized to create the whole organism.
• What type of microscope would be best for studying the structures found inside of cells?
• What are the three basic parts of the cell theory?
• According the cell theory, can you create a cell by combining molecules in a laboratory ? Why or why not?
• Give an example of a specialized cell.
• What is a tissue?
• What is the relationship between tissues and organs?

Cell Theory: A Core Principle of Biology

• Cell Biology
• Weather & Climate
• B.A., Biology, Emory University
• A.S., Nursing, Chattahoochee Technical College

Cell Theory is one of the basic principles of biology . Credit for the formulation of this theory is given to German scientists Theodor Schwann (1810–1882), Matthias Schleiden (1804–1881), and Rudolph Virchow (1821–1902).

The Cell Theory states:

• All living organisms are composed of cells . They may be unicellular or multicellular.
• The cell is the basic unit of life.
• Cells arise from pre-existing cells. (They are not derived from spontaneous generation .)

The modern version of the Cell Theory includes the ideas that:

• Energy flow occurs within cells.
• Heredity information ( DNA ) is passed on from cell to cell.
• All cells have the same basic chemical composition.

In addition to the cell theory, the gene theory , evolution , homeostasis , and the laws of thermodynamics form the basic principles that are the foundation for the study of life.

What Are Cells?

Cells are the simplest unit of matter that is living. The two primary kinds of cells are eukaryotic cells , which have a true  nucleus containing DNA and prokaryotic cells , which have no true nucleus. In prokaryotic cells, the DNA is coiled up in a region called the nucleoid.

Cell Basics

All living organisms in the kingdoms of life are composed of and depend on cells to function normally. Not all cells , however, are alike. There are two primary types of cells: eukaryotic and prokaryotic cells . Examples of eukaryotic cells include animal cells ,  plant cells , and fungal cells . Prokaryotic cells include bacteria and archaeans .

Cells contain organelles , or tiny cellular structures, that carry out specific functions necessary for normal cellular operation. Cells also contain DNA (deoxyribonucleic acid) and RNA (ribonucleic acid), the genetic information necessary for directing cellular activities.

Cell Reproduction

Eukaryotic cells grow and reproduce through a complex sequence of events called the cell cycle . At the end of the cycle, cells will divide either through the processes of mitosis or meiosis . Somatic cells replicate through mitosis and sex cells reproduce via meiosis. Prokaryotic cells reproduce commonly through a type of asexual reproduction called binary fission . Higher organisms are also capable of asexual reproduction . Plants, algae , and fungi reproduce through the formation of reproductive cells called spores . Animal organisms can reproduce asexually through processes such as budding, fragmentation, regeneration, and parthenogenesis .

Cell Processes: Cellular Respiration and Photosynthesis

Cells perform a number of important processes that are necessary for the survival of an organism. Cells undergo the complex process of cellular respiration in order to obtain energy stored in the nutrients consumed. Photosynthetic organisms including plants , algae , and cyanobacteria are capable of photosynthesis . In photosynthesis, light energy from the sun is converted to glucose. Glucose is the energy source used by photosynthetic organisms and other organisms that consume photosynthetic organisms.

Cell Processes: Endocytosis and Exocytosis

Cells also perform the active transport processes of endocytosis and exocytosis . Endocytosis is the process of internalizing and digesting substances, such as seen with macrophages and bacteria . The digested substances are expelled through exocytosis. These processes also allow for molecule transportation between cells.

Cell Processes: Cell Migration

Cell migration is a process that is vital for the development of tissues and organs . Cell movement is also required for mitosis and cytokinesis to occur. Cell migration is made possible by interactions between motor enzymes and cytoskeleton microtubules.

Cell Processes: DNA Replication and Protein Synthesis

The cell process of DNA replication is an important function that is needed for several processes including chromosome synthesis and cell division to occur. DNA transcription and RNA translation make the process of protein synthesis possible.

• 10 Facts About Cells
• What Is Cell Biology?
• Cell Biology Glossary
• Frequently Asked Biology Questions and Answers
• Biology Homework Help
• Genetics Basics
• Differences Between Plant and Animal Cells
• What Is an Organelle?
• Cytoskeleton Anatomy
• Laws of Thermodynamics as Related to Biology
• The Role of Cytoplasm in a Cell
• All About Photosynthetic Organisms
• Learn About the Different Types of Cells: Prokaryotic and Eukaryotic
• All About Animal Cells
• Euglena Cells

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Cell Theory Bundle

Do you need resources for your cell theory lesson? This bundle of resources is ready to use – engaging Google Slides activities, fun Doodle Notes and a self-grading Google Forms quiz! Download now.

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Engaging and time-saving resources for your cell theory lesson!

These resources are fun and ready to use. Assign the Google Slides activities to your Google Classroom and let students work through them! The slides are bright and illustrated to bring concepts to life.

Once students have completed the slides, the Doodle Notes are a perfect review activity or homework. Then you can easily assess student progress with the self-grading Google Forms Quiz. There are images to support some questions and increase engagement.

There are answer slides and with handy teacher guides so you know how to use these activities in your classroom.

Content covered:

• Components of Cell Theory
• History of Cell Theory – including these Scientists: Janssen, Hooke, Van Leeuwenhoek, Schleiden, Schwann and Virchow

My Cell Theory bundle includes:

• a literacy task
• video about cell theory
• storyboard to label
• true & false activity
• Answer Slides: for students to check and correct their work.
• Google Forms Quiz : self-grading to let you see how students are progressing.
• Doodle Notes : 2 pages of engaging Doodle Notes, perfect for reviewing content – there are 3 versions –  black and white, scaffolded, and color, plus a completed example & answer key presentation
• Teacher Guidance: how to assign and use these resources with your classes.

How do I use this bundle of resources?

• After checkout, you can immediately download your resources
• You’ll click the links in the PDF to make copies of the Google Slides and Quiz for your own Google Drive
• Simply assign these to your students on Google Classroom
• You’ll open the Doodle Notes ZIP file to see all of the versions of the Doodle Notes
• Pick which version you want students to complete and print one set for each student

And that’s it, your lesson prep is done – easy-peasy-lemon-squeezy!

Please note: You will need a Google Classroom set up for your students and each student will need their own device and an internet connection to access the Google Slides and Quiz. Most clipart and questions are secured and cannot be edited. The answers can be edited. There is a link to a YouTube video (not necessary for the activities but a nice introduction) – please check YouTube is not restricted in your school. My Doodle Notes are secure PDF files and are not editable. Doodle Notes is a trademarked term used with permission. Please visit  doodlenotes.org  for more info. 

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3.2: Foundations of Modern Cell Theory

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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 \(\PageIndex{1}\)). 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.

Science and Plagiarism

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

Exercise \(\PageIndex{1}\)

• 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

Exercise \(\PageIndex{2}\)

• 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 .

Exercise \(\PageIndex{3}\)

• 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?

Clinical Focus: Part 2

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 \(\PageIndex{6}\)).

Exercise \(\PageIndex{4}\)

What kinds of infectious agents are known to cause pneumonia?

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.
• 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.

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