The A - Z Of the Cell Theory - Med Articles


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Wednesday, January 29, 2020

The A - Z Of the Cell Theory


The cell theory is universally accepted historic scientific theory. The main points of the cell theory are that all organisms are made up of cells and these cells arise from the pre-existing cells. Another main claim of this theory is that the cell is the basic structural and functional unit of living organisms. The cell theory becomes possible with the invention of the microscope. In the 17th century, Robert Hooke was the first who discovered the cell with the help of a microscope and give rise to a new branch called cell biology. Cell biology is the branch of biology that deals with the study of the structure and function of the cells. Cell biology for better understanding is divided into many subtopics such as cell metabolism and cell cycle etc. Cell biology includes both prokaryotic and eukaryotic cells. After the discovery of the cell debate began among the scientist and lead to the cell theory. This theory was formulated in 1839.

cell theory
Cell theory (image of a nerve cell)

Matthias Schleiden and Theodor Schwann were the two main contributors of the cell theory however some other scientists like Rudolf Virchow also contributed to this theory. There are three main points of this theory that are that all living organisms are made up of one or more cell, new cell arises from the pre-existing cells and the last but the most important is that cell is the basic structural and functional unit of an individual. After the discovery of viruses that are noncellular entities made the first point of the theory disputed. The cell theory basically made the foundation of modern biology and plays a major role in various other discoveries such as Darwin's theory, Mendel law of inheritance, etc.

History of cell theory

In the 17th century, science expands both microscopically and macroscopically. The invention of the telescope made the cosmos visible while the invention of the microscope made the microscopic world visible to the human eye. Robert Hooke once stated about the new inventions at that time that "By means of the telescope there is nothing so far distant and by the help of the microscope, there is nothing so small as to escape of inquiry. The cell was first discovered by Robert Hooke in 1665 and he published his findings in the micrographia. The description that Robert Hooke published does not contain any indications about the nucleus and other cell organelles. And the reason for this is that the cell that was studied by Robert Hooke is actually a dead cell. In 1674 Anton van Leeuwenhoek was the first man who witnesses a live cell under the microscope.

After these discoveries debate began and two scientist Theodor Schwan and Matthias Schleiden proposed a theory called cell theory. This theory gives three main points that are,
a). All living organisms are composed of cells.
b). The cell is the basic unit of the individual.
C). New cells arise from pre-existing cells.

Main events that lead to the formulation of the cell theory

1.  In 1595 Jansen invents the first compound microscope.
2.  Robert Hooke discovered the cell in 1665.
3.  In 1674 Leeuwenhoek witnesses live cells under the microscope.
4.  In 1833 brown describe the cell nucleus.
5.  In 1838 two scientist Theodor and Schwan proposed cell theory. 
These five are the main events that lead to the formulation of the cell theory.

Scientists who contributed to cell theory

1. Zacharias Jansen

Zacharias Jansen was born in 1585 and lived most of his life in Middelburg. He is associated with the invention of the first compound microscope and telescope. The claims of the inventions were made 20 years after his death. Some people think that the association of Jansen with these inventions was the fabrication put forward by his son. To avoid confusion please note that we put the name of this scientist in the list to give you an idea that the cell theory gots its base from the invention of the compound microscope.

2. Robert Hooke

Robert Hooke was born in July 1635. Robert works In different fields of science and makes some amazing discoveries but here we will only discuss his discoveries related to the cell which makes the foundation of the cell theory. Robert Hooke discovered the first cell in the year 1665 by observing a piece of cork under a microscope. While observing the cork he noticed some pores and he coined the name cells to these pores. Robert Hooke published all of his research in his book Micrographia. Hooke also works on gravity, elasticity, and fossils.

3. Antonie van Leeuwenhoek

Van Leeuwenhoek was a Dutch businessman and scientist. He was known as the father of microbiology. Van was the first who see the living microscopic organism under a microscope. He works with microbes and makes a great contribution to the establishment of microbiology as a scientific discipline. Van made more than 500 lenses and also made some types of microscope that can magnify objects 500 times.

4.  Mathias Schleiden

Matthias Schleiden was born on April 5, 1804, in Germany. He was a botanist and cofounder of the cell theory. He studied plants under the microscope and stated that different parts of the plant are composed of cells. Matthias also recognized the importance of the nucleus that was discovered by Robert Brown in 1831. Mathias also sensed the connection of the nucleus with the cell divisions. Matthias Schleiden also accepts Charles Darwin's theory of evolution. He works along with the Theodor Schwan.

5.  Theodore Schwan

Theodore Schwan was born on 7th December 1810 in Germany. He is also the co-founder of the cell theory. He extends the cell theory to the animals and succeeded. Theodore also contributes to various other discoveries such as the discovery of Schwan cells in the peripheral nervous system and the organic nature of yeast. He invents the term metabolism.

