Cell theory is a biological theory. It states that all beings are made up of cells and that cells are the basic structural, fundamental and organisational unit of all organisms, and all cells emerge from pre-existing cells. The theory was first proposed in the mid-19th century. In other words, cells are the fundamental building blocks of all living organisms and the basic unit of reproduction. Modern cell theory was suggested to resolve the shortcomings of classical cell theory. Concluding all the findings on cells, many scientists, biologists, physiologists proposed the modern cell theory. With the continuous development in magnification techniques of microscopes, scientists can now precisely observe the cellular constituents under a microscope.

Earlier developments

The basic unit of all living organisms is the cell. The first cell was discovered in 1665 by English scientist Robert Hooke, and he published the book “Micrographia”, in which he illustrated the cell. 

While studying cork, Hooke noticed box-shaped structures that he named “cells” because they reminded him of rooms, cells in monasteries. 

Anton Van Leeuwenhoek first observed and illustrated a live cell. Due to the invention of the compound and the electron microscope, discovering the cell and its constituents could be possible.

Classical cell theory

Two of the three principles of cell theory were suggested based on these discoveries about plants and animals. 

• Every live entity contains one or more cells.
• The cell is life’s most fundamental unit.

The first cell theory was proposed by German botanist Matthias Jakob Schleiden in 1838 and German physiologist Theodor Schwann in 1839. Every structural part of a plant, according to Schleiden, is made up of cells or is the outcome of cells. According to Schwann, animals, like plants, are made up of cells or the products of cells in their structures.

Drawbacks of classical cell theory

The demerits of cell theory are-

• Viruses: Schleiden and Schwann could not explain the fundamentals of viral cells. They said that the body’s function is a coordinated function of a cell, which is false in the case of viruses because the viral machinery only gets activated inside the body of its host (bacteria, plants, and animals). A virus needs host machinery to complete its life cycle.
• Prokaryotic cells- Prokaryotes do not have a well-defined nucleus. Hence, cell theory fails in this case as well.
• Multinucleate condition- According to the cell theory, each cell has “a nucleus”, but specific cells like tapetal cells are multinucleated or coenocytic.
• Cells without nucleus- According to the cell theory, a nucleus is present in every cell, but human RBCs and sieve tubes of angiosperm are some cells that do not have a nucleus.

Modern cell theory

Modern cell theory is developed through the united efforts of several scientists. The two main objectives in modern cell theory are: all cells only come from some other cells (the principle of biogenesis). Organisms are made up of cells and their products, which are the basic fundamental units.

Postulates of modern cell theory

1. All living organisms are made up of one or more cells and the products of cells.
2. The basic fundamental, structural and functional unit of all living organisms is the cell.
3. The statement “Omnis cellula-e-cellula” was given by German scientist Rudolf Virchow in 1858, which means all the existing cells arise or develop from pre-existing cells.
4. The activity of an organism is related to the activity of the independent cell.
5.  The energy flow occurs within cells(Various metabolic reactions and biochemistry inside cells).
6. The cellular constituents are similar amongst the various individuals of similar species.
7. The genetic information is held by the DNA molecules, chromosomes. The genetic information is passed from one cell to the other.

Conclusion

The continuous development in cell theory has made understanding the cell and its structure very clear. Firstly, after the discovery of cells by Hooke, a basic idea of cellular constituents was made. Anton Van Leeuwenhoek observed a living cell under a microscope. Schleiden & Schwann formulated the classical cell theory based on their observations of plants and animals. Overcoming certain shortcomings of classical cell theory, modern cell theory was formulated. Rudolf Virchow (1855) first illustrated that when cells divide, the new cells are formed from pre-existing cells (Omnis cellula-e cellula).