Lesson Explainer: Cell Theory Biology

In this explainer, we will learn how to describe the principles of cell theory and explain the contributions of scientists to its development.

Cell theory is one of the unifying theories in biology. Cells are as important to biology as atoms are to chemistry and as gravity is to physics. What we call “cell theory” is a historical theory that is universally accepted today. It has three principles, or postulates, which are as follows:

  1. All living things are made of at least one cell.
  2. Cells are the basic unit of life.
  3. All cells come from preexisting cells.

These are statements that you may perceive as facts, but before the development of the microscope in the 1600s, scientists had no way of knowing what living things were made of. Let’s take a look at some of the scientists who contributed to cell theory and their work.

Theory: Cell Theory

  • Postulate 1: All living things are made of at least one cell.
  • Postulate 2: Cells are the basic unit of life.
  • Postulate 3: All cells come from preexisting cells.

Example 1: Recalling the Principles of Cell Theory

Which of the following is not a principle that cell theory is based on?

  1. Cells are the smallest structures found within an organism.
  2. All living organisms are made of one or more cells.
  3. All cells are made from preexisting cells.
  4. The basic functional unit of all living organisms is the cell.

Answer

Cell theory is a universally accepted historical theory based on three principles or postulates. Several scientists contributed to the development of cell theory from the early seventeenth century through the mid nineteenth century. A major technological contributor to the development of cell theory is the invention and production of simple and compound microscopes that allowed scientists to view the microscopic world clearly for the first time. The three postulates of cell theory are the following: all living things are made of at least one cell, the cell is the basic unit of life, and all cells come from preexisting cells. While a cell is the basic unit of life, and the smallest object within an organism to be considered independently alive, cells themselves are composed of smaller structures, called organelles. So, cells are not the smallest structures found within organisms.

The statement that is not a principle of cell theory is option A, cells are the smallest structures found within an organism.

The beginning of the historical development of cell theory begins with Robert Hooke, who coined the term “cell” in 1665. Hooke was an English scientist and architect. He built his own primitive compound microscope (a microscope with three sequential lenses) and, through it, made several observations that he published in a book entitled “Micrographia.” In the book, Hooke presented a drawing of what he observed through the microscope when examining a very thin slice from a cork used to close the top of a bottle. The material was filled with regular, open spaces that he named “cells” after the Latin word cellula, which means small room. The type of microscope Hooke used is illustrated in Figure 1.

Cork under the microscope

Figure2:AmicrographofcorkcellsunderalightmicroscopethatshowssomethingsimilartowhatHookerstobservedanddrewinhisbook.Micrographia

Hooke came up with the term “cell,” but he did not know that cells were alive. What Hooke observed, resembling what is shown in Figure 2, were the cell walls that remained in the piece of dried cork after the plant cells within them had died. As a matter of fact, Hooke was a proponent of the theory of “spontaneous generation” in which life simply arises when certain conditions are met. Spontaneous generation was a popular belief at the time. He observed some mold under his rudimentary microscope, and when he could not identify a reproductive apparatus, he concluded that the mold was born of warmth and moisture.

Fact: Robert Hooke (1635–1703)

Robert Hooke was an English scientist famous for observing a slice of cork under a microscope and coining the term “cell.”

The next step in the development of cell theory was to discover the fundamental living nature of cells. This was achieved by Antonie van Leeuwenhoek, who is often called the “father of microbiology.” Antonie van Leeuwenhoek was a Dutch businessman, politician, and microscope enthusiast. He learned to make his own glass lenses and built a simple microscope, basically single powerful magnifying glass lens, that was able to magnify objects to over 200 times their original size. He used this microscope to make many important observations, including of freshwater protozoans which he called “animalcules.” He also produced the first written description of bacteria by observing plaque scraped from his own teeth. We can imagine the surprise of a man from that time discovering that his own mouth was full of tiny organisms!

Van Leeuwenhoek published most of his findings in letters to the Royal Society of London. The society actually turned to English scientist Hooke to confirm the Dutch naturalist’s findings, since Van Leeuwenhoek had no formal education in the sciences. Hooke had trouble using the simple microscopes but managed to reproduce Van Leeuwenhoek’s work, which gave it credibility among the scientific community.