6.  Rudolf Virchow

Rudolf Virchow was born on 13 October 1831. He was a biologist, writer, editor, and politician. He makes a great contribution to the pathology and is known as the father of modern pathology. Rudolf Virchow concluded that where a cell exists there must have been pre-existing cells. He recognized the work of Robert Remak. Virchow correctly identifies the condition of leukemia. Virchow also coined the term embolism and thrombosis. Rudolf Virchow was the first man who analyzes the hair in a criminal investigation.

Cell theory complete explanation

The credit of cell theory is mainly given to the two scientists Theodor Schwan and Mathias Schleiden while the Rudolf Virchow also contributes to this theory but he is not as credited for their attribution toward the cell theory. Schleiden suggests that every part of the plant is composed of cells or their derivatives. Schleiden also stated that new cells were made by the process of crystallization that is now rejected by the modern cell theory.  Theodore Schwan also states that animals are also composed of cells. Due to the works of these two scientist three main points are derived that are called the cell theory.

The three main points are
A). All organisms are composed of cells.
B). The cell is the basic structural and functional unit of an organism.
C). New cells arise from pre-existing cells

Cell theory drawbacks

As we know that after the cell numerous inventions are made and many organisms are discovered at both micro level and macro level. New fields of study are emerged that extend and widen the study. Due to this advancement, various objections are made on cell theory. Some of them are,

1.  One of the founders of the cell theory named Schwan believed that cell could be spontaneously generated but later on various study shows that during growth cell duplicated themselves by division.
2.  In many organisms, there are many nuclei although their body is not differentiated into cells.
3. Both the main contributor to the cell theory did not know the mechanism of cell formation. One of them believes that cells developed from the cryptoblast while the second one believes that cells were developed spontaneously.

Modern cell theory

As we know that two scientists proposed the cell theory which makes the basis of modern cell theory but there are some serious objections raised on the cell theory.  To modify the cell theory two polish scientists present new ideas about the cell that today we recognized as modern cell theory. In 1852 Robert Remak proves that cells are not made spontaneously but the cells are derived from other cells through cell divisions. Three years later the same concept that cell arises from the cells was proved and recognized by a great pathologist Rudolf Virchow.

The main points of modern cell theory are
1.  All living individuals are composed of one or more cells.
2. New cells arise from the pre-existing by cell division.
3. The cell is the basic structural and functional unit of all organisms.
4. The activity of an organism depends on the activity of the cells present in the body.
5. Metabolism occurs within the cell.
6. DNA is present in the cell nucleus.
7. Cells of the same species are similar in chemical composition.
8. Genetic information is passed on from one cell to the other cell.

Endosymbiotic theory

After the cell theory, some scientist starts studying the role of cells in plant and animal while some scientist starts studying cell organelles. Due to these studies some new discoveries we're made. For example in 1831 Robert Brown observed the nucleus while in 1880 Andreas describes the chloroplast and identifies its role in starch formation during the process of photosynthesis. In 1905 Konstantin suggests that chloroplast derived from the ancestral photosynthetic bacteria. Konstantin also proposed that the ancestral bacteria live symbiotically inside a eukaryotic cell. He also suggests a similar origin for the nucleus. This suggestion becomes the endosymbiotic hypothesis. This hypothesis was put to experiment by an American scientist Wallin who studies the similarities between the mitochondria, chloroplast, and bacteria. Wallin published a series of papers and support the endosymbiotic theory.

The endosymbiotic hypothesis was ignored for a very long time because of the less information of scientists about the mitochondria and chloroplast. At the time of Wallins, scientists did not know that mitochondria and chloroplast contain it's own DNA. Later on, with the advancement, Lyn Margulis also study this theory and published his paper on the endosymbiotic theory.  Lynn provided evidence from genetics and fossils to prove the endosymbiotic theory. Lyn's ideas gained wide acceptance as new advanced discoveries are made. This is a general overview of the endosymbiotic theory.

What a theory?

A theory is an explanation of an event of the world that can be tested and verified repeatedly with scientific methods such as observation, evaluation, results, and measurements. Theories can also be verified with the experiment if possible. If the verification is not possible through the experiment then the theory is evaluated through abductive reasoning. Abduction is the form of logical inference with a set of observations which then seeks to give a simple explanation for the observation. So in short abductive reasoning is an inference to the best explanation. The term theory and scientific theory are different from each other. In everyday life, the theory is a speculative guess while the scientific theory is an explanation that has been tested and widely accepted.

