Cell theory, also known as cell doctrine, explains that all organisms consist of cells. The concept was formally established in 1839, and has acted as the basis of other biological concepts ever since. Originally, scientists by the names of Theodor Schwann and Matthias Schleiden formed cell theory based on three tenets, or principles. These tenets have since been revised and expanded to include six tenets.
Possibly the most fundamental concept in biology, cells make up the structure, physiology and organization of organisms. Simply put, that means without cells there would be no living things, plant, animal or otherwise. Eventually we were able to understand that all cell are either of the prokaryote type, meaning without nucleus, or eukaryote, meaning with a nucleus. Only the most simple organisms are prokaryotes, while fungi, protists, plants and animals are of the eukaryote type.
Cells can be viewed as distinct entities while simultaneously being building blocks in the construction of living things. The structure of cells follows an increasingly complex chain of chemicals starting from a single atom. Multiple atoms combine to form molecules, and many molecules make polymers and then complexes. A number of complexes create organelles which carry out individual functions of a cell. Lastly, of course, a collection of organelles comprise a single cell.
The third of the original tenets, Schwann and Schleiden thought cells formed through spontaneous generation, similar to crystals. Eventually this tenet was disproven, and a scientist named Rudolph Virchow correctly revised the tenet to state that “All cells only arise from pre-existing cells.” The modern understanding of cells explains that they divide to create new cells. One cell essentially splits to make two, and two to make four, etc. Spontaneous generation is not involved.
With the original three tenets as a foundation, modern cell theory amends the third tenet to explain new cells arrive from pre-existing cells by division. In addition, a new fourth tenet states cells contain hereditary information, also known as DNA, which is passed on as cells divide. The fifth tenet states that the chemical composition of all cells is basically the same. Finally, the sixth tenet tells us that the energy flow of life happens in cells through metabolic and biochemical processes. Today the cell is viewed as a unit of self-control, and descriptions of cells must include how genetic information is converted to structure.