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.

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.

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.

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.

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.

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.

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.

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.