The strength of theory depends on two factors which are the diversity of phenomena it explains and the second one is its simplicity. The theory can be modified or rejected if it does not match the new findings. But it does not mean that always theories are changed. The scientist also treats some less accurate theory as a theory because of its usefulness and simplicity. There are two types of theories that are constructive theories and principal theories. Constructive theory example is kinetic theory while the newton laws of motion are the principal theories. The essential criteria for the theory are that it must be observable and repeatable. The theory is supported by observation and experimental evidence. There is a misunderstanding that theory gradually upgrades into the laws when enough evidence is accumulated but in actual the theory always remains the theory and cannot change to law with the accumulation of new evidence. The theory is different from the hypothesis and it may be referred to as scientific fact.

Example of theories

1. Cell theory
2. Theory of evolution
3. Germ theory
4. Particulate inheritance theory
5. Dual inheritance theory
6. Collision theory
7. Kinetic theory of gases
8. Lewis theory
9. Molecular theory
10. Molecular orbital theory
11. Transition state theory
12. Valence bond theory
13. Atomic theory
14. Big Bang theory
15. Dynamo theory
16. Perturbation theory
17. Theory of relativity
18. Quantum field theory
19. Climate change theory
20. Plate tectonics theory
21. Origin of the moon theory
22. Moon illusion theory.

What is a cell?

As we know that Cell is the basic structural and functional unit of an individual. Cells are very small and cannot be seen with a naked eye. We can only see the cells with the help of an instrument called a microscope. All living things are made up of cells. The organisms are divided on the basis of the number of cells into various categories such as unicellular organisms and multicellular organisms. Unicellular organisms are composed of one cell and multicellular organisms are composed of many cells. On the basis of cells, organisms are also divided into prokaryotes and eukaryotes. There are different types of cells present in an individual.

For example, in humans, there are about two hundred different types of cells are present and about 20 different types of structures are present in these cells. Some of the cell organelles are the mitochondria, cell membrane, cell wall, Golgi apparatus, ribosomes, and many others. However, the cell wall is present only in the plant cell while it is absent in the animal cells. The cell contains a control center called the nucleus that contains DNA. Different cells perform different jobs in the body. The cell combines and makes tissues which then combines and makes an organ. The last and main point about the cells is that the cell dies. A study shows that every minute 96 million cells die while the same amount of cell divides and balances the number very accurately.

Below are some of the cell organelles with their short description. This is not a complete list but it gives you a basic understanding that what the cell structures or organelles are doing.

1. The nucleus is an important organelle that acts as a control center of the cell and stores DNA. The nucleus controls the growth and metabolism of the cell.
2. Ribosomes contain two subunits and are the protein factories of the cell.
3.  The endoplasmic reticulum is a membranous organelle present in the cytoplasm of a cell. There are two types of the endoplasmic reticulum that are rough endoplasmic reticulum and smooth endoplasmic reticulum.
4.  Golgi apparatus is also a membranous organelle that modifies the protein formed with the help of ribosome and endoplasmic reticulum. Another function of the Golgi apparatus is that it stores the processed proteins in vesicles and then transport it to the other areas of the cell.
5.  The chloroplast is only present in the plant cell and it contains a pigment called chlorophyll. The chloroplast plays a manor role in the process of photosynthesis.
6.  Mitochondria which is also known as the powerhouse of the cell is an oval-shaped structure present in the cytoplasm of the most eukaryotic cells. The main function of the mitochondria is to make ATP an energy-rich molecule.

What is a microscope?

As we know that the cell theory was made possible by the microscope but first we have to know what is a microscope. The microscope is an instrument that is used to see a small object that cannot be seen with a naked eye. The microscope made enlarge images of microscopic organisms. The study of objects with the help of a microscope is called microscopy. There are many microscopes but the first microscope that was invented was the light microscope. Microscopes are classified into light microscopes or electron microscopes. 

Light microscopes

Light microscopes also are known as optical microscopes are the first invented and most common type of microscopes. An optical microscope uses a beam of light. These types of microscope contain one or more lenses, refractive glass, and a focal plane. The focal plane is the area of the microscope on which the sample is placed. The function of the refractive glass is to focus the light.

Electron microscopes

These are the microscope that uses a beam of an electron instead of light. There are two types of electron microscopes that are scanning electron microscope and transmission electron microscope. These types of microscopes are very powerful.

Prokaryotic cells

These are the cells that lack a true nucleus and membrane-bounded organelles. Organisms that contain prokaryotic cells are very abundant and make up much of earth biomass. Prokaryotic cells are found in bacteria and Archaea. The organisms that contain prokaryotic cells are called prokaryotes. So bacteria and Archaea are prokaryotes. The reaction that occurs in prokaryotic cells generally takes place in the cytoplasm of the cells. These cells reproduce through binary fission. These cells have a nucleoid region and are simpler than the eukaryotic cells. Prokaryotic cells are surrounded by a membrane called cell membrane which separates the inner content of the cell from the outside environment.

