Lesson Video: Cell Theory Biology

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


Video Transcript

In this video, we will learn how to describe the principles of cell theory. We will dive into scientific history and explore the contributions of the scientists Robert Hooke, Antonie van Leeuwenhoek, Matthias Schleiden, Theodor Schwann, and Rudolf Virchow who are all critical in the development of cell theory.

Humans are always looking for answers. Thanks to this questioning behavior, Newton created the theory of gravity after an apple fell on his head and Darwin postulated the idea that organisms can change and evolve over time. One question that has been on the minds of scientists for years is what the smallest building block of an organism may be. If we were to take this apple, for example, which just a moment ago was part of the tree, and then break it down into its building blocks, we would see that it’s made up of cells. We take this idea for granted today, but the truth is it took hundreds of years to realize this to eventually form what we call cell theory.

Cell theory has three principles or postulates which are as follows. All living things are made of at least one cell. Cells are the basic unit of life. And all cells come from preexisting cells. These principles are universally accepted today. But before the development of the microscope in the 1600s, scientists had no way of knowing what living things were made of. Let’s now 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 starts with Robert Hooke. Hooke was an English scientist and architect who coined the term cell in 1665. He built his own primitive compound microscope much like the one shown here, which is a microscope with three sequential lenses. And using this microscope, he made several observations that he published in a book called Micrographia. In his book, he presented a drawing of what he observed through the microscope when examining a very thin slice from a cork such as the one used in wine bottles.

In this material, he noted that it was filled with regular open spaces that he named cells. This word comes from the Latin word cellula, which means small room. Hooke came up with the term cell, and he got famous for it. But he didn’t know that cells were alive. It’s understandable that he thought that cells were dead because what he observed were the cell walls that remained in the piece of dried cork after the plant cells within them had died. In fact, Hooke was a supporter of the theory of spontaneous generation in which life simply arises when certain conditions are met, for example, when a piece of meat begins to rot and maggots seemingly appear spontaneously out of nowhere.

Spontaneous generation was a popular belief at the time. He observed some mold under his rudimentary microscope. And when he couldn’t identify a reproductive apparatus, he concluded that the mold was born of warmth and moisture. We have learned a lot since then, but this kind of thinking was standard at the time. 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. Van Leeuwenhoek was a Dutch businessman, politician, and microscope enthusiast. He learned to make his own glass lenses and built a simple microscope. This was essentially a single powerful magnifying glass lens that was able to magnify objects to over 200 times their original size.

This microscope allowed him to observe a wide variety in microscopic life. This allowed him to make many important observations including the discovery of freshwater protozoa, which he called animalcules. He also produced the first written description of bacteria by observing plaques scraped from his own teeth. He published most of his findings in letters to the Royal Society of London. The society actually turned to the English scientist Robert Hooke to confirm these findings since van Leeuwenhoek had no formal training in the sciences. Robert Hooke had trouble using the simple microscopes, but he 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 who worked with compound microscopes again. 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 important in the production of new cells, and observed that new cells seem to arise from the nucleus of old cells. 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. With the same type of microscope, Schwann reproduced Schleiden’s observations about plants in the tissues of animals. He found that all animal tissues were made of cells that possess a nucleus. Schwann, backed by Schleiden, was then able to state that all living things are composed of cells and cell products, which is the foundation of modern cell theory. Schwann then made three more concluding statements. The cell is the unit of structure, function, and organization in living things. The cell is both a distinct entity and the building block in the construction of organisms and that living cells form in a way similar to the formation of crystals. However, this statement has since been disproven.

The last discovery in our timeline of cell theory, which is that all cells arise from preexisting cells, is credited to a German pathologist and politician named Rudolf Virchow. The Latin statement “omnis cellula e cellula,” meaning all cells come from cells, was part of the rise by Virchow but was actually first coined by a French scientist named Francois Vincent Raspail. In fact, there is evidence that the first scientist to discover that cells are produced by preexisting cells is another German scientist whose publications went largely unnoticed named Robert Remak. This idea of all cells come from cells is a rejection of the concept of spontaneous generation, which was a commonly held belief at the time.

The discovery that all cells come from cells gave rise to germ theory. This states that specific diseases are caused by certain microorganisms invading the body and that infections in hospitals can be significantly lowered by hand washing between patients. It’s interesting that Virchow, a scientist who specialized in the study of diseases, disagreed with the increasingly popular germ theory. He instead believed that diseases were caused by some imbalances and abnormal activities inside of cells, not by outside pathogens. The contributions of these scientists led to cell theory, which is really the foundation of modern biology. Today, the three postulates of cell theory are that all living things are made of at least one cell, that cells are the basic unit of life, and that all cells come from preexisting cells.

As a review, let’s look at the contributions that each of these scientists made to cell theory as a timeline. It begins in 1590 when the first microscope was made. Then in 1655, Robert Hooke sees the first cells in a cork. He describes them as looking like little rectangular rooms. Then in 1675, Antonie van Leeuwenhoek views tiny objects in pond water. In 1838, Matthias Schleiden concludes that all plants are made of cells. In 1839, Theodor Schwann proposes that all animals, and therefore all living things, are made of cells. And finally, in 1855, Rudolf Virchow concludes that all cells are formed from preexisting cells. All these contributions have been critical in forming cell theory as we know it today. Now let’s take a moment to apply what we’ve learned with a practice question.

Which of the following is not a principle that cell theory is based on? (A) Cells are the smallest structures found within an organism. (B) All living organisms are made up of one or more cells. (C) All cells are made from preexisting cells. Or (D) the basic functional unit of all living organisms is the cell.

This question is asking us about cell theory and which of the provided choices is not a principle that it’s based on. So what is cell theory exactly? Cell theory is the universally accepted theory that all living things are made up of cells. There are actually three principles or postulates of cell theory that were developed by numerous scientists from the early 17th century through to the mid-19th century. A major technological contributor to the development of cell theory was the invention and production of simple and compound microscopes. This allowed scientists to view the microscopic world clearly for the first time.

The three principles or postulates of cell theory are as follows. All living things are made of at least one cell, cells are the basic unit of life, and all cells come from preexisting cells. While the 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 such as the endoplasmic reticulum, the nucleus, and the mitochondrion. All of these smaller structures are called organelles, so cells are not the smallest structures found within the organisms. So therefore, the statement that is not a principle of cell theory is that cells are the smallest structures found within organisms.

Now let’s go over the key points that we covered in this video. Cell theory is a universally accepted theory that is made up of three principles or postulates. All living things are made of at least one cell, cells are the basic unit of life, and all cells come from preexisting cells. These three principles are based off the work of Robert Hooke, Antonie van Leeuwenhoek, Matthias Schleiden, Theodor Schwann, and Rudolf Virchow who contributed to cell theory between the early 17th century through to the mid-19th century.

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