Cell Theory Lesson

From Spontaneous Generation to Cell Theory

A very long time ago, during the 1600s, people believed in spontaneous generation. This theory stated that non-living things can generate or give life to living things. Two scientists helped disprove the theory of spontaneous generation. Italian doctor Francesco Redi used rotten meat to demonstrate that maggots could not spontaneously arise from the rotten meat. Redi put rotten meat in two identical jars. Flies were able to lay eggs in the jar that was left uncovered. Maggots were on the decaying meat in this jar. However, the second jar remained covered during the experiment. There were no maggots present on the meat in the second jar. Redi's experiment proved that rotten meat, a non-living thing, could not create flies. French chemist Louis Pasteur is widely known for his work in vaccination, pasteurization, and fermentation. Pasteur also performed an experiment that disproved spontaneous generation. He designed an experiment to test a broth enriched with bacteria and a broth that did not have bacteria. The broth had been boiled to kill any forms of bacteria or living thing. New bacteria did not grow in the boiled broth; but it did grow in the unheated broth. Pasteur was able to conclude that bacteria derived from existing bacteria. The outcome of his experiment helped convince people that living things did not generate from nonliving matter. This work paved the way to Cell Theory.

History of the Cell

Louis Pasteur's experiments led to the rejection of spontaneous generation. His work provided evidence to support another theory, the cell theory. The cell theory states the following:

All living things are composed of cells. They can be unicellular or multicellular.
The cell is the basic unit of structure and function in living things.
All cells arise from pre-existing cells.

The cell theory applies to all living things, from the unicellular amoeba to more complex organisms such as bears. How did the cell theory develop? You will learn about how a few scientists contributed to the development of the cell theory.

The first microscope was invented around 1590 by two Dutch eyeglass makers named Zacharias and Hans Janssen. It was a simple tube with lenses at one end. A microscope uses beams of light to magnify an object. A microscope is an instrument that makes it easier to see objects that are too small to be seen with the naked eye. Therefore the tiny details of objects that were not visible before can be seen through a microscope. In 1663, English scientist Robert Hooke invented a compound microscope. He was able to look at the tiny structures within a layer of cork from the bark of an oak tree. The empty spaces he saw were actually the cell walls of the plant cell. Hooke called these small spaces cells. Around 1674, a Dutch scientist name Anton van Leeuwenhoek designed a more powerful simple microscope to look at the specimens in pond water. He noticed that the microorganism could move. Leeuwenhoek was the first person to use a microscope to observe and record the movement of single-celled organisms, bacteria and protozoa. He obtained the organisms from pond water, rainwater, and even the human mouth. Theodor Schwann, Matthias Schleiden, and Rudolf Virchow were German scientists that also contributed to the development of the cell theory. Schledien's microscopic research led to his determination that plants are made of cells. Based on these findings and other microscopic discoveries, Schwann concluded that animals are made up of cells too. Virchow was a firm advocate against the theory of spontaneous generation. His research and the observations from previous scientists led him to propose that every cell originates from another cell.

Watch the following presentation to learn more about the history of cell theory.

Please watch the following videos to learn more about how cells were discovered using microscopes.

[CC BY 4.0] UNLESS OTHERWISE NOTED | IMAGES: LICENSED AND USED ACCORDING TO TERMS OF SUBSCRIPTION