Fact: Antonie van Leeuwenhoek (1632–1723)

Antonie van Leeuwenhoek was a Dutch scientist famous for making simple microscopes, which only had a single lens each, with high magnification that allowed him to observe a wide array of microscopic life.

Example 2: Recalling the Contributions of Antonie van Leeuwenhoek to the Development of Cell Theory

How did Antonie van Leeuwenhoek contribute to the development of cell theory?

  1. Van Leeuwenhoek stated that the new cells in an organism are produced by preexisting cells.
  2. Van Leeuwenhoek used a microscope to view a piece of cork that he determined was made up of small compartments he called “cells.”
  3. Van Leeuwenhoek used lenses to make simple microscopes that could view living organisms under greater than 200x magnification.
  4. Van Leeuwenhoek determined that all living organisms are composed of cells.
  5. Van Leeuwenhoek studied plant tissues and determined that plants are composed of cells.

Answer

Antonie van Leeuwenhoek was a Dutch microscope enthusiast who lived in the seventeenth and eighteenth centuries. He was a skilled lens maker and made his own simple microscopes that could magnify objects up to 270x their original size. These were basically very powerful, very tiny magnifying glasses that one would hold up to one’s eye in front of a powerful light source in order to view a specimen. Van Leeuwenhoek used his microscopes to view many different types of living cells from the protozoans found in pond water to living sperm cells. His work built on that of Robert Hooke and eventually led to discoveries made by Schleiden and Schwann.

This means that the correct answer is option C, Van Leeuwenhoek used lenses to make simple microscopes that could view living organisms under greater than 200x magnification.

The next major advancement in cell theory is attributed to Matthias Schleiden, whose work is often referenced to in conjunction with Theodore Schwann. In fact, Schleiden and his close friend Schwann are often called the “founders of cell theory.” Matthias Schleiden was a German botanist who was famous for studying plant structures under a microscope. In a book published in 1838, he stated that “all plants are composed of cells and the products of cells.” He also noted the importance of the nucleus, which seemed essential in the production of new cells, and observed that new cells seemed to arise from the nucleus of old cells. The photograph below is of a typical compound microscope built in the nineteenth century.

Antique microscope with clipping path

Figure4

Schleiden shared his observations about plants with his friend, Theodor Schwann. You may recognize his name from the Schwann cells in the nervous system, which are named after him. Schwann was a German physiologist particularly interested in the microscopic study of animal tissues. This field of research is called “histology,” and we can say that the theory and observations of Theodor Schwann created a foundation for modern histology.

Schwann reproduced Schleiden’s observations about plants in the tissues of animals. He found that all animal tissues were made of cells that possessed a nucleus. Schwann, backed by Schleiden, was then able to declare that “all living things are composed of cells and cell products,” which is the foundation of modern cell theory. He made three more conclusive statements:

  1. The cell is the unit of structure, function, and organization in living things.
  2. The cell is both a distinct entity and a building block in the construction of organisms.
  3. Living cells form in a way similar to the formation of crystals; however, this statement has subsequently been disproved.

Fact: Matthias Schleiden (1804–1881)

Matthias Schleiden was a German scientist famous for determining that all plants are made of cells. Along with Theodor Schwann, he is considered the father of cell theory.

Fact: Theodor Schwann (1810–1882)

Theodor Schwann was a German scientist famous for determining that all living organisms are made of cells. Along with Matthias Schleiden, he is considered the father of cell theory.

Example 3: Recalling the Contributions of Particular Scientists to a Tenet of Cell Theory

Which two scientists, involved in the development of cell theory, determined that living organisms are made up of cells?

