1.1 Cell theory
The word cell was introduced to biology in 1665 by Robert Hooke in his collection of microscopic drawings, called Micrographia, which included one of a thin slice of cork. He recorded the honeycomb structure of cork and referred to the compartments as cells by analogy to the cell of a prisoner or monk. The term, however, has been retained not to describe the voids remaining after the disintegration of cell cytoplasm (observed by Hooke) but the living contents normally resident between these plant cell walls. Today, the cell may be defined as the simplest integrated unit in living systems capable of independent survival.
By the early nineteenth century, recognition was given to cells as life forms and their organization into more complex multicellular organisms. In 1839, Theodor Schwann, a zoologist, published Mikroskopische Untersuchungen, which also contained figures supplied by Mathias Schleiden, a botanist, to record that plants and animals are composed of similar cells. Twenty years later, Rudolf Virchow announced āomnis cellula e cellulaā, i.e. all cells arise from pre-existing cells.
Despite the wide variety of cell types, most cells can be classified according to their size and complexity into one of two categories: prokaryotes or eukaryotes. These terms are derived from Greek, karyon meaning kernel as in a nut, pro- meaning before and eu- meaning well. The eukaryotes therefore contain a well-formed nucleus (Latin for nut) whereas the prokaryotes are devoid of a discrete nucleus since their genetic material is not enclosed by an intracellular membrane.
A definitive feature of prokaryotic cells is their lack of membrane-bound structures although layers of internal membranes may arise from the plasma membrane. In contrast, eukaryotic cells contain numerous membrane-bound organelles, e.g. mitochondria, chloroplasts, endoplasmic reticulum, Golgi apparatus and microbodies. Although the size of cells of both categories are variable, in general, prokaryotic cells range from 0.1 to 3 Ī¼m whereas the dimensions of most eukaryotic cells are 10 to 20 Ī¼m. Prokaryotic organisms may be subdivided into three groups: bacteria, mycoplasmas and cyanobacteria which differ morphologically.
During the 1970s, the dichotomy of cells into prokaryotic and eukaryotic began to be challenged by the discovery of a few classes of bacteria, called archaebacteria, which, although possessing the general structural features of prokaryotic cells, exhibit distinctive biochemical characteristics.