Eukaryotic cell

Eukaryotic cells are the cells that contain a true nucleus and membrane-bound organelles. The organisms in which the eukaryotic cells are found are plants, animals, fungi, etc. These cells are more complex than prokaryotic cells. The organisms that contain the eukaryotic cells are called eukaryotes. Some of the organelles that are found in the eukaryotic cells are Ribosomes, Golgi bodies, mitochondria, etc. As we know that eukaryotes have a true nucleus that contains the DNA. The nucleus is the control center of the cell and is surrounded by a nuclear envelope. The organelles that are present in the eukaryotic cells are supported by the cytoskeleton which consists of the microtubule, microfilaments, and intermediate filaments. The eukaryotic cells divide through mitosis and meiosis.


Mitosis is the process in which the parent cell divides into two daughter cells. Mitosis gives rise to genetically identical cells in which the number of chromosomes is the same as in the parent cell. In mitosis first, the DNA is replicated which is then followed by the cytokinesis. The mitosis is divided into various stages that are the prophase, prometaphase, metaphase, anaphase, and telophase. Mitosis only occurs in the eukaryotic cell while the prokaryotes lack this process. Prokaryotes divide through the process of binary fission. The process of mitosis may be different in different organisms. For example, some organisms undergo open mitosis whereas fungi undergo closed mitosis. Open mitosis is a type of mitosis in which the nuclear envelope breaks down before the chromosomes separate. While closed mitosis is a type of mitosis in which the chromosome divide within an intact cell nucleus.

Function of mitosis

The main function of mitosis relies on the maintenance of the chromosomal set. The main functions of the mitosis are,
1. Development and growth. The number of cells in the body of organisms increased by mitosis.
2. Cell replacement. In the body everyday million of cell dies and these cells are replaced through the process of mitosis.
3. Regeneration. There are certain individuals who can regenerate their body parts. This regeneration process is achieved through the process of mitosis.

Errors might occur during the process of mitosis. These changes especially occur during embryonic development. Error in the process of mitosis can create an aneuploid cell in which the cell has too few or too many of one or more chromosomes. None disjunction is the case in which the sister chromatids fail to separate during anaphase. In nondisjunction one cell receive both sister chromatids while others receive none. Endoreplication is another mitotic error in which the chromosomes duplicate but the cell doesn't divide. Endoreplication results in polyploid cells.  


Meiosis is a type of cell division by which the organisms produce the gamete such as sperm and egg cells. Meiosis involves two rounds of division and four cells are produced with the haploid number of chromosomes. like the process of mitosis errors also occurs in the meiosis that results in aneuploidy. Errors in meiosis can cause miscarriage and developmental disabilities. The process of meiosis or meiotic divisions begins with a diploid cell. The diploid cell first undergoes DNA-replication so each homologous chromosomes consist of two sister chromatids. These pairs then exchange genetic information through crossovers. After the crossovers, the homologs are divided into daughter cells by the spindle apparatus. After the first meiotic division, the cell then undergoes the second meiotic division in which the sister chromatids are divided into separate daughter cells. Meiosis only occurs in a eukaryotic cell while prokaryotic cells lack this process and they divide and increase their number through another type of process called binary fission.

The phases of meiosis are
1. Prophase I
2. Metaphase I
3. Anaphase I
4. Telophase I and Cytokinesis
5. Prophase II
6. Metaphase II
7. Anaphase II
8. Telophase II and Cytokinesis

Errors in meiosis

Normal separation of chromosomes is called disjunction while the abnormal separation of chromosomes is called nondisjunction. Nondisjunction in meiosis can cause trisomy or monosomy.  These errors in meiosis can occur in both meioses I and meiosis II. The conditions that occur due to errors in the meiosis are,
1.  Down syndrome. Trisomy of chromosomes 21
2.  Patau syndrome. Trisomy of chromosome 13
3.  Edwards syndrome. Trisomy of chromosome 18
4.  Turner syndrome. Lacking one X chromosome in female
5. Klinefelter syndrome. Extra X chromosome in males
6. Triple X syndrome. Extra X chromosome in female
7. Jacobs syndrome. Extra Y chromosome in males.

Difference between mitosis and meiosis

1. In mitosis 2 daughter cells are produced while in meiosis four daughter cells are produced.
2. The function of meiosis is the production of gametes while the function of mitosis is growth, repair, cellular reproduction, and asexual reproduction.
3. The daughter cells produced by mitosis are genetically identical to parent cells while the in meiosis daughter cells are not genetically identical to the parent cell.
4. In miosis crossing overs occurs while in mitosis it occurs very very rarely.
5. Cytokinesis in meiosis occurs in telophase I and telophase II while in mitosis it occurs only in telophase.

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