Cell Theory & Structure

Cell Theory & Structure
Cell theory has three basic generalizations, according to the University of California at Davis. These generalizations state that all organisms are composed of one or more cells, the cell is the smallest unit that has properties of life, and life continues as a result of division and growth of single cells.
The Facts
Cell theory is often referred to as cell doctrine. The concept that all organisms are made up of similar units called cells was originally formulated by Matthias Jakob Schleiden and Theodor Schwann in 1839. This theory was developed prior to other important theories in biology including Charles Darwin's theory of evolution in 1859.
History
In 1838, Schleiden, a botanist, and Schwann, a zoologist, had a conversation regarding their respective studies of cells. According to Charles Mallery of the Department of Biology, University of Miami, Schwann saw similarities between plant cells that Schleiden discussed and his own observations of animal tissue cells. They proceeded to Schwann's lab and wrote a book about plant and animal cells the following year.
Features
Schwann and Schleiden's conclusions regarding cells stated that cells are a unit of structure, organization and physiology in living things. Furthermore, the cell acts as both a unique entity and as a building block in organism construction, and cells result from free-cell formation that is similar to crystals. Today, biologists know that their first two statements are true, however the third it incorrect. In fact, it was Rudolph Virchow who suggested a more accurate description of cell generation by stating that "all cells only arise from pre-existing cells."
Considerations
While the three generalizations listed in the overview are the core tenets, cell theory has three other statements that add to a more complete picture of cell structure. These tenets state that cells contain hereditary information that passes on from cell to cell during cell division. Furthermore, all cells are considered of the same chemical composition and all energy flow of life (particularly metabolism and biochemical reactions) occur within the cells.
Structure
All cells are considered to have three things in common. Cells have a cell membrane that selects which materials are allowed to pass into and out of the cell. They also have cytoplasm, which is the part of the cell that is enclosed by the cell membrane, and DNA, which is the genetic material of the cell. Furthermore, cells are divided into two types, prokaryotic (simple cells like bacteria) or eukaryotic (more complex cells like plants).