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C4 - Cell Theory

I can paraphrase the main ideas of cell theory and explain the development of this theory


If you look at living matter with a microscope—even a simple light microscope—you will see that it consists of cells. Cells are the basic units of the structure and function of living things. They are the smallest units that can carry out the processes of life. All organisms are made up of one or more cells, and all cells have many of the same structures and carry out the same basic life processes. Knowing the structures of cells and the processes they carry out is necessary to understanding life itself.

Discovery of Cells

The first time the word cell was used to refer to these tiny units of life was in 1665 by a British scientist named Robert Hooke. Hooke was one of the earliest scientists to study living things under a microscope. The microscopes of his day were not very strong, but Hooke was still able to make an important discovery. When he looked at a thin slice of cork under his microscope, he was surprised to see what looked like a honeycomb. Hooke made the drawing in Figure below to show what he saw. As you can see, the cork was made up of many tiny units, which Hooke called cells.

Cork Cells. This is what Robert Hooke saw when he looked at a thin slice of cork under his microscope. What type of material is cork? Do you know where cork comes from?

Leeuwenhoek’s Discoveries

Soon after Robert Hooke discovered cells in cork, Anton van Leeuwenhoek in Holland made other important discoveries using a microscope. Leeuwenhoek made his own microscope lenses, and he was so good at it that his microscope was more powerful than other microscopes of his day. In fact, Leeuwenhoek’s microscope was almost as strong as modern light microscopes. Using his microscope, Leeuwenhoek discovered tiny animals such as rotifers. The magnified image of a rotifer in Figure below is similar to what Leeuwenhoek observed. Leeuwenhoek also discovered human blood cells. He even scraped plaque from his own teeth and observed it under the microscope. What do you think Leeuwenhoek saw in the plaque? He saw tiny living things with a single cell that he named animalcules (“tiny animals”). Today, we call Leeuwenhoek’s animalcules bacteria.

Microscopic Rotifer. Rotifers like this one were first observed by Aton van Leeuwenhoek. This tiny animal is too small to be seen without a microscope.

The Cell Theory

By the early 1800s, scientists had observed the cells of many different organisms. These observations led two German scientists, named Theodor Schwann and Matthias Jakob Schleiden, to propose that cells are the basic building blocks of all living things. Around 1850, a German doctor named Rudolf Virchow was studying cells under a microscope when he happened to see them dividing and forming new cells. He realized that living cells produce new cells through division. Based on this realization, Virchow proposed that living cells arise only from other living cells. The ideas of all three scientists—Schwann, Schleiden, and Virchow—led to the cell theory, which is one of the fundamental theories of biology. The cell theory states that:

  • All organisms are made of one or more cells.
  • All the life functions of organisms occur within cells.
  • All cells come from already existing cells.


Starting with Robert Hooke in the 1600s, the microscope opened up an amazing new world—the world of life at the level of the cell. As microscopes continued to improve, more discoveries were made about the cells of living things. However, by the late 1800s, light microscopes had reached their limit. Objects much smaller than cells, including the structures inside cells, were too small to be seen with even the strongest light microscope. Then, in the 1950s, a new type of microscope was invented. Called the electron microscope, it used a beam of electrons instead of light to observe extremely small objects. With an electron microscope, scientists could finally see the tiny structures inside cells. In fact, they could even see individual molecules and atoms. The electron microscope had a huge impact on biology. It allowed scientists to study organisms at the level of their molecules and led to the emergence of the field of molecular biology. With the electron microscope, many more cell discoveries were made. Figure below shows how the cell structures called organelles appear when scanned by an electron microscope.

Electron Microscope Image of Organelles. An electron microscope produced this image of a cell.

KQED: The World's Most Powerful Microscope

Lawrence Berkeley National labs uses a $27 million electron microscope to make images to a resolution of half the width of a hydrogen atom. This makes it the world's most powerful microscope. See http://www.kqed.org/quest/television/the-worlds-most-powerful-microscope and http://www.kqed.org/quest/slideshow/web-extra-images-from-the-worlds-most-powerful-microscope for more information.