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Biological cell

The cell is the basic unit of life. All known cells (except a few
specialized cell types) have certain basic components in common:

   * cytoplasm (the living substance which makes up most of the cell volume)
   * DNA, the genetic information that acts as a blueprint for the other
     components.
   * Proteins, the machinery of the cell.
   * Membranes, which separate the cell form its environment, work as a
     filter, and act as a communication relay with the outside.

They also share several abilities:

   * Reproduction by cell division.
   * Metabolism, the taking in of raw material, using it to build cell
     components or breaking it down for energy, and releasing byproducts.
   * Protein biosynthesis by transcription of DNA to RNA and then
     translation to protein.

Organisms vary from single cells which survive individually, through
colonial forms with multiple similar cells living together, to multicellular
forms in which cells are specialized and do not generally survive if
separated. There are 220 types of cells and tissues that make up the human
body.

There are two basic types of cells: prokaryotic and eukaryotic. Prokaryotic
cells are structurally simple. They are found only in single-celled and
colonial organisms. In the three-domain system of taxonomy, prokaryotic
cells are placed in the domains Archaea and Eubacteria. Eukaryotic cells
have organelles with their own cell membranes. Single-celled eukaryotic
organisms are very diverse, but many colonial and multicellular forms also
exist. (The multicellular kingdoms, Animalia, Plantae and Fungi, are all eukaryotic.)

Features of prokaryotic and eukaryotic cells

 Ê                           Prokaryotes               Eukaryotes

    typical organisms    bacteria            protists, fungi, plants,
                                             animals
      typical size       ~ 1-10 um           ~ 10-100 um

     type of nucleus     nucleoid region; no real nucleus with double
                         real nucleus        membrane

           DNA           circular            linear molecules (chromosomes)
                                             with histone proteins

 RNA-/protein-synthesis  coupled in          RNA-synthesis inside the core
                         cytoplasm           protein synthesis in cytoplasm
        ribosomes        50S+30S             60S+40S
                                             highly structured by
 cytoplasmatic structure very few structures intercellular membranes and a cytoskeleton

      cell movement      flagella made of    flagella and cilia made of
                         flagellin           tubulin
      mitochondria       none                one to several dozen
      chloroplasts       none                in algae and plants
                                             single cells, colonies, higher
      organization       usually single      organisms with specialized
                         cells
                                             cells
                                             Mitosis (core division)
      cell division      simple division     Cytokinesis (cytoplasmatic division)

Prokaryotic cells

   * The cytoplasm of prokaryotes (the liquid which makes up most of the
     cell volume) is diffuse and granular due to ribosomes (protein
     factories) floating in the cell.
   * The plasma membrane (a phospholipid bilayer) separates the interior of
     the cell from its environment and serves as a filter and communications
     beacon.
   * Most prokaryotes (except for the bacterial Mycoplasma and the
     Ureaplasma of the archaea) have a cell wall. It consists of
     peptidoglycan and acts as an additional barrier against exterior
     forces. It also prevents the cell from "exploding" of osmotic pressure
     against a hypotonic environment.
   * The prokaryotic DNA is a circular molecule. Even without a real
     nucleus, the DNA is somehow condensed in a nucleoid. Prokaryotes can
     have extrachromosomal DNA organized in circular molecules called
     plasmids. Plasmids can carry additional functions, such as antibiotics
     resistance.
   * Some prokaryotes have flagella which enable them to move actively
     instead of passively drifting.

Eukaryotic cells

   * The cytoplasm of eukaryotes does not appear as granular as that of
     prokaryotes, since an important part of the ribosomes are bound to the
     endoplasmatic reticulum.
   * The plasma membrane resembles that of prokaryotes in function, with
     minor differences in the setup. Cell walls may or may not be present.
   * The eukaryotic DNA is organized in one or more linear molecules, called
     chromosomes, which are highly condensed (e.g. folded around histones).
     All chromosomal DNA is stored in the cell nucleus, separated from the
     cytoplasm by a membrane. Some eukaryotic organelles can contain some
     DNA.
   * Eukaryotes can become mobile using cilia or flagella. The flagella are
     more complex than those of prokaryotes.

History

   * 1665 : Robert Hooke discovers cells in cork, then in living plants
     using the first microscopes.
   * 1839 : Theodor Schwann discovers that plants and animals are made of
     cells, concluding that cells are a common unit of structure and
     development, thus founding cell theory. Schwann cells are named after
     him.
   * The belief that life forms are able to occur spontaneously ("generatio
     spontanea") is contradicted by Louis Pasteur (1822-1895).
   * Rudolph Virchow states that cells always emerge from cell divisions
     ("omnis cellula ex cellula").

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