Cell

cell: nuclei stained blue, mitochondria stained red, and F-actin, a component of microfilaments, stained green. Cell imaged on a fluorescent microscope.]]

In biology, the **cell** is the basic structure of organisms. All living things are made up of cells, from the organs in the human body to the parts of plants. All cells are made by the division of cells that already exist ( cells). 

The environment outside the cell is separated from the cytoplasm inside the cell by the cell membrane. Inside some cells, parts of the cell stay separate from other parts. These separate parts are called organelles (like small organs). They each do different things in the cell. Examples are the nucleus (where DNA is), and mitochondria (where chemical energy is converted). Prokaryotes do not have membrane-bound organelles like Eukaryotes.

There are around 37 trillion (3.7 x 1013) cells in the human body. Organisms (living things) which are made up of multiple cells are eukaryotes.

Most cells are small, the largest just being seen by the human eye (egg cell). They need to be small in order to work as they should. In things like cells and mechanical engineering (and other types of mechanic work), the smaller and more simple things are usually means they can survive better. This means that a lot of the thing things are as small as they can be.
## Regions of prokaryotic cell 
The prokaryotic cell has three regions made up of the envelope, cytoplasmic region and flagella (or pili), called:

* **Cell envelope:** This consists of the plasma membrane covered by a cell wall. Some bacteria have a third layer called a capsule. Virions might have an envelope called the capsid. Flagella usually helps the cell move which cilia the cell move other cells (across its surface). Prokaryotic organisms evolved before eukaryotic organisms, so at one point the world consisted of nothing but prokaryotic organisms.

### Viruses 
There are also viruses, which are biological organisms. Viruses violate the rules that we have for cells (living things) so are not classed as living things.
# All living things are made of cells.
# The cell is the basic unit of structure and function in all organisms.
# Every cell comes from another cell that lived before it.
There is a 4th idea, because of advancements in biology: Cells carry genetic information in the form of deoxyribonucleic acid (DNA). This genetic material is passed on from parent to daughter cell.

Cells were discovered by Robert Hooke (1635–1703). He used a compound microscope with two lenses to look at the structure of cork, and to look at leaves and some insects. He did this from about 1660, and reported it in his book *Micrographica* in 1665. He named cells after the Latin word *cella*, meaning room. He did this because he thought cells looked like small rooms in a cathedral. It is not clear who first saw animal cells, Malpighi, Jan Swammerdam (1637–1680) or Antonie van Leeuwenhoek (1632–1723).p17

Leeuwenhoek's discoveries and drawings of 'little animalcules' opened up a whole new world for naturalists. Protozoa, and microorganisms generally were discovered, and the discoveries about them are still going on today. Christian Gottfried Ehrenberg's book *Die Infusionsthierchen* summarised what was known in 1838. Lorenz Oken (1779–1851) in 1805 wrote that infusoria (microscopic forms) were the basis of all life. 

The idea that cells were the basis of the larger forms of life came in the 18th century. Finding out who did the work has taken some time:
:"The work of the Czech Jan Purkyně (1787–1869) and his student and collaborator Gabriel Valentin (1810–1883) was unjustly denigrated by the nationalistic Germans. They have a claim to some priority in the cell theory".Chapter 9 Johannes Müller (1801–1858) also made great contributions. "It was, however, his student Theodor Schwann (1810–1882) and Matthias Schleiden (1804–1881) who got the credit for the *cell theory*, despite the fact that some of their observations were not correct, and their credits to previous workers were "a travesty".p97 

## Cell reproduction 
The body cells of animals divide by simple mitotic cell division. Sexual reproduction is ancestral in eukaryotes, and in animals it is carried out by specialised sex cells. They are produced by a process called meiosis. 

Prokaryotic cells reproduce using binary fission, where the cell simply splits in half. For both mitosis and binary fission the cell must replicate (copy) all its genetic information (DNA) so that each new cell will have a copy.

## Related pages 
*Cytology
*Plant cell

## References