The concepts of the cell and that life comes from pre-existing cells were introduced in 1800 by the invention of the microscope. 

The classic cell theory was introduced by Theodor Schwann, Rudolf Virchow, and Matthias Schleiden; it mainly consists of three parts. 

The first part says all living organisms are made from the basic unit, a cell. In the second part, the cell is defined as the basic unit of life. The third part introduces the concept that cells come from pre-existing cells. 

In later years,  many other concepts were also introduced by scientists and it led to the development of the modern cell theory, which introduced the hereditary properties and organelles.

What are cells?

The simplest or the basic unit of all living things is a cell. There are mainly two kinds of cells: eukaryotic cells and prokaryotic cells. 

The one that has a true nucleus with DNA is a eukaryotic cell, while the one that does not have a true nucleus is a prokaryotic cell. 

In prokaryotic cells, the DNA is coiled up in a region called the nucleoid. In order to function properly, cells are required for all living organisms.

Classic cell theory

In the book Micrographia, Robert Hook described cells for the first time in history in 1670. 

A honeycomb-like structure was observed when he sliced a piece of cork and observed it under his microscope. This was then given the name cell. 

In 1674, Leeuwenhoek was the first person to observe under the microscope a living cell from a bacterium. He also drew sketches of those cells he observed. 

Cell theory is not a single theory developed by one scientist; it is a generalisation of many scientists’ observations and inferences. The theory was mainly proposed by, or the ones who were recognised for those observations, are Theodor Schwann, Rudolf Virchow, and Matthias Schleiden. And it mainly includes three tenets or parts. 

Three tenets

1. In 1838 and 1839, Schwann and Schleiden put forward the first tenant – one or more cells are present in all living organisms.
2. In 1838 and 1839, Schwann and Schleiden put forward another or second tenet by depicting a cell as the basic structural and functional unit of life. Schleiden also put forward that new cells are developing from the old cells mainly from the nucleus. This proposal was further corrected as cells arising from pre-existing cells by Rudolf Virchow and were then called the theory of cell lineage.
3. In 1858, in the last tenet, Rudolf Virchow proposed that cells are derived from the cell division of pre-existing cells.

Modern cell theory

With the advancements in science and molecular biology, some other points were also included in the classic cell theory. They are:

1. During cell division, the hereditary information is also passing from cell to cell.
2. The same chemical composition and metabolic activities are present in all the cells.
3. Physiological functions and chemical functions are carried out inside the cells; energy flow occurs within the cells.
4. The activity of a cell also depends on the activities of subcellular structures within the cells such as organelles, plasma membrane, etc.

The classic cell theory explanation

According to cell theory, all living or biological organisms are combinations of cells and all life forms arose from the pre-existing cells. It is one of the unifying principles of biology as well. From these principles, we can say that only one biological cell is enough to create an organism. And the functional unit of cells is the organelles.

Scientists’ contribution to cell theory

• Anton van Leeuwenhoek: In 1674, he was the first to observe single-celled organisms now known as prokaryotes.
• Matthias Schleiden: A German botanist who discovered that the basic unit of all plants is composed of cells.
• Theodore Schwann: A German biologist and zoologist who discovered that all animal cells are composed of cells.
• Rudolf Virchow: In 1855, he proposed the biogenic law in which he proposed that all cells of living things were produced from pre-existing cells.

Conclusion

The development of cell theory has led to the explanation of the basic unit of life. The cell is the basic structural, developmental, and functional unit of life. The cell is also the reproductive unit of life. 

The classical cell theory has three important tenets and some additions and modifications are present in the modern cell theory. Modern cell theory introduced the concept of hereditary information DNA, the metabolism of cells, and so on. 

Cell theory has contributed to many theories in the science field. The existence of life on earth is now well-explained with this theory.