  1. Virchow and Schwann
  2. Virchow and Hooke
  3. Schleiden and Schwann
  4. Hooke and Van Leeuwenhoek

Answer

The observation that all living things are made of cells was made by two scientists who worked in conjunction with each other. Matthias Schleiden was a nineteenth century German botanist who used microscopes to study the structure of plants. He concluded that all plants were made of cells. He also noticed that all the cells in plants had a nucleus and that new cells seemed to arise from the nucleus of old cells. Schleiden asked his friend Theodor Schwann, a German physiologist, to compare his findings to what he was observing in animal tissues. Schwann, after concluding that all animal tissues were indeed made of cells, was able to theorize that all living organisms were likely made of at least one cell.

So, the two scientists who determined that living organisms are made of cells were Schleiden and Schwann. Therefore, the correct answer is option C, Schleiden and Schwann.

The last discovery in our timeline of cell theory, which is all cells arise from preexisting cells, is credited to a German pathologist and politician named Rudolf Virchow. The statement “Omnis cellula e cellula,” meaning “all cells come from cells,” while popularized by Virchow, was actually first coined by a French scientist named François-Vincent Raspail. In fact, there is evidence that the first scientist to discover that the origin of cells is by the division of preexisting cells is another German scientist, whose publications went largely unnoticed, named Robert Remak. This idea is a rejection of the concept of spontaneous generation that was commonly held at the time. Interestingly, Virchow, a scientist who specialized in the study of diseases, disagreed with the increasingly popular germ theory, which states that specific diseases are caused by certain microorganisms invading the body, and the accompanying practice of hand washing. He believed that diseases were caused by some imbalances and abnormal activities inside the cells, not by outside pathogens. He also believed that microorganisms were merely attracted to diseased tissue as their natural habitat where they subsequently take up residence. A timeline of the major discoveries in the development of cell theory is shown in Figure 5.

Fact: Rudolf Virchow (1821–1902)

Rudolf Virchow was a German scientist famous for determining that all new cells in an organism are produced by preexisting cells.

Example 4: Recalling the Sequence of Events in the Development of Cell Theory

Which of the following is the correct sequence of events for the development of cell theory?

  1. Manufacturing and using simple microscopes determining that all living organisms are comprised of cells determining that plants are comprised of cells understanding that new cells are produced by preexisting cells
  2. Manufacturing and using simple microscopes determining that plants are comprised of cells determining that all living organisms are comprised of cells understanding that new cells are produced by preexisting cells
  3. Determining that plants are comprised of cells determining that all living organisms are comprised of cells manufacturing and using simple microscopes understanding that new cells are produced by preexisting cells
  4. Manufacturing and using simple microscopes understanding that new cells are produced by preexisting cells determining that plants are comprised of cells determining that all living organisms are comprised of cells

Answer

Several scientists contributed to the development of cell theory. Some of these contributions, listed in chronological order, are the following:

  • Robert Hooke uses a compound microscope to coin the term “cell.”
  • Antonie van Leeuwenhoek uses a simple microscope to observe microscopic life.
  • Matthias Schleiden observes that all plants are made of cells.
  • Theodor Schwann concludes that all organisms are made of cells.
  • Robert Virchow states that all cells come from preexisting cells.

None of these discoveries would have been possible without the preceding invention of the light microscope, a device that uses glass lenses to magnify the image of very small objects, making them visible to the naked eye.

So, the correct sequence of events is option B, manufacturing and using simple microscopes determining that plants are comprised of cells determining that all living organisms are comprised of cells understanding that new cells are produced by preexisting cells.

Cell theory is the foundation of modern biology. Today, the three tenets of cell theory are considered to be scientific facts. Each of these scientists, Hooke, Van Leeuwenhoek, Schleiden, Schwann, and Virchow, made many valuable contributions to contemporary biology in addition to contributions to cell theory. We can also see that modern microbiologists have continued to build on this work, using what we have learned about cells to develop germ theory, evolutionary theory, modern embryology, and so much more.

Let’s summarize what we have learned about cell theory.

Key Points

  • Cell theory states that all organisms are made of at least one cell, the cell is the basic unit of life, and all cells arise from preexisting cells.
  • Major contributors to cell theory are Robert Hooke, Antonie van Leeuwenhoek, Matthias Schleiden, Theodor Schwann, and Rudolf Virchow.

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