Cell Theory- Definition, History, Modern, Exceptions

According to the cell theory, all biological organisms are made up of cells, the basic building blocks of life, and all life evolved from preexisting life.

It is the cell theory that emphasizes the unity underlying the diversity of forms, i.e., the cellular organization of all life forms.

Cell Theory
Cell Theory

The cell theory explains that all life, whether unicellular, colonial, or multicellular in an organization, is punctuated by periods of minimal organization, whether spores or zygotes. 

Schleiden & Schwann were the first to introduce the idea of the cell theory in 1839, and it has remained the cornerstone of modern biology ever since. The cell theory continues to be the dominant theory of biology despite the numerous concepts that ultrastructural research and modern molecular biology have introduced.

Progression of Cell Theory

The discovery of cells would not have been possible without the advancement of the microscope. Objects that are too small to be seen with the naked eye are magnified with the help of a microscope.

Robert Hooke was the first person to coin the term cell (L., Cella = hollow space) in 1665. He used a custom-built compound microscope to look at a tiny slice of dry cork that had been cut from a larger piece. He then published a collection of essays under the title Micrographia which described cork as a honeycomb of chambers or “cells”. It is now recognized that the chambers or cells are void spaces left behind after the living components of the cell have broken down. 

The development of a more sophisticated microscope by Anton van Leeuwenhoek in 1673 led to his observation of numerous minute “animalcules” in water. Additionally, he conducted more research on sperm and red blood cells. Nevertheless, Leeuwenhoek’s findings on bacteria and spermatozoa were largely disregarded for a long time.

Marcello Malpighi and Nehemiah Grew performed in-depth analyses of plant cells and confirmed that cellular structures are present throughout the entire plant body. 

Malpighi referred to cells as “utriculi” and “saccule” in his 1671 publication “Anatome plantarum.” Grew used the terms “bladders”, “cells” and “pore” in his book “The Anatomy of Plants” and offered several illustrations of plant material that show he was aware of the cellular structure.

In 1838, a German botanist, Matthias Schleiden noticed that all plants are made up of many types of cells that constitute the plant’s tissues after studying many plants. 

Theodore Schwann (1839), a British zoologist, researched many animal cell types around the same time and found that cells had a thin outer layer known as the “plasma membrane.” Similarly, he also concluded that the presence of a cell wall is a distinctive feature of plant cells based on his research on plant tissues.

Based on this, Schwann proposed the hypothesis that the bodies of animals and plants are composed of cells and products of cells. He summarized his observations into three conclusions about cells:

  1. The cell is the unit of structure, physiology, and organization in living things.
  2. The cell retains a dual existence as a distinct entity and a building block in the construction of organisms.
  3. Cells form by free-cell formation, similar to the formation of crystals.

However, Schwann’s third conclusion stating that cells formed similarly to crystals, was discounted as this observation refers to the spontaneous generation of life. Schwann’s theory also did not explain how new cells were formed.

Rudolf Virchow, a German pathologist, first explained that cells divide and new cells are formed from pre-existing cells and famously wrote “omnis cellula-e cellula.” He modified the hypothesis of Schleiden and Schwann to give the cell theory a final shape.

Therefore, the cell theory states,

  1. All organisms are composed of one or more cells.
  2. The cell is the basic unit of life in all living things.
  3. All cells are produced by the division of pre-existing cells.

With his “swan-neck” experiment in 1865, Louis Pasteur further provided experimental proof in support of Virchow’s extension of the cell hypothesis.

Modern Cell Theory

The original cell theory proposed by Schleiden and Schwann is supplemented by a few additional principles in the modern version; the three basic components of cell theory, plus four additional statements:

  1. The cell pass information from cell to cell during cell division using DNA.
  2. All cells have basically the same chemical composition and metabolic activities.
  3. All cells have basically the same chemical & physiological functions (movement, digestion etc.)
  4. Cell activity depends on the activities of structures within the cell (organelles, nucleus, plasma membrane).

Understanding the functioning of cells in both healthy and ill conditions paves the way for the creation of novel vaccinations, more potent medications, superior plants, and a greater understanding of how all living things function.

Exception of Cell Theory

Cell theory does not have a universal application, i.e., certain living organisms do not have true cells.

Viruses do not easily fit into the parameters of a true cell. They lack a plasma membrane and metabolic machinery for energy production and the synthesis of proteins. 

The protozoan Paramecium, the fungus Rhizopus, and the algae Vaucheria are a few examples of additional organisms that do not fall inside the cell theory’s purview. All of these organisms have bodies made up of a single, undivided mass of protoplasm that lacks any cellular organization and has several nuclei.


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