The cell (from the Latin cellula small room) is the structural unit, functional and reproductive constituting all or part of a living being (except viruses). Each cell is a living entity, which in the case of multicellular organisms, works independently, but coordinated with others. The same type of cells are present in tissues themselves together in bodies.

The cell theory implies the unity of all life: all living things are composed of cells whose basic structure is common and homeostasis of internal environment, environment physico-chemical and controlled environment for developing cells of 'species.

History of concept
* 1665: Robert Hooke discovers cells in cork dead, these cells are thought to himself cells of a monastery, hence the name. He then observed the cells in living plants, using the first microscopes.
* 1839: Theodor Schwann discovered that plants and animals are all made of cells, concluding that the cell is the common unit of structure and development, which founded the cell theory. He gave his name to Schwann cells.
* The belief that life forms can appear spontaneously (spontaneous generation) is disproved by Louis Pasteur (1822-1895).
* 1858: Rudolf Virchow stated that cells arise from the result of cell division ( "omnis cellula ex cellula), which is based in cellular terms the question of the egg and the chicken. It is precisely this part which is contested by proponents of creationism or its latest incarnation, intelligent design.

Cell theory
1. The cell is the constituent unit of living organisms. It is also the functional unit.
2. The body depends on the activity of cells or tissues grouped to provide different functions.
3. The biochemical activities of cells are coordinated and determined by certain structures found inside cells.
4. The proliferation of cells allows the maintenance of organisms and their multiplication.
5. This theory was formulated in 1838 by Schleiden and Schwann the cell is living unit (everything is living cell). This theory also suggests the presence of organelles within these cells.

The cell machinery to produce the order
The cell is a highly organized state of matter that maintain order while being subject to the principles of thermodynamics requires the establishment of structures that use energy, material exterior (one creates the order in the cell but overall entropy increases), the cell is a thermodynamically open system.

An enclosed conducting trade with the outside
The cell is a spatial unit, delimited by a membrane. This, far from being a tight deadline, is an area of exchange for the establishment of flow.

The plasma membranes, despite their diversity have exceptions (some thermophilic archaea possess a single layer of lipids), the same structure:

* A phospholipid bilayer composed of amphiphilic lipids, which is a basic filter allowing the passage of hydrophobic substances, inhibiting that of hydrophilic.
* Transmembrane proteins and peripheral roles various (transfers, transport, signal transduction ...)

The membrane acts not only as a filter, that is to say, by allowing certain molecules according to the concentration difference (mistakenly called concentration gradient) but also using energy (osmotic, chemical ...) to encourage the flow endergonic. It also allows the passage of light, heat ... As surface contact with the outside, it also ensures the transmission of information necessary for the responsiveness of the cell to environmental changes and coordination with other cells.

The plasma membrane creates an air space closed in constant exchange with the surrounding environment.

Compartmentation: development of microenvironments with specific properties
The presence of a biological membrane surrounding a space, either the cytoplasm or an organelle of light, will help in controlling the exchange of macromolecules, ions (and any other molecule) the establishment of conditions favoring certain reactions to others: by varying the different physico-chemical factors (pH, ion concentration ...), the nature of enzymes and products, their number ...

This environment allows the biosynthesis and degradation of organic molecules, thus maintaining a highly organized structure with a constant recycling of molecules that form.

This compartmentalization is particularly pronounced in the case of eukaryotes: it allows the functional specialization of different organelles (the composition of their light is different, they are the seat of different reactions: they will thus be able to support the production of this product in a compartment , its destruction in another).

The cell flow organized matter and energy
This basic structure (membrane organizing exchanges surrounding a compartment, rather than specific chemical reactions) will enable the establishment and maintenance of flows of matter, energy, information ... through the cell. So there is a real organization of cell-external trade, which will allow the "system unit" (as defined in thermodynamics) to maintain its highly organized structure.

Finally, the cell is not a stable and unchanging but rather a dynamic entity requiring a constant supply of matter and energy to allow its operation and maintenance of its structure: the only organization persists, its constituents being in perpetual renewal.

The resting potential of the cell determines the potential difference (-70 mV), with the interior of the cell negative.

Read also Basal Cell Carcinoma