Cell Theory History

Before microscopes were invented, people believed that diseases were caused by curses and supernatural spirits. They had no idea that organisms such as bacteria existed. As scientists began using microscopes, they quickly realized they were entering a new world—one of microorganisms.

Microscopes enabled scientists to view and study cells, the basic units of living organisms.

Light microscopes

The microscope Anton van Leeuwenhoek used in the 1600s is considered a simple light microscope because it contained one lens and used light to view objects. Over the next 200 years, scientists greatly improved microscopes by grinding higher quality lenses and developing the compound light microscope. Compound light microscopes use a series of lenses to magnify objects in steps. These microscopes can magnify objects up to about 1500 times. As the observations of organisms viewed under a microscope expanded, scientists began to draw conclusions about the organization of living matter.

Electron Microscopes

The microscopes we have discussed so far use a beam of light and can magnify an object up to about 1500 times its actual size. Although light microscopes continue to be valuable tools, scientists knew that another world, which they could not yet see, existed within a cell. In the 1930s and 1940s, a new type of microscope, the electron microscope, was developed. This microscope uses a beam of electrons instead of light to magnify structures up to 500000 times their actual size, allowing scientists to see structures within a cell.

There are two basic types of electron microscopes. Scientists commonly use the scanning electron microscope (SEM) to scan the surfaces of cells to learn their three-dimensional shape. The transmission electron microscope (TEM) allows scientists to study the structures contained within a cell.  New types of microscopes and new techniques are continually being designed. For example, the scanning tunneling microscope (STM) uses the flow of electrons to create computer images of atoms on the surface of a molecule. New techniques using the light microscope have increased the information scientists can gain with this basic tool. Most of these new techniques seek to add contrast to structures within the cells, such as adding dyes that stain some parts of a cell, but not others.

Comparison between LM, TEM, and SEM

Pollen grain under SEM (left) and TEM (right)

The Cell Theory

Robert Hooke was an English scientist who lived at the same time as van Leeuwenhoek. Hooke used a compound light microscope to study cork, the dead cells of oak bark. In cork, Hooke observed small geometric shapes, like those shown in Figure. Hooke gave these box shaped structures the name cells because they reminded him of the small rooms monks lived in at a monastery.

Several scientists extended Hooke’s observations and drew some important conclusions. In the 1830s, the German scientist Matthias Schneider observed a variety of plants and concluded that all plants are composed of cells. Another German scientist, Theodor Schwann, made similar observations on animals.
And finally, in his study of human diseases, the German physician Rudolf Virchow (1821–1902) noted that all cells come from other cells. These three observations were combined to form a basic theory about the cellular nature of life. 
Today, the cell theory has four essential parts, which are summarized in the following.

The cell theory is made up of three main ideas:

1.   All organisms are composed of one or more cells. An organism may be a single cell, such as the organisms van Leeuwenhoek saw in water. Others, like the plants and animals with which you are most familiar, are multicellular, or made up of many cells.

2.    The cell is the basic unit of structure and organization of organisms. Although organisms such as humans, dogs, and trees can become very large and complex, the cell remains the simplest, most basic component of any organism.

3.     All cells come from preexisting cells. Before the cell theory, no one knew how cells were formed, where they came from, or what determined the type of cell they became. The cell theory states that a cell divides to form two identical cells.

4.    Cells contain hereditary material, which they pass to their offspring during division.

Developments in Cell Biology

The discovery of cells and the development of the cell theory happened at the beginning of a revolutionary time in the history of science. Before the invention of the microscope, many questions about what makes up living and nonliving things could not be answered. Once cells could be observed, these questions could be explored. Scientists could then turn their attention to finding out how cells function.

The Cellular Basis of Life

Microscopes helped biologists clarify our definition of life. All living things share several basic characteristics. All living things consist of organized parts, obtain energy from their surroundings, perform chemical reactions, change with time, respond to their environments, and reproduce. In addition, living things must be able to separate their relatively constant internal environment from the ever-changing external environment. The ability to maintain a constant internal environment, called homeostasis.